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Human Body > IV. Myology > Development of the Muscles

2. Development of the Muscles

FIG. 373– Transverse section of human striped muscle fibers. x 255.
Both the cross-striated and smooth muscles, with the exception of a few that are of ectodermal origin, arise from the mesoderm. The intrinsic muscles of the trunk are derived from the myotomes while the muscles of the head and limbs differentiate directly from the mesoderm.
The Myotomic Muscles.—The intrinsic muscles of the trunk which are derived directly from the myotomes are conveniently treated in two groups, the deep muscles of the back and the thoraco-abdominal muscles.
  The deep muscles of the back extend from the sacral to the occipital region and vary much in length and size. They act chiefly on the vertebral column. The shorter muscles, such as the Interspinales, Intertransversarii, the deeper layers of the Multifidus, the Rotatores, Levatores costarum, Obliquus capitis inferior, Obliquus capitis superior and Rectus capitis posterior minor which extend between adjoining vertebræ, retain the primitive segmentation of the myotomes. Other muscles, such as the Splenius capitis, Splenius cervicis, Sacrospinalis, Semispinalis, Multifidus, Iliocostalis, Longissimus, Spinales, Semispinales, and Rectus capitis posterior major, which extend over several vertebræ, are formed by the fusion of successive myotomes and the splitting into longitudinal columns.
  The fascia lumbo-dorsalis develops between the true myotomic muscles and the more superficial ones which migrate over the back such as the Trapezius, Rhomboideus, and Latissimus.
  The anterior vertebral muscles, the Longus colli, Longus capitis, Rectus capitis anterior and Rectus capitis lateralis are derived from the ventral part of the cervical myotomes as are probably also the Scaleni.
  The thoraco-abdominal muscles arise through the ventral extension of the thoracic myotomes into the body wall. This process takes place coincident with the ventral extension of the ribs. In the thoracic region the primitive myotomic segments still persist as the intercostal muscles, but over the abdomen these ventral myotomic processes fuse into a sheet which splits in various ways to form the Rectus, the Obliquus externus and internus, and the Transversalis. Such muscles as the Pectoralis major and minor and the Serratus anterior do not belong to the above group.
The Ventrolateral Muscles of the Neck.—The intrinsic muscles of the tongue, the Infrahyoid muscles and the diaphragm are derived from a more or less continuous premuscle mass which extends on each side from the tongue into the lateral region of the upper half of the neck and into it early extend the hypoglossal and branches of the upper cervical nerves. The two halves which form the Infrahyoid muscles and the diaphragm are at first widely separated from each other by the heart. As the latter descends into the thorax the diaphragmatic portion of each lateral mass is carried with its nerve down into the thorax and the laterally placed Infrahyoid muscles move toward the midventral line of the neck.
Muscles of the Shoulder Girdle and Arm.—The Trapezius and Sternocleidomastoideus arise from a common premuscle mass in the occipital region just caudal to the last branchial arch; as the mass increases in size it spreads downward to the shoulder girdle to which it later becomes attached. It also spreads backward and downward to the spinous processes, gaining attachment at a still later period.
  The Levator scapulæ, Serratus anterior and the Rhomboids arise from premuscle tissue in the lower cervical region and undergo extensive migration.
  The Latissimus dorsi and Teres major are associated in their origin from the premuscle sheath of the arm as are also the two Pectoral muscles when the arm bud lies in the lower cervical region.
  The intrinsic muscles of the arm develop in situ from the mesoderm of the arm bud and probably do not receive cells or buds from the myotomes. The nerves enter the arm bud when it still lies in the cervical region and as the arm shifts caudally over the thorax the lower cervical nerves which unite to form the brachial plexus, acquire a caudal direction.
The Muscles of the Leg.—The muscles of the leg like those of the arm develop in situ from the mesoderma of the leg bud, the myotomes apparently taking no part in their formation.
The Muscles of the Head.—The muscles of the orbit arise from the mesoderm over the dorsal and caudal sides of the optic stalk.
  The muscles of mastication arise from the mesoderm of the mandibular arch. The mandibular division of the trigeminal nerve enters this premuscle mass before it splits into the Temporal, Masseter and Pterygoideus.
  The facial muscles (muscles of expression) arise from the mesoderm of the hyoid arch. The facial nerve enters this mass before it begins to split, and as the muscle mass spreads out over the face and head and neck it splits more or less incompletely into the various muscles.
  The early differentiation of the muscular system apparently goes on independently of the nervous system and only later does it appear that muscles are dependent on the functional stimuli of the nerves for their continued existence and growth. Although the nervous system does not influence muscle differentiation, the nerves, owing to their early attachments to the muscle rudiments, are in a general way indicators of the position of origin of many of the muscles and likewise in many instances the nerves indicate the paths along which the developing muscles have migrated during development. The muscle of the diaphragm, for example, has its origin in the region of the fourth and fifth cervical segments. The phrenic nerve enters the muscle mass while the latter is in this region and is drawn out as the diaphragm migrates through the thorax. The Trapezius and Sternocleidomastoideus arise in the lateral occipital region as a common muscle mass, into which at a very early period the nervus accessorius extends and as the muscle mass migrates and extends caudally the nerve is carried with it. The Pectoralis major and minor arise in the cervical region, receive their nerves while in this position and as the muscle mass migrates and extends caudally over the thorax the nerves are carried along. The Latissimus dorsi and Serratus anterior are excellent examples of migrating muscles whose nerve supply indicates their origin in the cervical region. The Rectus abdominis and the other abdominal muscles migrate or shift from a lateral to a ventrolateral or abdominal position, carrying with them the nerves.
  The facial nerve, which early enters the common facial muscle mass of the second branchial or hyoid arch, is dragged about with the muscle as it spreads over the head and face and neck, and as the muscle splits into the various muscles of expression, the nerve is correspondingly split. The mandibular division of the trigeminal nerve enters at an early time the muscle mass in the mandibular arch and as this mass splits and migrates apart to form the muscles of mastication the nerve splits into its various branches.
  The nerve supply then serves as a key to the common origin of certain groups of muscles. The muscles supplied by the oculomotor nerve arise from a single mass in the eye region; the lingual muscles arise from a common mass supplied by the hypoglossal nerve.
Striped or Voluntary Muscle.—Striped or voluntary muscle is composed of bundles of fibers each enclosed in a delicate web called the perimysium in contradistinction to the sheath of areolar tissue which invests the entire muscle, the epimysium. The bundles are termed fasciculi; they are prismatic in shape, of different sizes in different muscles, and are for the most part placed parallel to one another, though they have a tendency to converge toward their tendinous attachments. Each fasciculus is made up of a strand of fibers, which also run parallel with each other, and are separated from one another by a delicate connective tissue derived from the perimysium and termed endomysium. This does not form the sheath of the fibers, but serves to support the bloodvessels and nerves ramifying between them.
  A muscular fiber may be said to consist of a soft contractile substance, enclosed in a tubular sheath named by Bowman the sarcolemma. The fibers are cylindrical or prismatic in shape (Fig. 373), and are of no great length, not exceeding, as a rule, 40 mm. Huber  (*78 has recently found that the muscle fibers in the adductor muscle of the thigh of the rabbit vary greatly in length even in the same fasciculus. In a fasciculus 40 mm. in length the fibers varied from 30.4 mm. to 9 mm. in length. Their breadth varies in man from 0.01 to 0.1 mm. As a rule, the fibers do not divide or anastomose; but occasionally, especially in the tongue and facial muscles, they may be seen to divide into several branches. In the substance of the muscle, the fibers end by tapering extremities which are joined to the ends of other fibers by the sarcolemma. At the tendinous end of the muscle the sarcolemma appears to blend with a small bundle of fibers, into which the tendon becomes subdivided, while the muscular substance ends abruptly and can be readily made to retract from the point of junction. The areolar tissue between the fibers appears to be prolonged more or less into the tendon, so as to form a kind of sheath around the tendon bundles for a longer or shorter distance. When muscular fibers are attached to skin or mucous membranes, their fibers become continuous with those of the areolar tissue.
FIG. 374– Striped muscle fibers from tongue of cat. x 250.
  The sarcolemma, or tubular sheath of the fiber, is a transparent, elastic, and apparently homogeneous membrane of considerable toughness, so that it sometimes remains entire when the included substance is ruptured. On the internal surface of the sarcolemma in mammalia, and also in the substance of the fiber in frogs, elongated nuclei are seen, and in connection with these is a little granular protoplasm.
  Upon examination of a voluntary muscular fiber by transmitted light, it is found to be marked by alternate light and dark bands or striæ, which pass transversely across the fiber (Fig. 374). When examined by polarized light the dark bands are found to be doubly refracting (anisotropic), while the clear stripes are singly refracting (isotropic). The dark and light bands are of nearly equal breadth, and alternate with great regularity; they vary in breadth from about 1 to 2μ. If the surface be carefully focussed, rows of granules will be detected at the points of junction of the dark and light bands, and very fine longitudinal lines may be seen running through the dark bands and joining these granules together. By treating the specimen with certain reagents (e. g., chloride of gold) fine lines may be seen running transversely between the granules and uniting them together. This appearance is believed to be due to a reticulum or network of interstitial substance lying between the contractile portions of the muscle. The longitudinal striation gives the fiber the appearance of being made up of a bundle of fibrils which have been termed sarcostyles or muscle columns, and if the fiber be hardened in alcohol, it can be broken up longitudinally and the sarcostyles separated from each other (Fig. 375.) The reticulum, with its longitudinal and transverse meshes, is called sarcoplasm.
FIG. 375– A. Portion of a medium-sized human muscular fiber. Magnified nearly 800 diameters. B. Separated bundles of fibrils, equally magnified. a, a. Larger, and b, b, smaller collections. c. Still smaller. d, d. The smallest which could be detached.
  In a transverse section, the muscular fiber is seen to be divided into a number of areas, called the areas of Cohnheim, more or less polyhedral in shape and consisting of the transversely divided sarcostyles, surrounded by transparent sarcoplasm (Fig. 373).
FIG. 376– Diagram of a sarcomere. (After Schäfer.) A. In moderately extended condition. B. In a contracted condition. k, k. Membranes of Krause. H. Line or plane of Hensen. S.E. Poriferous sarcous element.
  Upon closer examination, and by somewhat altering the focus, the appearances become more complicated, and are susceptible of various interpretations. The transverse striation, which in Fig. 374 appears as a mere alternation of dark and light bands, is resolved into the appearance seen in Fig. 375, which shows a series of broad dark bands, separated by light bands, each of which is divided into two by a dark dotted line. This line is termed Dobie’s line or Krause’s membrane (Fig. 376, k), because it was believed by Krause to be an actual membrane, continuous with the sarcolemma, and dividing the light band into two compartments. In addition to the membrane of Krause, fine clear lines may be made out, with a sufficiently high power, crossing the center of the dark band; these are known as the lines of Hensen (Fig. 376, H).
  Schäfer has worked out the minute anatomy of muscular fiber, particularly in the wing muscles of insects, which are peculiarly adapted for this purpose on account of the large amount of interstitial sarcoplasm which separates the sarco-styles. In the following description that given by Schäfer will be closely followed.
  A sarcostyle may be said to be made up of successive portions, each of which is termed a sarcomere. The sarcomere is situated between two membranes of Krause and consists of (1) a central dark part, which forms a portion of the dark band of the whole fiber, and is named a sarcous element. This sarcous element really consists of two parts, superimposed one on the top of the other, and when the fiber is stretched these two parts become separated from each other at the line of Hensen (Fig. 376, A). (2) On either side of this central dark portion is a clear layer, most visible when the fiber is extended; this is situated between the dark center and the membrane of Krause, and when the sarcomeres are joined together to form the sarcostyle, constitutes the light band of the striated muscular fiber.
  When the sarcostyle is extended, the clear intervals are well-marked and plainly to be seen; when, on the other hand, the sarcostyle is contracted, that is to say, when the muscle is in a state of contraction, these clear portions are very small or they may have disappeared altogether (Fig. 376, B). When the sarcostyle is stretched to its full extent, not only is the clear portion well-marked, but the dark portion—the sarcous element—is separated into its two constituents along the line of Hensen. The sarcous element does not lie free in the sarcomere, for when the sarcostyle is stretched, so as to render the clear portion visible, very fine lines, which are probably septa, may be seen running through it from the sarcous element to the membrane of Krause.
  Schäfer explains these phenomena in the following way: He considers that each sarcous element is made up of a number of longitudinal channels, which open into the clear part toward the membrane of Krause but are closed at the line of Hensen. When the muscular fiber is contracted the clear part of the muscular substance is driven into these channels or tubes, and is therefore hidden from sight, but at the same time it swells up the sarcous element and widens and shortens the sarcomere. When, on the other hand, the fiber is extended, this clear substance is driven out of the tubes and collects between the sarcous element and the membrane of Krause, and gives the appearance of the light part between these two structures; by this means it elongates and narrows the sarcomere.
  If this view be true, it is a matter of great interest, and, as Schäfer has shown, harmonizes the contraction of muscle with the ameboid action of protoplasm. In an ameboid cell, there is a framework of spongioplasm, which stains with hematoxylin and similar reagents, enclosing in its meshes a clear substance, hyaloplasm, which will not stain with these reagents. Under stimulation the hyaloplasm passes into the pores of the spongioplasm; without stimulation it tends to pass out as in the formation of pseudopodia. In muscle there is the same thing, viz., a framework of spongioplasm staining with hematoxylin—the substance of the sarcous element—and this encloses a clear hyaloplasm, the clear substance of the sarcomere, which resists staining with this reagent. During contraction of the muscle—i.e., stimulation—this clear substance passes into the pores of the spongioplasm; while during extension of the muscle—i.e., when there is no stimulation—it tends to pass out of the spongioplasm.
  In this way the contraction is brought about: under stimulation the protoplasmic material (the clear substance of the sarcomere) recedes into the sarcous element, causing the sarcomere to widen out and shorten. The contraction of the muscle is merely the sum total of this widening out and shortening of these bodies.
Vessels and Nerves of Striped Muscle.—The capillaries of striped muscle are very abundant, and form a sort of rectangular network, the branches of which run longitudinally in the endomysium between the muscular fibers, and are joined at short intervals by transverse anastomosing branches. In the red muscles of the rabbit dilatations occur on the transverse branches of the capillary network. The larger vascular channels, arteries and veins, are found only in the perimysium, between the muscular fasciculi. Nerves are profusely distributed to striped muscle. Their mode of termination is described on page 730. The existence of lymphatic vessels in striped muscle has not been ascertained, though they have been found in tendons and in the sheaths of the muscles.
  Ossification of muscular tissue as a result of repeated strain or injury is not infrequent. It is oftenest found about the tendon of the Adductor longus and Vastus medialis in horsemen, or in the Pectoralis major and Deltoideus of soldiers. It may take the form of exostoses firmly fixed to the bone—e.g., “rider’s bone” on the femur—or of layers or spicules of bone lying in the muscles or their fasciæ and tendons. Busse states that these bony deposits are preceded by a hemorrhagic myositis due to injury, the effused blood organizing and being finally converted into bone. In the rarer disease, progressive myositis ossificans, there is an unexplained tendency for practically any of the voluntary muscles to become converted into solid and brittle bony masses which are completely rigid.
Note 78.  Anat. Rec., 1916, 11. [back]
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