Vascular system - Encyclopedia

GEOGRAPHICAL NAMES

VASCULAR SYSTEM. I. Anatomy. - The circulatory or blood vascular apparatus consists of the central pump or heart, the arteries leading from it to the tissues, the capillaries, through the walls of which the blood can give and receive substances to and from the tissues of the whole body, and the veins, which return the blood to the heart. As an accessory to the venous system, the lymphatics, which open finally into the great veins, help in returning some of the constituents of the blood. Separate articles are devoted to the heart, arteries, veins and lymphatic system, and it only remains here to deal with the capillaries.

The blood capillaries form a close network of thin-walled tubules from 2 0 1 -0-6to 3 o l ooof an inch in diameter, permeating, with a few exceptions, the whole of the body, and varying somewhat in the closeness of its meshwork in different tparts. In the smallest capillaries, in which the arteries end and from which the veins begin, the walls are formed only of somewhat oval endothelial cells, each containing an oval nucleus and joined to its adjacent cells by a serrated edge, in the interstices of which is a small amount of intercellular cement, easily demonstrated by staining the preparation with nitrate of silver. Here and there the cement substance is more plentiful, and these spots when small are known as stigmata, when large as stomata. As the capillaries approach the arteries on the one hand and the veins on the other they blend and become larger, and a delicate connective tissue sheath outside the endothelium appears, so that the transition from the capillaries into the arterioles and venules is almost imperceptible; indeed, the difference between a large artery or vein and a capillary, apart from size, is practically the amplification and differentiation of its connective tissue sheath.

The first appearance of a vascular system is outside the body of the embryo in the wall of the yolk sac, that is to say, in the mesoderm or the middle one of the three embryonic layers.

The process is a very early one and in the chick is seen to begin at the end of the first day of incubation. The first occurrence is a network made up of solid cords of cells forming in certain places solid cell masses called the blood islands of Pander. The central cells of these islands divide by karyokinesis and gradually float away into the vessels which are now being formed by fluid from the exterior, finding its way into the centre of the cell cords and pressing the peripheral cells flat to form the endothelial lining. These free cells from the blood islands are known as erythroblasts and are the primitive corpuscles of the foetal blood. They have a large reticular nucleus and at first are colourless though haemoglobin gradually develops within them and the blood becomes red (see BLooD). The erythroblasts continue to multiply by karyokinesis in early foetal life, especially in the liver, spleen, bone marrow and lymphatic glands, though later on their formation only occurs in the red bone marrow. In most of the erythroblasts the nucleus soon becomes contracted, and the cell is then known as a normoblast, while ultimately the general view is that the nucleus disappears by extrusion from the cell and the non-nucleated red blood plates or erythrocytes remain. The leucocytes or white blood corpuscles appear later than the red, and are probably formed from lymphoid tissue in various parts of the body. The blood vessels thus formed in the so-called vascular area gradually travel along the vitelline stalk into the body of the embryo, and two vessels larger than the rest are formed one on each side of the stalk. These are the vitelline veins, which, as they pass towards the caudal end of the embryo, become the two primitive aortae, and these fuse later on to form the heart. After the inversion of the pericardial region and formation of the head fold (see Coelom And Serous Membranes) the front of the developing heart becomes the back, and the vitelline veins new enter it from behind. It must be understood that most of our knowledge of the early history of the blood vessels is derived from the study of lower mammals and birds, and that this is being gradually checked by observations on human embryos and on those of other primates. It seems probable that in these mammals, owing to the small size of the yolk sac, the vessels of the embryo establish an early communication with those of the chorion before the vitelline veins are formed (see Quain's Anatomy, vol. i., London, 1908). The later stages of the embryology of the vascular system are sketched in the articles on Heart, Arteries, Veins and Lymphatic System (q.v.). (F. G. P.) II. History Of Discovery Galen, following Erasistratus (ob. 280 B.C.) and Aristotle, clearly distinguished arteries from veins, and was the first to overthrow the old theory of Erasistratus that the arteries contained air. According to him, the vein arose from the liver in two great trunks, the vena porta and vena cava. The first was formed by the union of all the abdominal veins, which absorbed the chyle prepared in the stomach and intestines, and carried it to the liver, wher it was converted into blood. The vena cava arose in the liver, divided into two branches, one ascending through the diaphragm to the heart, furnishing the proper veins of this organ; there it received the vena azygos, and entered the right ventricle, along with a large trunk from the lungs, evidently the pulmonary artery. The vena azygos was the superior vena Cava, the great vein which carries the venous blood from the head and upper extremities into the right auricle. The descending branch of the great trunk supposed to originate in the liver was the inferior vena cava, below the junction of the hepatic vein. The arteries arose from the left side of the heart by two trunks, one having thin walls (the pulmonary veins), the other having thick walls (the aorta). The first was supposed to carry blood to the lungs, and the second to carry blood to the body. The heart consisted of two ventricles, communicating by pores in the septum; the lungs were parenchymatous organs communicating with the heart by the pulmonary veins. The blood-making organ, the liver, separates from the blood subtle vapours, the natural spirits, which, carried to the heart, mix with the air introduced by respiration, and thus form the vital spirits; these, in turn carried to the brain, are elaborated into animal spirits, which are distributed to all parts of the body by the nerves.' Such were the views of Galen, taught until early in the 16th century.

Jacobus Berengarius of Carpi (ob. 1530) investigated the structure of the valves of the heart. Andreas Vesale or Vesalius (1514-1564) contributed largely to anatomical knowledge, especially to the anatomy of the circulatory organs. He determined the position of the heart in the chest; 1 See Burggraeve's Histoire de l'anatomie (Paris, 1880).

he studied its structure, pointing out the fibrous rings at the bases of the ventricles; he showed that its wall consists of layers of fibres connected with the fibrous rings; and he described these layers as being of three kinds - straight or vertical, oblique, and circular or transverse. From the disposition of the fibres he reasoned as to the mechanism of the contraction and relaxation of the heart. He supposed that the relaxation, or diastole, was accounted for principally by the longitudinal fibres contracting so as to draw the apex towards the base, and thus cause the sides to bulge out; whilst the contraction, or systole, was due to contraction of the transverse or oblique fibres. He showed that the pores of Galen, in the septum between the ventricles, did not exist, so that there could be no communication between the right and left sides of the heart, except by the pulmonary circulation. He also investigated minutely the internal structure of the heart, describing the valves, the columnae corneae and the musculi papillares. He described the mechanism of the valves with much accuracy. He had, however, no conception either of a systemic or of a pulmonary circulation. To him the heart was a reservoir from which the blood ebbed and flowed, and there were two kinds of blood, arterial and venous, having different circulations and serving different purposes in the body. Vesalius was not only a great anatomist: he was a great teacher; and his pupils carried on the work in the spirit of their master. Prominent among them was Gabriel Fallopius (1523-1562), who studied the anastomoses of the blood vessels, without the art of injection, which was invented by Frederick Ruysch (1638-1731) more than a century later. Another pupil was Columbus. (Matthieu Reald Columbo, ob. 1560), first a prosector in the anatomical rooms of Vesalius and afterwards his successor in the chair of anatomy in Padua; his name has been mentioned as that of one who anticipated Harvey in the discovery of the circulation of the blood. A study of his writings clearly shows that he had no true knowledge of the circulation, but only a glimpse of how the blood passed from the right to the left side of the heart. In his work there is evidently a sketch of the pulmonary circulation, although it is clear that he did not understand the mechanism of the valves, as Vesalius did. As regards the systemic circulation, there is the notion simply of an oscillation of the blood from the heart to the body and from the body to the heart. Further, he upholds the view of Galen, that all the veins originate in the liver; and he even denies the muscular structure of the heart.'. In 1553 Michael Servetus (1511-1553), a pupil or junior fellow-student of Vesalius, in his Christianismi Restitutio, described accurately the pulmonary circulation? Servetus perceived the course of the circulation from the right to the left side of the heart through the lungs, and he also recognized that the change from venous into arterial blood took place in the lungs and not in the left ventricle. Not so much the recognition of the pulmonary circulation, as that had been made previously by Columbus, but the discovery of the re- spiratory changes in the lungs constitutes Servetus's claim to be a pioneer in physiological science.

Andrea Cesalpino (1519-1603), a great naturalist of this period, also made important contributions towards the dis-. covery of the circulation, and in Italy he is regarded as the real discoverer. 3 Cesalpino knew the pulmonary circulation. Further, he was the first to use the ' An interesting account of the views of the precursors df Harvey will be found in Willis's edition of the Works of Harvey, published by the Sydenham Society. Compare also P. Flourens, Histoire de la decouverte de la circulation du sang (Paris, 1854), and Professor R. Owen, Experimental Physiology, its Benefits to Mankind, with an Address on Unveiling the Statue of W. Harvey, at Folkestone, 6th August 1881. See Willis, Servetus and Calvin (London, 1877).

' A learned and critical series of articles by Sampson Gamgee in the Lancet, in 1876, gives an excellent account of the controversy as to whether Cesalpino or Harvey was the true discoverer of the circulation; see also the Harveian oration for 1882 by George Johnston (Lancet, July 1882), and Professor G. M. Humphry, Journ. Anat. and Phys., October 1882.

term " circulation," and he went far to demonstrate the systemic circulation. He experimentally proved that, when a vein is tied, it fills below and not above the ligature. The following passage from his Quaestiones Medicae (lib. v. cap. 4, fol. 125), quoted by Gamgee, shows his views: " The lungs, therefore, drawing the warm blood from the right ventricle of the heart through a vein like an artery, and returning it by anastomosis to the venal artery (pulmonary vein), which tends towards the left ventricle of the heart, and air, being in the meantime transmitted through the channels of the aspera arteria (trachea and bronchial tubes), which are extended near the venal artery, yet not communicating with the aperture as Galen thought, tempers with a touch only. This circulation of the blood (huic sanguinis circulationi) from the right ventricle of the heart through the lungs into the left ventricle of the same exactly agrees with what appears from dissection. For there are two receptacles ending in the right ventricle and two in the left. But of the two only one intromits; the other lets out, the membranes (valves) being constituted accordingly." Still Cesalpino clung to the old idea of there being an efflux and reflux of blood to and from the heart, and he had confused notions as to the veins conveying nutritive matter, whilst the arteries carried the vital spirits to the tissues. He does not even appear to have thought of the heart as a contractive and propulsive organ, and attributed the dilatation to " an effervescence of the spirit," whilst the contraction - or, as he termed it, the " collapse " - was due to the appropriation by the heart of nutritive matter. Whilst he imagined a communication between the termination of the arteries and the commencement of the veins, he does not appear to have thought of a direct flow of blood from the one to the other. Thus he cannot be regarded as the true discoverer of the circulation of the blood. More recently Ercolani has put forward claims on behalf of Carlo Ruini as being covery the true discoverer. Ruini published the first edition of circa- of his anatomical writings in 1598, the year William Harvey entered at Padua as a medical student. This claim has been carefully investigated by Gamgee, who has come to the conclusion that it cannot be maintained.4 The anatomy of the heart was examined, described and figured by Bartolomeo Eustacheo (c. 1500-1574) and by Julius Caesar Aranzi or Arantius (c. 1530-1589), whose name is associated with the fibro-cartilaginous thickenings on the free edge of the semilunar valves (corpora Arantii). Hieronymus Fabricius of Acquapendente (1537-1619), the immediate predecessor and teacher of Harvey, made the important step of describing the valves in the veins; but he thought they had a subsidiary office in connexion with the collateral circulation, supposing that they diverted the blood into branches near the valves; thus he missed seeing the importance of the anatomical and experimental facts gathered by himself. At the time when Harvey arose the general notions as to the circulation may be briefly summed up as follows: the blood ebbed and flowed to and from the heart in the arteries and veins; from the right side at least a portion of it passed to the left side through the vessels in the lungs, where it was mixed with air; and, lastly, there were two kinds of blood - the venous, formed originally in the liver, and thence passing to the heart, from which it went out to the periphery by the veins and returned by those to the heart; and the arterial, containing " spirits " produced by the mixing of the blood and the air in the lungs - sent out from the heart to the body and returning to the heart by the same vessels. The pulmonary circulation was understood so far, but its relation to the systemic circulation was unknown. The action of the heart, also, as a propulsive organ was not recognized. It was not until 1628 that Harvey announced his views to the world by publishing his treatise De Motu Cordis et Sanguinis. His conclusions are given in the following celebrated passage: " And now I may be allowed to give in brief my view of the circulation of the blood, and to propose it for general adoption. Since all things, both argument and ocular demonstration, show that the blood passes through the lungs and heart by the auricles and 4 Gamgee, " Third Historical Fragment," in Lancet, 1876.

Harvey. ventricles, and is sent for distribution to all parts of the body, where it makes its way into the veins and pores of the flesh, and then flows by the veins from the circumference on every side to the centre, from lesser to the greater veins, and is by them finally discharged into the vena cava and right auricle of the heart, and this in such a quantity, or in such a flux and reflux, thither by the arteries, hither by the veins, as cannot possibly be supplied by the ingestor, and is much greater than can be required for mere purposes of nutrition, it is absolutely necessary to conclude that the blood in the animal body is impelled in a circle, and is in a state of ceaseless motion, that this is the act or function which the heart performs by means of its pulse, and that it is the sole and only end of the motion and contraction of the heart " (bk. x. ch. xiv. p. 68).

Opposed to Caspar Hofmann of Nuremberg (1571-1623), Veslingius (Vesling) of Padua (1598-1649), and J. Riolanus the younger, this new theory was supported by Roger Drake, .a young Englishman, who chose it for the subject of a graduation thesis at Leiden in 1637, by Werner Rolfinck of Jena (1599-1673), and especially by Descartes, and quickly gained the ascendant; and its author had the satisfaction of seeing it confirmed by the discovery of the capillary circulation, and universally adopted. The circulation in the capillaries circula- between the arteries and the veins was discovered by Marcellus Malpighi (1628-1694) of Bologna in 1661. He saw it first in the lungs and the mesentery of a frog, and the discovery was announced in the second of two letters, Epistola de Pulmonibus, addressed to Borelli, and dated 1661.1 Malpighi actually showed the capillary circulation to the astonished eyes of Harvey. Anthony van Leeuwenhoek (1632-1723) in 1673 repeated Malpighi's observations, and studied the capillary circulation in a bat's wing, the tail of a tadpole and the tail of a fish. William Molyneux studied the circulation in the lungs of a water newt in 1683.2 The idea that the same blood was propelled through the body in a circuit suggested that life might be sustained by renewing the blood in the event of some of it being lost. About 1660 Lower, a London physician (died 1681), succeeded in transferring the blood of one animal directly from its blood vessels into those of another animal. This was first done by passing a " quill " or a " small crooked pipe of silver or brass " from the carotid artery of one dog to the jugular vein of another. 3 This experiment was repeated and modified by Sir Edmund King (1629-1709), Thomas Coxe (1615-1685), Gayant and Denys with such success as to warrant the operation being performed on man, and accordingly it was carried out by Lower and King on the 23rd of November 1667, when blood from the arteries of a sheep was directly introduced into the veins of a man. 4 It would appear that the operation had previously been performed with success in Paris.

The doctrine of the circulation being accepted, physiologists next directed their attention to the force of the heart, the pressure of the blood in the vessels, its velocity, and the phenomena of the pulse wave. Giovanni velocity Alphonso Borelli (1608-1679) investigated the circulaton during the lifetime of Harvey. He early conceived the design of applying mathematical principles to the explana tion of animal functions; and, although he fell into many errors, he must be regarded as the founder of animal mechanics. In his De Motu Animalium (1680-85) he stated his theory of the circulation in eighty propositions, and in prop. lxxiii., founding on a supposed relation between the bulk and the strength of muscular fibre as found in the ventricles, erroneously concluded that the force of the heart was equal to the pressure of a weight of 180,000 lb. He also recognized and figured the spiral arrangement of fibres in the ventricles. The question was further investigated by James. Keill, a Scottish physician (1673-1719), who in his Account of Animal Secretion, the Quantity of Blood in the Human Body, and Muscular Motion (1708) attempted to estimate the velocity of blood in the aorta, and gave it at 52 ft.

1 See his Opera Omnia, vol. i. p. 328.

2 Lowthorp, Abridgement of Trans. Roy. Soc., 5th ed. vol. iii.

p. 230.

3 Ibid. p. 231.4 Ibid. p. 226.

per minute. Then, allowing for the resistance of the vessels, he showed that the velocity diminishes towards the smaller vessels, and arrived at the amazing conclusion that in the smallest vessels it travels at the rate of 4 in. in 278 days, - a good example of the extravagant errors made by the mathematical physiologists of the period. Keill further described the hydraulic phenomena of the circulation in papers communicated to the Royal Society and collected in his Essays on Several Parts of the Animal Oeconomy (1717). In these essays, by estimating the quantity of blood thrown out of the heart by each contraction, and the diameter of the aortic orifice, he calculated the velocity of the blood. He stated (pp. 84, 87) that the blood sent into the aorta with each contraction would form a cylinder 8 in. (2 oz.) in length and be driven along with a velocity of 156 ft. per minute. Estimating then the resistances to be overcome in the vessels, he found the force of the heart to be " little above 16 oz.," - a remarkable difference from the computation of Borelli. Keill's method was ingenious, and is of historical interest as being the first attempt to obtain quantitative results; but it failed to obtain true results, because the data on which he based his calculations were inaccurate. These calculations attracted the attention not only of the anatomico-physiologists, such as Haller, but also of some of the physicists of the time, notably of Jurin and D. Bernoulli. Jurin (died 1750) gave the force of the left ventricle at 9 lb 1 oz., and that of the right ventricle at 6 lb 3 oz. He also stated with remarkable clearness, considering that he reasoned on the subject as a physicist, without depending on experimental data gathered by himself, the influence on the pulse induced by variations in the power of the heart or in the resistance to be overcome.' The experimental investigation of the problem was supplied by Stephen Hales (1677-1761), rector of Teddington in Middlesex, who in 1708 devised the method of estimating the force of the heart by inserting a tube into a large artery and observing the height to which the blood was impelled into it. Hales is the true founder of the modern experimental method in physiology. He observed in a horse that the blood rose in the vertical tube, which he had connected with the crural artery, to the height of 8 ft. 3 in. perpendicular above the level of the left ventricle of the heart. But it did not attain its full height at once: it rushed up about half-way in an instant, and afterwards gradually at each pulse 12, 8, 6, 4, 2, and sometimes 1 in. When it was at its full height, it would rise and fall at and after each pulse 2, 3 or 4 in.; and sometimes it would fall 12 or 14 in., and have there for a time the same vibrations up and down at and after each pulse as it had when it was at its full height, to which it would rise again after forty or fifty pulses. 6 He then estimated the capacity of the left ventricle by a method of employing waxen casts, and, after many such experiments and measurements in the horse, ox, sheep, fallow deer and dog, he calculated that the force of the left ventricle in man is about equal to that of a column of blood 72 ft. high, weighing 512 lb, or, in other words, that the pressure the left ventricle has to overcome is equal to the pressure of that weight. When we contrast the enormous estimate of Borelli (180,000 lb) with the under-estimate of Keill (16 oz.), and when we know that the estimate of Stephen Hales (1677-1761), as corroborated by recent investigations by means of elaborate scientific appliances, is very near the truth, we recognize the far higher service rendered to science by careful and judicious experiment than by speculations, however ingenious. With the exception of some calculations by Dan Bernoulli (1700-1782) in 1748, there was no great contribution to haemadynamics till 1808, when two remarkable papers appeared from Thomas Young (1773-1829). In the first, entitled " Hydraulic Investigations," which appeared You g s in the Phil. Trans., he investigated the friction and dis charge of fluids running in pipes and the velocity of rivers, the 5 Jones, Abridgement of Phil. Trans. (3d ed., 3749), vol. v. p. 223. See also for an account of the criticisms of D. Bernoulli the elder and others, Haller's Elementa Physiologiae, vol. i. p. 448.

Hales, Statical Essays, containing Haemastatics, &c. (1733), vol. ii. p. I.

resistance occasioned by flexures in pipes and rivers, the propagation of an impulse through an elastic tube, and some of the phenomena of pulsations. This paper was preparatory to the second, " On the Functions of the Heart and Arteries," - the Croonian lecture for 1808 - in which he showed more clearly than had hitherto been done (1) that the blood pressure gradually diminishes from the heart to the periphery; (2) that the velocity of the blood becomes less as it passes from the greater to the smaller vessels; (3) that the resistance is chiefly in the smaller vessels, and that the elasticity of the coats of the great arteries comes into play in overcoming this resistance in the interval between systoles; and (4) that the contractile coats do not act as propulsive agents, but assist in regulating the distribution of blood.1 The next epoch of physiological investigation is characterized by the introduction 'of instruments for accurate measurement, and the graphic method of registering phenomena, now so largely used in science. 2 In 1825 appeared E. and Wilhelm Weber's (1804-1891) Wellenlehre, and in 1838 Ernest Weber's (1795-1878) Ad Notat. Anatom. et Physiolog. i., both of which contain 'an exposition of E. H. Weber's schema of the circulation, a scheme which presents a true and consistent theory. In 1826 Jean Louis Marie Poiseuille invented the haemadynamometer. 3 This was adapted with a marker to a recording cylinder by Ludwig in 1847, so as to form the instrument named by Alfred Volkmann (1801-1877) the kymograph. Volkmann devised the haemadromometer for measuring the velocity of the blood in 1850; for the same purpose Vierordt constructed the haematachometer in 1858; Chauveau and Pierre Lortet (1792-1868) first used their haemadromograph in 1860; and lastly, Ludwig and Dogiel obtained the best results as regards velocity by the " stream-clock " in 1867. As regards the pulse, the first sphygmograph was constructed by Karl Vierordt (1818-1884) in 1856; and Etienne Marey's form, of which there are now many modifications, appeared in 1860. In 1861 Jean Chauveau (b. 1827) and Marey obtained tracings of the variations of pressure in the heart cavities (see below), by an experiment which is of great historical importance. During the past twenty-five years vast accumulations of facts have been made through the instruments of precision above alluded to, so that the conditions of the circulation, as a problem in hydrodynamics, have been thoroughly investigated. Since 1845, when the brothers Weber discovered the inhibitory action of the vagus, and 1858, when Claude Bernard (1813-1878) formulated his researches showing the existence of a vaso-motor system of nerves, much knowledge has been acquired as to the relations of the nervous to the circulatory system. The Webers, John Reid (1816-1895), Claude Bernard and Carl Ludwig (1809-1849) may be regarded as masters in physiology equal in standing to those whose researches have been more especially alluded to in this historical sketch. The Webers took the first step towards recognizing the great principle of inhibitory action; John Reid showed how to investigate the functions of nerves by his classical research on the eighth pair of cranial nerves; Claude Bernard developed the fundamental conception of vaso-motor nerves; and Ludwig showed how this conception, whilst it certainly made the hydraulic problems of the circulation infinitely more complicated than they were even to the scientific imagination of Thomas Young, accounted for some of the phenomena and indicated at all events the solidarity of the arrangements in the living being. Further, Ludwig and his pupils used the evidence supplied by some of the phenomena of the circulation to explain even more obscure phenomena of the nervous system, and they taught pharmacologists how to study in a scientific manner the physiological action of drugs. (J. G. M.) See Miscellaneous Works, ed. Peacock (2 vols., London, 1855).

2 See Marey, La Methode graph. dans les sc. expel'. (Paris, 1878).

3 Magendie's Journal, vol. viii. p. 272.

xxv11.30 Physiology The unicellular animal immersed in water absorbs nurittive matter and oxygen, and excretes waste materials with its whole surface. Owing to the small mass of the protozoa the metabolic products can penetrate throughout the whole. With the evolution of the multicellular organs of the metazoa and the division of physiological labour a circulatory mechanism became of immediate need. A double-layered animal like the common water polype Hydra can exist, it is true, without such a mechanism, but communities of polypes, such as the sponges, form channels for the circulation of water. With the development of the three-layered animal the coelom or body cavity arose by the splitting of the mesoderm, and it was in this body cavity that the evolution of the circulatory system took place, an evolution which finally became perfected in the higher members of the metazoa into a closed vascular system filled with red blood. The evolution of the red matter, haemoglobin, as a special carrier of oxygen was necessitated by the increasing mass and muscular activity of the higher animal, in comparison with the size of the oxygenabsorbing surface - the gill or lung. The blood vascular system of the invertebrata such as the Arthropoda and Insecta, is not generally a closed system, but consists of a pulsatile heart whence proceed arteries which open into lacunar spaces forming part of the coelom. The lacunae exist between the organs and tissues of the body, and the blood from these spaces is returned to a venous sinus whence the heart draws its supply through valved openings. The movements of the animal help to return the blood from the tissue spaces to the heart, while the heart by its rhythmic contraction drives the blood into the arteries. Somewhere in the course of this system are placed the gills and renal organs, and it appears to be a matter of indifference whether the gills be placed on the arterial or venous side of the system, both arrangements being found in different types. In some types (mussel, earthworm), the whole blood passes through the renal organs at each circulation, in others (crayfish) only parts. In the earthworm the vascular system is closed, the arteries and veins being connected by capillaries in place of lacunae. The movement of tissue juices may be maintained by physico-chemical forces alone, e.g. by the forces of osmosis and adsorption, as is seen in the movements of sap in the vascular bundles of plants, in the streaming of protoplasm in the plant cell and in the marvellous rhythmic to-and-fro movements of the richly granular juice contained in the veins of the spreading protoplasmic sheet of myxomycetes. Such agencies come into play in the lacunar or capillary part of the circulation of the metazoa and are assisted by the movements of the body wall and of the alimentary organs. The evolution of a special pumping organ, the heart, associated with the aeration of the body fluids in the gills, led to the perfection of the efficient system of circulation which is found in the vertebrata.

The blood is to be regarded as alive in as strict a sense as any other component of the living body. It is a tissue consisting of mobile elements - the blood corpuscles - and a plasma - a colloidal albuminous fluid which is analogous to the more solid intercellular material of other tissues. The primary sources of its elements are the blood-forming organs - the bone marrow, the haemolymph and lymphatic glands and other lymphatic tissue, and the spleen. It circulates as the middleman between the tissues, conveying from the alimentary canal the products of digestion - sugar, fat, aminoacids and salts; oxygen from the lungs; carbonic acid, urea and other waste products of the tissues to the lungs and kidneys; internal secretions from one organ to another; and acts not only as a carrier, but deals with the material remitted to it on the way. One other function of the blood, a most important one, must not be omitted, that of defence against the invasion of bacteria and their toxins, and other parasites.

The blood is contained in a continuous system of vessels; arteries lead from the heart and divide into a multitude of capillary vessels, and these lead into the veins which finally pass back to the heart. The heart is to be regarded as a double organ, each half consisting of an auricle and a ventricle. The fight half contains dark venous blood which has been returned from the body and is sent to the lungs: the left heart contains the bright oxygenated blood which has been returned from the lungs and is distributed to the body. There are thus two circulations - the one pulmonary, from the right side of the heart to the pulmonary artery and thence to the capillaries of the lungs and to the left heart by the pulmonary veins - the other systemic, from the left side of the heart, by the aorta, to the arteries and capillaries of the body tissues and organs, whence the blood returns by the veins to the right side of the heart. A schematic representation is given of the circulatory system in the accompanying diagram. The venous blood flows into the right auricle (RA) from the superior vena cava and the inferior vena cava. The right ventricle (RV) drives through the lungs the blood received from the right auricle. The right auriculo-ventricular valve, or tricuspid, and the pulmonary semilunar valve are represented directing the flow of blood in this direction. From the pulmonary capillaries the blood returns by the pulmonary veins (PV) into the left auricle (LA), and so through the left auriculo-ventricular or mitral valve into the left ventricle (LV). By the left ventricle the blood is driven through the aortic semilunar valve, and is distributed to the systemic arteries, and so to the capillaries of the various organs and back to the veins. The muscular wall of the auricles and that of the right ventricle are much thinner than that of the left ventricle. This is so, because the energy required of the left ventricle must exceed that of the right ventricle, inasmuch as the resistance in the systemic system exceeds that in the pulmonary circuit.

The heart fills with venous blood during its expansion or diastole, and forces the blood into the arteries during its contraction or systole. The large arteries are of less capacity than the corresponding veins, and their walls are essentially extensile and elastic. The pulmonary arteries are especially extensile structures. The small arteries and arterioles are essentially muscular tubes and can vary considerably in diameter. The arterioles open into the capillaries, and these are so numerous that each organ may be regarded as a sponge full of blood. The skeletal muscles and the muscular walls of the viscera at each contraction express the blood within them, and materially influence the circulation. The whole muscular system, as well as the heart, must therefore be regarded as a pump to the vascular system. The capillary wall is composed of a single layer of flattened cells, separating the blood within from the tissues without. Through this layer, which is of extraordinary tenuity, there takes place an exchange of material between the blood and the tissues, an exchange which depends on the physico-chemical conditions which characterize the living state of the cells. The phenomena of adsorption and osmosis come into play here, but the conditions still await complete elucidation. The veins are of larger calibre than the corresponding arteries, and have tough and inextensile walls. Their walls are muscular, and contract on local stimulation. The veins are not, as a rule, distended with blood to their full potential capacity. The latter is so great that the whole blood of the body can collect within the veins.

The heart and lungs are placed within the thoracic cavity (T), the floor of which is formed by the muscular diaphragm (D); the heart is itself enclosed in a tough inextensile bag, the pericardium (P), the function of which is to check overdilatation of the heart. The pericardium bears to the muscular wall of the heart the same relation as the leather case of a football does to the bag within. In particular, it prevents over-distension of the heart during muscular efforts.

The abdominal organs and blood vessels are encompassed by the muscular wall of the abdomen (A), and may be regarded as enclosed in a sphere of muscle. Above is the dome of the diaphragm (T), and below the basin-like levator ani, closing the outlet of the pelvis; in front are the recti muscles, behind the quadrati lumborum and the spine; while the oblique and transverse muscles complete the wall at either side. The brain is enclosed in a rigid and unyielding box of bone - the cranium, while the limbs are encompassed by the extensile and, in health, taut and elastic skin.

The heart's energy is spent in maintaining a pressure of blood in the elastic arteries, and by the difference of pressure in the arteries and veins the blood is kept flowing through the capillaries into the veins. The movements of the body and particularly of respiration help to return the blood from the capillaries and veins back to the heart, valves being set in the veins to direct the blood in this direction. The blood is a viscous fluid and its viscosity varies; it is propelled by a heart which varies both in rate and energy; it circulates through a system of muscular and elastic arteries and veins, which varies in capacity and may alter in elasticity. The width of bed through which it flows varies greatly at different parts of the circuit, and the resistance offered to the moving blood is very much greater in the capillary-sized vessels than in the large arteries and veins. The blood continually varies, both in quantity and in quality, as it effects exchanges through the capillary walls with the tissues. The problems of the FIG. I. - General Cou rse of Circulation and some of the Principal Vessels. H', right ventricle; H, left ventricle; A, A, A, aorta; h, part of left auricle; P, pulmonary artery, going to lungs; P, pulmonary veins; v, ascending or lower vena cava; e, trachea or wind-pipe; p, p', bronchial tubes; a', a, right and left carotid arteries; v, v', veins from root of neck (internal jugular and subclavian), joining to form descending or upper vena cava; i, hepatic artery 1, hepatic vein; I, superior mesenteric artery, going to mesentery and bowels; L, portal vein, going to liver; k', renal artery; k, renal vein; V, inferior vena cava, splitting into the two iliac veins, v, v.

FIG. 2. - Scheme of the Circulation of the Blood in Man, standing erect. The venous system is stippled. C, rigid cranial wall; N, muscles and cutaneous wall of neck; T, thoracic wall; A, muscular and cutaneous wall of abdomen; D, diaphragm; L, muscles and cutaneous wall of limbs; P, pericardium; AO, aorta; S. V. C, I. V. C, venae cavae; P.V, portal vein; V, valves in veins of neck, or legs; RA, LA, right and left auricles; RV, LV, right and left ventricles.

circulation are thus far from simple. They resolve themselves mainly into a consideration of (1) the physiology of the heart; FIG. 3. - The Thoracic Viscera. In this diagram the lungs are turned to the side, and the pericardium removed to display the heart. a, upper, a', lower lobe of left lung; b, upper, b', middle, lower lobe of right lung; c, trachea; d, arch of aorta; e, superior vena cava; f, pulmonary artery; g, left, and h, right auricle; k, right, and 1, left ventricle; m, inferior vena cava; n, descending aorta; I, innominate artery; 2, right, and 4, left common carotid artery; 3, right, and 5, left subclavian artery; 6, 6, right and left innominate vein; 7 and 9, left and right internal jugular veins; 8 and 10, left and right subclavian veins; II, 12, 13, left pulmonary artery, bronchus and vein; 14, 15, 16, right pulmonary bronchus, artery and vein; 17 and 18, left and right coronary arteries.

(2) the physical characters of the circulation; (3) the control of the heart and vessels by the nervous system.

A. Keith, in Journal of Anatomy and Physiology. FIG. 5. - Showing the Attachments of the Heart. a, a, auricular base of ventricle; c, c, aortic base of ventricles; d, d, arterial mesocardium; e, e, venous mesocardium; f,ascending aorta; g, pulmonary aorta; h, superior vena cava; i, inferior vena cava, perforating diaphragm and pericardium; 1, m, n, structures at the root of the lung - bronchus, pulmonary artery, and pulmonary veins; o, vortex at apex; p, pectinate musculature of right auricle; r, superficial musculature of right ventricle.

The Action of the Heart. The permanent position and general arrangements of the heart are described in a separate article, and it is only necessary here to allude to certain points of physiological importance. The substance of the heart is composed of a special kind of muscular tissue which must be regarded as a syncytium in which no distinct and separate cells occur, a complex plexus of branching and anastomosing fibres, forming one functional whole. The fibres are nucleated, have a cross-striated structure and are surrounded by delicate connective tissue sheaths. The cross-striations are due to the primitive fibrils which as in skeletal muscle are differentiated into alternate doubly and singly refracting substances. These fibrils are embedded in a granular nucleated sarcoplasm. Between the bundles of fibres are thin layers of connective tissue containing closely spun networks of capillaries. The muscle of the auricles consists of a circular layer common to both and a deeper layer separate for each chamber. The auriculo-ventricular ring consists of connective tissue surrounding the auriculo-ventricular orifices and separating the auricular from the ventricular muscle with the exception of an important band, the auriculo-ventricular bundle. The superficial fibres of the ventricles appear to have origin in the auriculo-ventricular ring, to wind about the heart spirally and to end in the tendons of the papillary muscles or pass up to the ring again on the inner surface of the heart. The middle layers consist of bundles of fibres running more or less circularly round the ventricles.

The greater part of the hear', lies free in the pericardial sac. The pericardium is reflected from the wall of the sac on to the wall of the heart and attaches the heart at the point where the venae cavae and aorta leave the sac. This part of the pericardium gives a fixation point to the auricles, for it is attached to the roots of the lungs and thereby to the thoracic wall, to the diaphragm and to the structures at the root of the neck. On opening the chest the normal fulcra for the movements of the auricles are lost, and this renders it difficult to record the exact movements of the heart. The attached part of the heart is called the base, and the venous part of the base is the beginning and the arterial part the end of the tube, coiled on itself, from which in the embryo the heart develops. The longitudinal and circular muscle fibres of the ventricles are antagonists. The circular fibres by their contraction tend to lengthen the apex-base diameter, the longitudinal fibres resist this and the two together wring the blood out of the heart. The apex is maintained as a fixed point by this antagonistic action, and thus the longitudinal fibres are enabled to expand the auricles by pulling down the floor of these chambers. This action is important, as it contributes to the filling of the auricles simultaneously with the emptying of the ventricles. Tracings of the jugular pulse give evidence of such action.

In the case of the auricles the longitudinal musculi pectinati not only help the circular fibres to expel the blood, but draw up the base of the ventricle to meet its load of blood. Thus the base of the ventricular part (or floor of the auricles) is pulled up during auricular systole, and down during ventricular systole. The posterior and upper borders of the left auricle lie against the unyielding structures of the posterior mediastinum, the pulmonary artery and bronchi, the floor and anterior part in contact with the base of the ventricle and ascending aorta respectively. The latter parts alone are free to move during systole. Thus the left ventricular base is drawn up and the aorta back on auricular systole (A. Keith).

As regards the valves of the heart - (i) the tricuspid guards the right auriculo-ventricular opening, and consists of three flaps of fibrous tissue, covered, like all the internal surfaces of the heart, with the smooth shining membrane, the endocardium. The flaps are continuous at their base, forming an annular From Hill's Manual of Physiology, by permission of Edward Arnold.

FIG. 4. - Diagram of Chambers of Heart and Large Vessels.

A, Vena cava, superior.

B, Vena cava, inferior.

C, Pulmonary artery.

D, Aorta.

E, Right auricle.

F, Right ventricle.

G, Left auricle, into which open the four pulmonary veins.

H, Left ventricle.

The arrows point the course of the blood.

membrane surrounding the opening. The bicuspid or mitral the consists of two cusps and guards the left auriculo-ventricular opening. The under surface and free edge of each cusp of these FIG. 6. - Cavities of the Right Side of the Heart. a, superior, and b, inferior vena cava; c, arch of aorta; d, pulmonary artery; e, right, and f, left auricular appendage; g, fossa ovalis; h, Eustachian valve; k, mouth of coronary vein; 1, m, n, cusps of the tricuspid valve; o, o, papillary muscles; p, semilunar valve; q, corpus Arantii; r, lunula.

valves are attached by chordae tendinae to two papillary muscles; these are pillars of muscle which rise up from the inner surface of the ventricles.

The edges of these valves which come into opposition are exceedingly thin and delicate, while the outer parts, which bear the full systolic pressure of the blood, are tough. The cardiac muscle, by its contraction, limits the size of the auriculo-ventricular orifices and so maintains the competency of the valves. It is the papillary muscles and chordae tendineae which pull down the diaphragm formed by the closed valves (the floor of the auricles), thus expanding the auricles and enabling the valvular as well as the muscular parts of the wall of the ventricles to approach together and wring out the blood. The thin, moist, film-like edges of the valves of the heart come into perfect apposition and prevent all leakage, while the fibrous parts give strength and support. The ventricles are never completely emptied, for some blood remains in contact with the auriculo-ventricular valves up to the end of systole and ensures Left anterior cusp of pulmonary valve Left posterior cusp of pulmonary valve Left posterior cusp of aortic valve Left coronary artery Anterior cusp of mitral valve Posterior cusp of mitral valve_, Left ventricle a ^ From Young and Robinson, Cunningham's Text-Book of Anatomy. FIG. 7. - The Bases of the Ventricles of the Heart, showing the au aortic and pulmonary orifices and their valves.

their closure. Incompetency of the valves may arise when the right heart is greatly dilated. The aortic and pulmonary valves consist of three semilunar, pocket-shaped cusps. A fibrous nodule is placed centrally in the free edge of each cusp, whence numerous tendinous fibres radiate to the attached borders of the cusp. The rest of the free edges which come into apposition are thin and delicate. Opposite the cusps are bulgings of the aortic walls - the sinuses of Valsalva. From the anterior one arises the right coronary artery and from the left posterior, the left coronary artery, these vessels supply the substance of the heart with blood. Eddies formed in the sinuses during the period of systolic output bring the semilunar valves into apposition, so that they close without noise or jar at the moment when the intraventricular becomes less than the aortic pressure. The auriculo-ventricular valves are likewise floated up by eddies, and brought into apposition at the moment the intraventricular pressure surmounts that in the auricles.

The heart in size is about equal to the closed fist of a man. The average weight of the heart in the new-born baby is about 24 grms., in the adult 300 grms. The percentage which the heart weight bears to the whole body is. in the new-born and 0.46 in the adult. While the whole body increases in weight 21 -fold, the heart increases only 12.74-fold (Vierordt, Karl, 1818-1884). The average weight of the male and female heart is almost the same. The average volume of the whole heart is about 270 c.c. The capacity, estimated by filling the heart with wax, is for each auricle about 100-150 c.c., and 150-230 c.c. for each ventricle. There are considerable sources of error in such measurements. The muscle of the left ventricle is about 1.6 cm. in thickness, and of the right ventricle 0.5 cm. The left ventricle has twice From Hill's Manual of Physiology, by permission of the muscular mass of Edward Arnold. the right. The cirFIG. 8. - Position of the Valves of the Heart cumference of the left in Systole and Diastole. auriculo-v e n t r i c ul a r orifice is about 14.0 cm.; of the right, about 12.5 cm.; of the aortic orifice, 8o cm.; of the pulmonary orifice, 9o cm. The average diameter of the vena cava superior is about 23 mm.; of the vena cava inferior, 34 mm.; of each of the four pulmonary veins about 13-14 mm. of the pulmonary artery, 28 mm.; of the aorta, 32 mm.

The physiologist or physician has many means at his disposal of examining the heart's action. By palpation with the hand over the region of the heart, its stroke, the cardiac impulse, can be felt. By auscultation with the ear directly, or with use of the stethoscope the sounds of the heart can be heard. By percussion the anatomical limits of the organ can be defined. The cardiac impulse can be recorded by tambour methods of registration, the heart sounds by means of the microphone and capillary electrometer, while the volume and movements of the heart can be studied with the help of the Röntgen rays The impulse is caused by the sudden hardening of the muscular mass of the ventricles against the wall of the thorax. It is synchronous with the beginning of systole. The position at which the impulse is felt varies with changing posture cardiac of the body, as different parts of the mpu s. Conus arteriosus thorax come in turn in contact with the ventricle. In the supine position it is usually to be felt in the fifth intercostal space 32 inches from the midsternal line. The chest wall is driven out by the systole only where the heart muscle touches it; at other places it is slightly drawn in. This indrawing is attributed valve to the expulsion of the blood out of the thorax by the left ventricle. The thorax is a closed cavity and the vacuum therein produced by systolic output into the arteries of the head, limbs and abdomen is filled by (I) the drawing of air into the lungs, (2) the drawing of venous blood into the great veins and right auricle, (3) the slight indrawing of the chest wall. The impulse is recorded by placing small cup, or receiving tambour, over the spot where it is most evident, and connecting the inside of the cup by a tube to a recording tambour. The cup can be closed by a rubber dam, or an air-tight junction can be effected by pressing it upon the skin. The stroke of the heart is transmitted as a wave of compression to the air within the system of tambours. The recording tambour is brought to write on a drum, moved by clockwork, and covered with a paper smoked with lamp-black. From the record so obtained we can obtain information as to the time relations of the heart-beat, but no accurate information as to its energy or amount of contraction.

From Young and Robinson, Cunningham's Text-Book of Anatomy. FIG. 9. - The Relation of the Heart to the Anterior Wall of the Thorax.

I, II, III, Iv, v, vi, the upper six costal cartilages.

The movements of the heart consist of a series of contractions which succeed each other with a certain rhythm. The period of contraction is called the systole and that of relaxation the diastole. The two auricles contract and relax synchronously, and these movements are followed by the synchronous contraction and relaxation of the ventricles. the Finally, there is a short period when the whole heart is in heart. diastole. The whole series of movements is known as the cardiac _ Right posterior cusp of aortic valve _Anterior (infundibular) cusp of tricuspid valve _Right (marginal) cusp of tricuspid valve _Posterior (septal) cusp of tricuspid valve Right ventricle riculo-ventricular, Right anterior cusp of pulmonary valve Right coronary artery Anterior cusp of aortic cycle. Taking 75 as the average number of heart-beats per minute, each cardiac cycle will occupy. 8 seconds. Of this period auricular systole occupies .1 second auricular diastole occupies. 7 „ ventricular systole occupies 3 „ ventricular diastole occupies. 5 „ In 1861 Chauveau and Marey obtained direct records of the heart of a horse, and determined the sequence and duration of the events happening in the heart, and measured the endo-cardiac pressure by an instrument termed the cardiac sound. The sound - a twoway tube - was pushed down the jugular vein until the orifice of one tube lay in the right ventricle and of the other in the right auricle. The tubes were connected with recording tambours which wrote on a moving drum covered with smoked paper.

Another tambour was used to record the cardiac impulse. The tracings so obtained (fig. to) teach us the following facts: (1) The auricular contraction is less sudden than the ventricular, and lasts only a very short time, as indicated by the line ab. The ventricle, on the other hand, contracts suddenly and forcibly and remains contracted a considerable time, as shown by the line c' d' and by the flat top to the curve which succeeds d'. (2) The auricular movement precedes the ventricular, and the latter coincides with the impulse of the apex against the wall of the chest. (3) The contraction of the auricle influences the pressure in the ventricle as shown by the small rise a'b', and that of the ventricle influences the pressure in the auricle somewhat as shown by the waves cd. Much labour has been spent in the contrivance of rapidly acting spring pressure gauges, freed as far as possible from inertia, in order to investigate more exactly the changes of intracardiac pressure, which were first described by Chauveau and Marey. As the intraventricular pressure FIG. 10. - Tracings from the Heart of a Horse, by Chauveau and Marey. The upper tracing is from the right auricle, the middle from the right ventricle, and the lowest from the apex of the heart. The horizontal lines represent time, and the vertical amount of pressure. The vertical dotted lines mark coincident points in the three movements. The breadth of one of the small squares represents one-tenth of a second.

may rise 150 mm. of mercury in one-tenth of a second, it is no easy matter to contrive an instrument which will respond as rapidly and yet yield an accurate result without overshooting the mark. The final result of a most careful inquiry is the confirmation in almost every point of Chauveau and Marey's pressure curves. Karl Hi.irthle's differential manometer has proved to be an instrument of great value and precision. A double-bored tube cannula is introduced so that one tube reaches the right auricle and the other the right ventricle. In observations on the left side of the heart, one tube is placed in the left ventricle and the other in the aorta, and each of these tubes is brought into connexion with a tambour. The two tambours are placed one on either side of the fulcrum of a lever. This lever works against a light spring, which in its turn sets in motion a writing-style. The style records the pressure changes on a drum covered with smoked paper. By this means there can be recorded the exact moment at which the auricular pressure exceeds that in the ventricle, that is to say, the moment when the auriculo-ventricular valves open; likewise the moment when the ventricular pressure becomes greater than that in the auricles, and the auriculo-ventricular valves shut. Similarly, there can be recorded the moment when the intraventricular pressure exceeds that in the aorta and the semilunar valves open, and the moment at which the diastole of the ventricle begins, when the aortic pressure becomes the greater, and the semilunar valves shut. The smoothness with which the heart works is shown by the fact that neither the opening nor the closing of the valves is marked by any peak or point on the pressure curves.

The absence of a mechanism for preventing regurgitation of blood from the auricles of birds and mammals is remarkable, for in fishes, amphibia and reptiles this is effected by valves guarding the sino-auricular junction. In the warm-blooded vertebrata with the appearance of the diaphragm the sinus becomes merged into the right auricle, and the venous cistern formed by the superior and inferior venae cavae, the innominate, iliac, hepatic and renal veins takes the place of the sinus.

Six pairs of valves prevent regurgitation from this cistern, viz. those placed in thee common. femoral, the sub-clavian and jugular veins. The cistern when filled holds some 400 c.c. of blood; in the liver there is some 500 c.c. of blood, and this can be expressed into the cistern by abdominal pressure; in the portal venous system, when distended, another 500 c.c. may be held, which can be expressed through the liver into the cistern. A large volume of blood is thus at the disposal rena cave of the heart for it to draw on during diastole. Respiration by the aspirating action of the thorax sucks this blood into the heart, while the inspiratory descent of the diaphragm squeezes the abdominal contents and forces blood from the liver and cistern into the heart. These forces take the place of the sinus and are far more efficient. The intra-abdominal pressure may be raised on bending or straining till it becomes equivalent to the pressure of a column of mercury 80-10o mm. high (Keith). Under such conditions the pericardium prevents the right side of the heart being over-distended with venous blood.

With these facts in view, we can now describe the complete course of a cardiac cycle. We will start at the moment when the blood is pouring from the venae cavae and pulmonary veins into the two auricles. The auricles are relaxed and their cavities open into the ventricles by the funnelshaped apertures formed by the dependent segments of the tricuspid and mitral valves. The blood passes freely through these apertures into the ventricles. The small positive pressure which is always present in the venous cistern (aided by the respiratory forces) From Diseases of the Heart, by James Mackenzie, M.D., by permission.

FIG. 12. - Tracings of the Jugular Pulse Apex Beat, Carotid and Radial Pulses. The perpendicular lines represent the time of the following events. 1, the beginning of the auricular systole; 2, the beginning of ventricular systole; 3, the appearance of the pulse in the carotid; 4, the appearance of the pulse in the radial; 5, the closing of the semilunar valves; 6, the opening of the tricuspid valves.

is at this time filling the right heart, while the positive pressure in the pulmonary veins is filling the left heart. The auricular systole now takes place. The circular muscle bands compress the blood out of the auricles into the ventricles, while the longitudinal bands aid in this and pull up the base of the ventricles to meet the load of blood. As the contraction starts from the mouths of the venae cavae, and sweeps towards the ventricles, there can NHN?aiN? ' 'MEW  ?[?

Nnnn((/O¦. ??

'NM MN?r¦?1 EMU 'A ' 'Wwii I¦mNill ' 'ME M .t?._?N A. Keith, in Journal of Anatomy and Physiology. 'FIG. 11. - Diagram of the Venous Cistern from which the Heart is filled. The abdominal or infradiaphragmatic part of the cistern is indicated in black; the thoracic or supra-diaphragmatic is stippled.

occur but little regurgitation of blood into the venous cistern, but the cessation of flow into the auricle during its systole does produce a slight rise of pressure in the cistern, as is shown by tracings taken from the jugular pulse. The function of the auricles is to rapidly complete the filling of the ventricles.

The auriculo-ventricular valves are floated up and brought into apposition by eddies set up in the blood which streams into the ventricles, and close without noise or jar at the moment when the intra-ventricular pressure exceeds in the least that in the auricles. The systole of the ventricles immediately following that of the auricles closes the auriculo-ventricular valves, and as the intra-ventricular pressure rises above that in the pulmonary artery and aorta respectively the semilunar valves open and the blood is expelled; these elastic vessels are in their turn expanded by the expulsive force of the heart so as to receive the blood. The papillary muscles, by contracting synchronously with the muscular wall of the ventricles, pull down and flatten the dome-like diaphragm formed by the closed auriculo-ventricular valves, thus shortening the longitudinal diameter of the ventricles, while at the same time they enlarge the auricles and so help to fill these cavities. The outflow of blood from the ventricles is rapid at first. It becomes slower as the big arteries become distended and the pressure of blood rises within them, and ceases finally when the pressure becomes equal to that in the ventricles. As the outflow diminishes the semilunar pockets are filled by eddies of blood, and their thin edges are brought nearer and nearer, until finally they come into apposition. The closure is effected without jar or noise at the moment when the outflow ceases and the ventricles begin to expand. The heart, as a good pump should, works with the least possible jar. During the contraction of the ventricles blood has been pouring from the veins into the auricles, and directly the ventricular systole ceases the auriculo-ventricular valves open, and the blood begins to fill the expanding ventricular cavities. For a brief moment the ventricles remain dilated and at rest, then the auricles contract again, and the cycle of changes, once more, is repeated. During the first period of ventricular systole - the period of rising tension - all the valves are closed and the ventricle is getting up pressure. This period has been measured and is found to occupy 02 "--04". The second period is that of systolic output, and lasts about .2", that is, from the moment when the semilunar valves open to the moment when they close.

The upstroke of the pulse curve taken in the aorta, or in the carotid artery in man, can be taken as marking the moment so that either only a rise or a fall of pressure is recorded. In the right ventricle of the dog the maximal pressures recorded equalled 35mm. of mercury, in the left ventricle 114-135 mm., in the auricles 2-20 mm. (Michael Jdger, 1795-1838). A negative pressure, of considerable amount but of very fleeting duration, sometimes occurs in the ventricles at the beginning of diastole. This is produced by the elastic rebound of the fleshy columns of the inner wall of the heart, which become pressed together as the blood is wrung out of the ventricular cavities. The entry of the first few drops of blood from the auricles abolishes this negative pressure, and it has no important influence on the filling of the heart.

When the ear is applied over the cardiac region of the chest, or a stethoscope is employed, two sounds are heard, the first, heard most intensely over the apex, is a duller and longer sound than the second, which is shorter and sharper and is heard best over the base of the heart. The syllables lub, dupe express fairly well the characters of the two sounds, and the accent is on lub when the stethoscope is over the apex, thuslub-dupp - lub-dupp - lub-dupp, and on the second sound when over the base, thus - tub-dupp - lub-dupp - lub-dupp. The sounds of the heart have been successfully recorded by means of the microphone. Hi rthle inserted the microphone in the primary circuit of an E. Du Bois-Reymond induction coil, and placed the nerve of a frog-muscle preparation in the secondary circuit. The muscle, being attached to a lever, recorded its contraction on a revolving drum at the moment when the sound of the heart reached the microphone and closed the primary circuit. A capillary electrometer can be inserted in place of the frog-muscle indicator, and the movements of the electrometer photographed on a sensitized plate moved by clockwork (Willem Einthoven). Each sound gives rise to a succession of vibrations of the mercury meniscus of the capillary electrometer. The first sound is formed of many component tones derived from the sudden tension, and consequent vibration, of the ventricular muscle, and of the auriculo-ventricular valves with their chordae tendineae. The first sound can be resolved by a trained musical ear into two tones, one deep and the other high. The deeper tone alone is heard on the contraction of the excised and bloodless heart, while the higher tone is produced by throwing the auriculo-ventricular valves into tension (John Berry Haycraft). In the cold-blooded animal, such as the turtle, the heart muscle does not become tense rapidly enough to produce a sound (Allen). This sound is not produced by fluid friction as the blood rushes through the arterial orifices, for the velocity of outflow is too small to produce in this way any noise. Nor is it produced by sudden opening of the semilunar valves, for these open quietly and without jar at the moment when the intra-ventricular pressure rises above that in the aorta.

The second sound of the heart is produced by the tension of the semilunar valves in the aorta and pulmonary artery at the moment when the ventricles pass into diastole. These valves close without any jar or shock so soon as the arterial pressures rise to the slightest degree above that in the ventricles. In the next moment the ventricles dilate, and the valves, no longer supported on one side, become taut. The elastic vibrations of the walls of the distended arteries probably share in the production of this sound.

When the sounds and the impulse are recorded together the record shows that the first sound begins about ooi sec. before the cardiagram marks the beginning of systole, and for the first o06 sec. of its duration this sound is heard only over the apex. Over the base of the heart the first sound is heard just at the time when the semilunar valves open and the output begins. The first sound ceases before the ventricular contraction is over, for it is the sudden tension, not the continuance of contraction, that causes it. The beginning of the second sound marks the sudden tension of the semilunar valves which immediately follows their closure.

For practical purposes it is important to bear in mind what is happening in the heart whilst one listens to its sounds. During the first sound we have (I) contraction of the ventricles, closure of the auriculo-ventricular valves and impulse of the apex against the chest; (2) rushing of the blood into the aortic and pulmonary artery, and filling of the auricles. With the second sound we have closure of the semilunar valves from the elastic recoil of the aorta and pulmonary artery, relaxation of the ventricular walls, opening of the auriculo-ventricular valves so as to allow the passage of blood from auricle to ventricle, and diminished pressure of apex against chest wall. With the long pause there are (1) gradual refilling of the ventricle from the auricle, and (2) contraction of the auricle so as to entirely fill the ventricle. The sound of the tricuspid valve is heard loudest at the junction of the lower right costal cartilages with the sternum, of the mitral over the apex beat, of the aortic semilunar valves in the direction of the aorta where it comes nearest to the surface at the second right costal cartilage, and of the valves of the pulmonary orifice over the third left costal cartilage, to the left and external to the margin of the sternum. The sounds are changed in character by valvular lesion or muscular weakness of the heart, and afford important signs to the physician. Murmurs are produced by eddies setting some part of the membranous walls or valve flaps in vibration.

If a stethoscope be placed over a large artery, a murmur will be From Further Advances in Physiology, by permission.

FIG 13. - Diagrammatic representation of the Cardiac Cycle and of the Carotid and Jugular Pulses in relation to standard movements. The scale of abscissae is I mm. to i oa sec.

S.C. =semilunar valve closure' when the semilunar valves open, O. = auriculo - ventricular pen, valves open. The broken lines while the dicrotic notch on the indicate those portions of the pulse curve marks their closure. respective curves over which The second sound of the heart there is doubt or controversy. occurs immediately after their closure, and can be used to mark the time of this event on the impulse curve.

The intra-ventricular pressure curve may rise or fall during the output period according to the state of the peripheral resistance. If the carotid pulse be recorded synchronously with the impulse curve, the time relations can be determined for the human heart. The beginning of the upstroke of the impulse curve marks the beginning of systole, that of the pulse curve marks the opening of the semilunar valves, and the dicrotic notch, which precedes the dicrotic wave, marks the closure of these valves and the end of the output. The first sound of the heart is synchronous with the upstroke of the impulse curve. The maximal systolic pressure exerted by the heart varies with the degree of diastolic filling and with the obstruction to outflow. The heart responds to the latter by a greater output of energy, and this it does with little loss in rapidity of action. The total fluid pressure to which the wall of the heart is submitted rapidly increases as the radii of curvature become greater. Hence the greater energy required of a dilated heart, its tendency to hypertrophy and liability to fail. By its reserve power the heart may throw out three or even six times the volume of the normal output per minute, and may maintain its output when the aortic pressure is twice its normal value.

The maximal and minimal pressures have been accurately recorded in the heart by a manometer fitted with a valve arranged heard, caused by the blood rushing through the vessel narrowed by the pressure of the instrument. The fluid escapes into a wider portion of the vessel beyond the point of pressure, and the sound is caused by the eddies set up there throwing the membranous wall of the vessel into vibration. Such a sound is heard over an aneurism. The placental bruit heard during pregnancy is a sound of this kind, arising from pressure on the uterine arteries. In cases of insufficient aortic valves a double blowing murmur may be heard, the first being due to the rush of blood into the vessel, and the second to the regurgitation of the blood back into the ventricle. These murFIG. 14. - Scheme of a Cardiac Cycle. murs are produced by eddies The inner circle shows what events of branous blood setti into the memb occur in the heart, and the outer r parts n a o vibration.

the relation of the sounds and Occasionally a murmur is silences to relation of events. produced by the displacement of air in the bronchial vessels by the beat of the heart, and may simulate the murmur of aortic incompetence. By placing a stethoscope over the jugular vein on the right above the collar bone a murmur is heard, the bruit de diable, particularly if the subject turn his head to the left. This is held to be due to the vibration of the blood in the jugular vein rushing from the dilated to the contracted part. It is more marked during auricular diastole and during inspiration.

In the lower vertebrates, as the frog, the heart is directly nourished by the blood which fills the cavities in its sponge-like structure. In the warm-blooded vertebrates there is a special arrange ment of coronary vessels. The two coronary arteries (right and left) originate at the root of the aorta from the sinuses of of Valsalva. Their branches penetrate the muscular sub stance and end in a rich plexus of capillaries. From these arise the radicles of the coronary veins which open into the right auricle by the coronary sinus and other small veins. These openings are valved. The heart in contracting exerts a greater pressure than that of the coronary arteries, and so arrests the flow in these during the height of systole, and squeezes the blood within the coronary capillaries and veins on into the right auricle. On diastole the coronary system fills again. Sudden occlusion of any large part of the coronary arteries produces irregular and inco-ordinate contractions, followed by death of the heart. Gradual occlusion of the coronary arteries by degenerative changes in advanced life is one of the causes of the distressing form of cardiac distress known as angina pectoris. The work of the left ventricle is calculated by the formula W = VP +mv 2 , where V =volume of blood in c.c. expelled of per beat, P = mean pressure in aorta, m = mass of the blood expelled on systole, and v=the velocity imparted to it.

The volume of the output has been determined directly by inserting the stromuhr in the ascending aorta (Robert Adolf Tigerstedt), and indirectly by determining (1) how much oxygen is absorbed per minute, (2) the difference in the oxygen content of the arterial and venous blood, (3) the number of heart beats. If woo c.c. of oxygen are absorbed from the air breathed in a minute, and the arterial blood contains to % more oxygen than the venous, it is clear that loo X loo c.c. of blood must have passed through the lungs in that time, and if the heart beat too times, the output for each beat would be 100 c.c. From the determinations made on animals the output is calculated for man to be 60-100 c.c. The velocity of the output can be calculated if the volume of the output is known, the duration of the period of output, and the diameter of the aorta. The pressure is measured with a manometer. The velocity is much greater at the orifice than in the aorta, for the blood can flow from the aorta during the whole cardiac cycle, while the whole of it must escape through the orifice into the aorta during the period of output. The work spent on maintaining the velocity is not, however, more than 4 1 0 of the whole and is generally neglected in the calculation. The output is not greater than 60-10o c.c. (3 oz.) (Tigerstedt, Nathan Zuntz), and the mean arterial pressure in a healthy man, determined by the sphygmometer, is not more than 110 mm. of mercury (L. Hill). The work of the right heart can be reckoned to be a that of the left, for the pressure in the pulmonary artery does not exceed 30 mm. The total work of the heart during the day may be taken as equal to 20,000 kilogr.- metres, and this would be equivalent to 50 calories out of the total 2500 calories which a man takes in as food. A labourer does about 150,000 kilogrm.-metres of external work a day. The work of the heart is increased two or three times over during severe muscular labour. It has been estimated that the heart requires per diem, to maintain its energy, an amount of solid food (water-free) equal to the weight of solids in the heart itself, i.e. about 60 grms. of sugar or proteid. 30 c.c. of blood must be circulated per minute through the coronary arteries of a dog to maintain the vigour of the heart.

The use of oxygen per grm. of weight per minute is high for the heart. Thus for the whole body of the dog there was used 017 c.c. per grm. per min., for the heart 045-083, The and for the active secretory glands 07-1.0 (Barcroft artificial and Dixon). It has long been known that the heart of circuiafrog or tortoise can be kept beating normally for hours tion of after removal from the body, if it is provided with an artificial circulation of blood or a suitable solution of salts. Sydney, _Ringer worked out the necessary ingredients of this solution to be Sodium chloride o '7 Potassium „. 0.03% Calcium „.. 0.025% The excised mammalian heart can be kept beating in the same way provided the nutritive fluid is oxygenated and the heart kept at body temperature. A solution containing one-third defibrinated blood and two-thirds Ringer's salt solution is most suitable. .A mammalian heart thus was restored to activity 7 days after death. The beat of the heart of a child was restored 20 hours after death from pneumonia. The excised heart of a cat was kept beating for 4 days. The heart of a monkey was restored after freezing the body of the animal. The nerves of the excised heart retain their action for some time if the nutritive fluid is immediately circulated through the coronary arteries. Thus the heart's action can be conveniently studied when taken from the body of a mammal.

The cause of the heart beat has naturally been one of the most continued objects of inquiry, and the point of view shifts with each advance of our experimental methods, and the wider extension of the inquiry throughout the animal world. H. Allen in 1757 was the first to announce that the activity of the heart is not dependent on its connexion with the nervous system. The excised heart, properly fed, con tinues to beat. The heart of a dog continued to work effectively and the animal to keep in health for months after division of all the nerves passing to the heart. The heart, it is true, is controlled and influenced constantly by the nervous system - attuned to the general needs of the body - but this control is not essential to life. The above dog, when exercised, became fatigued quickly, owing to the lack of the nervous control of the heart. When in 1848 Robert Remak discovered that groups of nerve cells are contained in the heart of the frog, the causation of the beat was attributed to the activity of these ganglia.

Confirmation of this view was found in the experiment of Hermann Stannius which demonstrates that the apex of the heart ceases to beat rhythmically if physiologically separated from the rest of the heart by ligature or momentary application of a clamp. The sinus, on the other hand, which contains ganglion cells, continues its beat as before when separated. Further experiment has shown that the beat of the heart cannot be ascribed to the rhythmic activity of the ganglion cells, which in the mammalian heart lie scattered in the base of the heart, in the neighbourhood of the venous opening and in the auriculo-ventricular groove. That this is so is shown by the fact that every strip of heart muscle, whether free of ganglion cell or not, is capable of rhythmic activity under suitable conditions (Walter Gaskell, 1847-, Theodor Wilhelm Engelmann, Alfred Wm. Porter). The inherent power of rhythmic contraction is most clearly seen in the embryonic heart, for the pulsation of the chick's heart became visible by the 24th to 48th hour of incubation, while the migration of the ganglion cells into the heart from the sympathetic system does not take place until the sixth day (His.). The heart muscle is pervaded by a network of nerve fibrils, and the supporters of the neurogenic theory have had to fall back upon this network as the cause of the beat. The " myogenic " theorists place the causation in the muscle itself.

The pulsating " umbrella " of the jelly-fish is formed of a network of nerve fibril and contractile elements, and this can be excited to contract by irritating any one of the sensory endings of the nervous network which are situated on the edge of the " umbrella." In the manifestation of a " refractory period " the " umbrella " behaves like the heart. Against this view we may cite the experiment of Julius Bernstein (1839), who clamped off the apex of the frog's heart to destroy the physiological continuity, kept the animal alive till the nerve network had degenerated and then found the apex could be mechanically excited to contract. Moreover, skeletal muscle-fibres can be thrown into rhythmic contraction by the application of a suitable solution of salts (Wilhelm Biedermann, 1854), and it is probable that heart muscle is excited to rhythmic activity by such means. At any rate the beat is profoundly affected by varying slightly the nature and percentage of salts supplied in the nutritive fluid. Carlson has recorded experiments upon the heart of the horseshoe crab (Limulus) which show that its beat at any rate depends on the integrity of the median nerve (and its ganglion cells) which runs down the heart. On the other hand, Gaskell has shown that any small bridge of heart muscle left connecting the auricle and ventricle of the tortoise heart will transmit the wave of contraction, while if the nerve passing from of sinus to ventricle be left, and the muscular connexions entirely severed, no wave passes. In contradistinction to cross-striated muscle, the structural unit of the heart is not also a functional unit, for the heart-cells are, from the earliest stage of development, joined together by branches into networks and bands so as to form one functional whole, and hence excitation of any one part leads to the contraction of the whole. The first part to begin to functionate in the embryo is the venous end, and the waves of contraction passing thence .spread over the developing ventricular segment. The muscle-cells of the ventricles are thicker, less sarcoplasmic and more clearly striated than the auricular muscle, which is more embryonic in structure. The contraction lasts longer in the ventricular than in the auricular muscle, while the automatic rhythm not only persists longer in the auricles, but is of greater frequency, as is clearly seen when the cavities of the heart are divided from each other. The venous orifices of the heart are least sensitive to injury, beat longest after death, and are the first to recover after arrest. Owing to the more powerful automatism of the venous extremity, the contraction normally proceeds thence, and, passing as a peristaltic wave over the auricles and ventricles, finally reached the arterial orifices. This peristaltic form of contraction is invariable in all periods of development and in all hearts, both of invertebrate and vertebrate animals. The peristalsis may, with difficulty, be artificially reversed by the application of a powerful rhythmic stimulus to the ventricular end. Antiperistalsis does not, however, take place easily, because the comparatively slow excitatory process in the ventricle has little effect on the auricular muscle. The latter, by initiating more rapid contraction-waves, over-dominates the former. The frequency of the whole heart is accelerated by warming the auricles, while the period of systole is alone shortened on warming the ventricles.

The sequence in the beat of the three chambers of the heart is attributed by Gaskell to the delay that occurs in the excitatory wave passing through the muscular connexions in the sino-auricular and auriculo-ventricular junctions. He showed that such delay could be imitated by moderately clamping a strip of heart muscle; the compressed part transmitted the wave less readily, so that the part above and below the clamp contracted in sequence.

In the mammalian heart there has recently been discovered a remarkable remnant of primitive fibres persisting in the neighbourhood of the venous orifices (representing the sinus). These fibres are in close connexion with the vagus and sympathetic nerves, and form the sino-auricular node of A. Keith and Martin Flack. If this l3 ' node is squeezed by a clamp, it prevents the effect of excitation of j 4 3 .,'? the vagus reaching th ? r. n = 5heart. The auricle and ventricles of the mam 6 malign heart are connected ?, -' i through the septum by a remarkable bundle of muscle fibres which is believed to convey the ex citatory wave from the FIG.15. - The Right Auricle andVentricle one cavity to the other. of a Calf's Heart, exposed to show the The root of this auriculocourse and connexions of the auriculoventricular bundle lies in ventricular bundle. I, central cartilthe right auricle, the main age exposed by dissection; 2, the part is buried in the intermain bundle; 3, auricular fibres from ventricular septum; its which the main bundle arises; 4, branches and twigs are right septal division; 5, moderator distributed to all parts of band; 6, a cusp of the tricuspid valve; either ventricle; the papil7, posterior group of the musculi lary muscles and fleshy papillaries; 8, orifice of the coronary columns, in particular, sinus; 9, above orifice of the inferior receive a direct supply. vena cava (Io); II, orifice of the The muscle fibres are of superior vena cava; 12, septal wall a peculiar type, known of the right auricle; 13, appendix of as the cells of Purthe right auricle; 14, septal wall of kinje. By this bundle it the inf undibulum; 15, beginning of is believed every part of the pulmonary artery; 16, apex of the ventricle is brought the right ventricle. (After A. Keith, in into synchronous contrac- Journal of Anatomy and Physiology.) tion. To its degeneration has been ascribed certain cases of disturbed cardiac rhythm, when the ventricle no longer follows the sequence of auricle.

The evidence of such degeneration is, at present, not convincing. The contraction of the heart, like that of other muscle, is accom panied by an electrical change. The part in contraction is at different potential to the part at rest. Thus an electrical wave accompanies the wave of contraction. This has been studied by means of the capillary, or the string, electrometer (Sir John Scott Burdon-Sanderson and Page, Einthoven, Gotch). The photographic records obtained with these instruments afford us a most beautiful method of recording the rhythm of normal and abnormal hearts in man, for they can be obtained by connecting the right hand and left foot of a patient with the instrument. Einthoven, by making A R - V A R - V A R - V FIG. 16. - Electrical Changes of Heart. A, diphasic variation of auricle; R - V, diphasic variation of ventricle. R = base negative; V = apex negative to base. After auricular contraction the ventricular is delayed - an example of arhythmia. (Einthoven.) The string galvanometer is the best method for elucidating disorders of cardiac rhythm.

use of the telephone wires, recorded in his laboratory the electrical changes of the hearts of patients seated in a hospital 2 m. away.

The heart during the period of systole is refractive to artificial excitation, but its susceptibility returns with diastole. The force and amplitude of any cardiac contraction depend on the previous activity of the heart and on such physical conditions as the degree of diastolic filling, the resistance to systolic outflow, temperature, &c., but are independent of the strength of the artificial stimulus so long as the latter is efficient. Owing to the refractory period, the slow rate of contraction and the independence of the amplitude of contraction on the strength of stimulus, the heart under ordinary conditions cannot be thrown, by rapidly repeated excitation, into a complete state of tetanic spasm. The refractory period can be shortened by heat (40° C.), or by calcium and sodium salts until tetanus is obtainable. The cardiac muscle is rich in sarcoplasm, and on this depends its power of slow, sustained contraction. The heart-muscle, besides rhythmically contracting, possesses " tone," and this tone varies with the conditions of metabolism, temperature, &c. Chloroform, for example, produces a soft dilated, strychnine, adrenalin or ammonia a tonically contracted heart. The mammalian heart ceases to beat at temperatures below 7° C. and above 44° C., and passes into " heat rigor " at 45° C.

In 1845 the brothers Weber made the astonishing discovery that the vagus nerve, when excited, slowed or even arrested the action of the heart. This was the first proof of the existence of inhibitory nerves. The cardiac inhibitory nerves have since been found in all classes of vertebrates and in many invertebrates. Some years later v. Beyeld (1862) and Moses and Il'ya Cyon (1843) discovered the existence of nerve fibres which, when excited, augmented and accelerated the beat of the heart. These nerves arise from 1-5 thoracic anterior spinal nerve roots and have their " cell stations " in the first thoracic and inferior cervical ganglia, whence they pass to the heart partly in company with the cardiac branches of the vagus, and partly as separate twigs. The vagus cardiac fibres arise by the middle of the lowermost group of vagus roots, and have their cellstations in the ganglion cells of the heart. These ganglion cells lie chiefly in the sub-pericardial tissue in the posterior wall of the auricles between and around the orifices of the venae cavae and pulmonary veins and between the aorta and pulmonary artery. The minute structure of these ganglia and the terminations of the nerves have been studied particularly by Dogiel. The inhibitory fibres arise from a centre in the spinal bulb which is in tonic action and constantly bridles the heart's action. When the vagi are divided the frequency of the heart increases and the blood pressure rises. The vagus centre is reflexly excited by the inhalation of chloroform, ammonia or other vapour irritant to the air passages, also by the want of oxygen in the blood in asphyxia. It may be excited by irritation of the abdominal nerves, e.g. a blow on the abdomen, and by increased pressure in the cerebral vessels. The acceleratory and augmenting fibres OWNIRIAINIII/Mlaf Medulla 'r' x Heart FIG. 17. - The origins of pneumogastric and vasomotor systems are in medulla, that of the sympathetic in upper portion of cord. The arrows indicate direction of nerve currents. In the heart R represents a reflex centre, I an inhibitory centre and A an accelerating centre.

likewise have their centre in the spinal bulb, and are in tonic action, the vagal centre. The vagus nerve works directly on the cardiac muscle, and produces some change (signalized by a positive variation in the electrical state of the heart) which results in a depression of the x x excitability, the con ductivity, the force and the frequency of the heart. After the vagal arrest the heart beats more forcibly, owing, it is thought, to the greater accumulation of contractile material dur ing the period of rest.

The converse of all these effects occurs on stimu lation of the accelerator nerves. Excitation of 20 these nerves may excite to renewed efforts an excised heart which has just ceased to beat after withdrawal of the supply of nutritive solution. Hence it is thought by some that the accelerator nerves tonically exert a sustaining influence on the heart.

The alkaloid atropin paralyses the vagal nerve endings in the heart, while nicotine paralyses the ganglion cells. Muscarin obtained from poisonous fungi slows and finally arrests the heart. Adrenalin, the active principle of the medulla of the supra-renal glands, augments its power. Chloroform depresses it and in poisonous dose throws the heart into paralytic dilatation. A great many of the cardiac vagal fibres convey impulse to the spinal bulb (centripetal), and reflexly influence the heart frequency, the breathing and the tonus of the blood vessels. In particular certain fibres, termed depressor (discovered by Ludwig and Cyon, 1866), cause dilatation of the arterioles and a fall of arterial pressure by inhibiting the tonic action of the vaso-motor centre in the spinal bulb. The depressor fibres arise from the root of the aorta, and overdistension of this part excites them, as evidenced not only by the above effect, but also by the electrical variation (action current) which has been observed passing up the depressor nerve. Sensory impressions originating in the heart do not as a rule enter into consciousness. They are carried by the cardiac nerves to the sympathetic ganglia, and thence to the upper thoracic region of the spinal cord, where they come into relation with the sensory nerves from the pectoral region, upper limb, shoulder, neck and head. The impressions are not felt in the heart, but referred to these sensory cutaneous nerves. Thus cardiac pain is felt in the chest wall and upper limbs and particularly on the left side. The function of the cardiac nerves is to co-ordinate the beat of the heart with the needs of the body and to co-ordinate the functions of other organs with the needs of the heart. For example, an undue rise of arterial pressure, induced, let us say, by compression of the abdomen, excites the centre of the vagus and produces slowing of the heart and a consequent lowering of arterial pressure. The heart of a mammal, however, continues to functionate after a section of all the branches of the cardiac plexus has been made, so that the nervous control and co-ordination of the heart are not absolutely essential to the continuance of life.

Water flowing through a tube from a constant head of pressure encounters a resistance occasioned by the friction of the moving water particles against each other and against the stationary layer that wets the wall of the tube. Part of factors the potential energy of the head of pressure is spent in endowing the fluid with kinetic energy, g gy, the greater part in circula- overcoming this resistance is rubbed down into heat. The narrower the tube is made, the greater the friction, until finally the flow ceases, the total energy being then insufficient to overcome the resistance.

The resistance may be measured at any point in the tube by inserting a side tube in the vertical position. The water rises to a certain height in the side tube, indicating the head of pressure spent in overcoming the resistance between the point of measurement and the orifice. If the lower end of the side tube is bent thus and inserted so that its orifice faces the stream, the water will rise higher than it did in the first case. The extra rise indicates the head of pressure spent in maintaining the velocity of flow. Such a method has been used to measure the velocity of flow in the vascular system (Napoleon Cybulski). When a stream of water is transmitted intermittently by the frequent strckes of a pump through XXVII. 30 a a long elastic rubber tube, the fluid does not issue in jets as it would in the case of a rigid tube, but flows out continuously. The elastic tube is distended by the force of the pump, and its elasticity maintains the outflow between the strokes. The continuous outflow here depends on the elasticity of the tube and the resistance to flow.

In the vascular system an area of vessels of capillary size is placed between the large arteries and veins. This area opposes a great resistance to flow. The arteries also are extensile elastic tubes. The effect of the peripheral resistance, as it is called, is to raise the pressure on the arterial side and lower it on the venous. The resistance to flow is situated chiefly, not in the capillaries, but in the small arteries, where the velocity is high; for " skin friction "- that is, the friction of the moving concentric layers of blood against one another and against the layer which wets the wall of these blood vessels is proportional to the surface area and to the viscosity of the blood - is nearly proportional to the square of the velocity of flow, and is inversely proportional to the sectional area of the vessels. Owing to the resistance to the capillary outflow, the large arteries are expanded by each systolic output of the heart, and the elasticity of their walls comes into play, causing the outflow to continue during the succeeding diastole of the heart. The conditions are such that the intermittent flow from the heart is converted into a continuous flow through the capillaries. If the arteries were rigid tubes, it would be necessary for the heart to force on the whole column of blood at one and the same time; but, owing to the elasticity of these vessels, the heart is saved from such a prolonged and jarring strain, and can pass into diastolic rest, leaving the elasticity of the distended arteries to maintain the flow. As a result of disease, the elastic tissue may degenerate and the arteries become rigid. Besides the saving of heart-strain, there are other advantages in the elasticity of the arteries. It has been found that an intermittently acting pump maintains a greater outflow through an elastic than through a rigid tube; that is to say, if the tubes be of equal bore. The four chief factors which co-operate in producing the conditions of pressure and velocity in the vascular system are - (I) the heart-beat, (2) the peripheral resistance, (3) the elasticity of the arteries, (4) the quantity of blood in the system. Suppose the body to be in the horizontal position and the vascular system to be brought to rest by, say, excitation of the vagus nerve and arrest of the heart. A sufficiency of blood to distend it collects within the venous cistern. The arterial system, owing to its elasticity and contractility, empties. If the heart now begin to beat, blood is taken from the venous system and is driven into the arterial system. The arteries receive more blood than can escape through the capillary vessels, and the arterial side of the system becomes distended, until equilibrium is reached, and as much blood escapes into the venous side per unit of time as is delivered by the heart. The flow in the capillaries and veins has now become a constant one and if the side pressure be measured it will be found to fall from the arteries to the capillaries, and from the capillaries to the venae cavae. In the large arteries there is a large side pressure which rises and falls with the pulses of the heart. The pulse waves spread out over a wider and wider area as the arteries branch. They finally die away in the arterioles. An increase or decrease in the energy of the heart-beat will increase or decrease respectively the velocity of flow and pressure of the blood. An increase or decrease in the total width of the arterioles respectively will lessen or raise the resistance; increase or decrease the velocity; lower or raise the blood pressure. A loss of blood, other conditions remaining the same, would cause a decrease in pressure and velocity. As a matter of fact, such a loss is compensated for by the adjustability of the vascular system. Tissue lymph passes from the tissues into the blood, and the blood vessels of the limbs and abdomen constrict, and thus the pressure is kept up, and an efficient circulation maintained through the brain, lungs and coronary vessels of the heart.

The whole vascular system is lined within by a layer of flattened cells, the endothelium; each cell is exceedingly thin and cemented to its fellows by a wavy border of an interstitial protoplasmic substance. The endothelium affords a smooth surface along which the blood can flow with of ease. Outside it there exists in the arteries and veins blood a middle and an external coat. The middle coat varies l greatly in thickness and contains most of the non-striated muscle-cells, which in the smaller arteries and arterioles form a particularly well developed band. In the larger arteries (fig. 19) a great deal of yellow elastic tissue, together with some white, fibrous tissue, pervades the middle coat. At the inner and outer border of this coat the elastic fibres fuse to form an internal and external fenestrated membrane. This coat endows the arteries with extensibility, elasticity and contractility. The outside coat consists mostly of white fibrous tissue and not only protects the arteries, but by its rigidity prevents over-distension. In the veins (fig. 20), where the middle coat is somewhat thinner and contains less elastic tissue, the outer coat consists mostly of muscle-fibres. The valves of the veins are formed of fibrous and elastic tissue covered with endothelium. As the arterioles branch into capillaries the muscular and elastic elements become less and less, until. in the capillaries themselves there is left only the layer of endothelium, supported by some stellate connective tissue cells. The.