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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.

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Chapter VIII. The Development of the Muscular System

Two forms of muscular tissue exist in the human body, the striated tissue, which forms the skeletal muscles and is under the control of the central nervous system, and the non-striated, which is controlled by the sympathetic nervous system and is found in the skin, in the walls of the digestive tract, the blood-vessels and lymphatics, and in connection with the genito-urinary apparatus. In the walls of the heart a muscle tissue occurs which is frequently regarded as a third form, characterized by being under control of the sympathetic system and yet being striated; it is, however, in its origin much more nearly allied to the non-striated than to the striated form of tissue, and will be considered a variety of the former.

The Histogenesis of Non-striated Muscular Tissue

With the exception of the sphincter and dilator of the pupil and the muscles of the sudoriparous glands, which are formed from the ectoderm, all the non-striated muscle tissue of the body is formed by the conversion of mesenchyme cells into muscle-fibers. The details of this process have been worked out by McGill for the musculature of the digestive and respiratory tracts of the pig and are as follows: The mesenchyme surrounding the mucosa in these tracts is at first a loose syncytium (Fig. 117, m) and in the regions where the muscle tissue is to form a condensation of the mesenchyme occurs followed by an elongation of the mesenchyme cells and their nuclei, so that the muscle layers become clearly distinguishable from the neighboring undifferentiated tissue (Fig. 117, mm). Fibrils of two kinds then begin to appear in the cytoplasm of the muscle cells. Coarse fibrils (f.c) make their appearance as rows of granules, which enlarge and increase in number until they finally fuse to form homogeneous

Fig. 117. - Longitudinal Section of the Lower Part of the Oesophagus of a Pig Embryo of 15 mm, Showing the Histogenesis of the Non-striated Musculature.

b, Basement membrane; e, epithelium; /.c, coarse fibril;//., fine fibril; ga, ganglion of Auerbach's plexus; gm, ganglion of Meissner's plexus; m, mesenchyne; mm, muscularis mucosae; pb, protoplasmic bridge; vf, varicose fibril. - (McCill.)

fibrils that are at first varicose, but later become of uniform caliber. Fine fibrils (/./) which are homogeneous from the first, make their appearance after the coarse ones and in some cases seem to be formed by the splitting of the latter. They are scattered uniformly throughout the cytoplasm of the muscle cells and increase in number as development proceeds, while the coarse fibrils diminish and may be entirely wanting in the adult tissue.

Some of the mesenchyme cells in each muscle sheet fail to undergo the differentiation just described and multiply to form the interstitial connective tissue, which usually divides the muscle cells into more or less distinct bundles. Traces of the original syncytial nature of the tissue are to be seen in the intercellular bridges that occur between the non-striated muscle cells of many adult forms.

The cells from which the heart musculature develops are at first of the usual well defined embryonic type, but, as development proceeds, they become irregularly stellate in form, the processes of neighboring cells fuse and, eventually, there is formed a continuous mass of protoplasm or syncytium in which all traces of cell boundaries are lacking (Fig. 118). While the individual cells, or myoblasts as they are termed, are still recognizable, granules appear in their cytoplasm, and these arrange themselves in rows and unite to form slender fibrils, which at first do not extend beyond the limits of the myoblasts in which they have appeared, but later, as the fusion of the cells proceeds, are continued from one cell territory into the other

Fig. 118. - Section through the Heartwall of a Duck Embryo of Three Days. - (M. Heidenhain.)

through considerable stretches of the syncytium, without regard to the original cell areas.

The fibrils multiply, apparently by longitudinal division, and arrange themselves in circles around areas of the syncytium (compare Fig. 119). As the multiplication of the fibrils continues those newly formed arrange themselves around the interior of each of the original circles and gradually occupy the entire cytoplasm, or sarcoplasm as it may now be termed, except immediately around the nuclei where, even in the adult, a certain amount of undifferentiated sarcoplasm persists. The fibrils when first formed are apparently homo

Fig. 119. - Cross-section of a Muscle prom the Thigh of a Pig Embryo 75 mm.

Long. A, Central nucleus; B, new peripheral nucleus. - (Macallum.)

geneous, but later they become differentiated into two distinct substances which alternate with one another throughout the length of the fibril and produce the characteristic transverse striation of the tissue. Finally stronger interrupted transverse bands of so-called cement substance appear, dividing the tissue into areas which have usually been regarded as representing the original myoblasts, but are really devoid of significance as cells, the tissue remaining, strictly speaking, a syncytium.

The Histogenesis of Striated Muscle Tissue

The histogenesis of striated or voluntary muscle tissue resembles very closely that which has just been described for the heart muscle. There is a similar formation of a syncytium by the fusion of the cells of the myotomes, an appearance of granules which unite to form fibrils, an increase of the fibrils by longitudinal division and a primary arrangement of the fibrils around the periphery of areas of sarcoplasm (Fig. 119), each of which represents a muscle fiber. In addition there is an active proliferation of the nuclei of the original myoblasts, the new nuclei arranging themselves more or less regularly in rows and later migrating from their original central position to the periphery of the fibers, and, in the limb muscles, the development is further complicated by a process of degeneration which affects groups of muscle fibers, so that bundles of normal fibers are separated by strands of degenerated tissue in which the fibrils have disappeared, the nuclei have become pale and the sarcoplasm vacuolated and homogeneous. Later the degenerated tissue seems to disappear entirely and mesenchymatous connective tissue grows in between the persisting fibers, grouping them into bundles and the bundles into the individual muscles.

So long as the formation of new fibrils continues, the increase in the thickness of a muscle is probably due to a certain extent to an increase in the actual number of fibers, which results from the division of those already existing. Subsequently, however, this mode of growth ceases, the further increase of the muscle depending upon an increase in size of its constituent elements (Macallum).

The Development of the Skeletal Muscles

It has already been pointed out that all the skeletal muscles of the body, with the exception of those connected with the branchial arches, are derived from the myotomes of the mesodermic somites, even the limb muscles possibly having such an origin, although the cells of the tissue from which the muscles of the limb buds form lack an epithelial arrangement and are indistinguishable from the somatic mesenchyme which forms the axial cores of the limbs.

The various fibers of each myotome are at first loosely arranged, but later they become more compact and are arranged parallel with one another, their long axes being directed antero-posteriorly. This stage is also transitory, however, the fibers of each myotome undergoing various modifications to produce the conditions existing in the adult, in which the original segmental arrangement of the fibers can be perceived in comparatively few muscles. The exact nature of these modifications is almost unknown from direct observation, but since the relation between a nerve and the myotome belonging to the same segment is established at a very early period of development and persists throughout life, no matter what changes of fusion, splitting, or migration the myotome may undergo, it is possible to trace out more or less completely the history of the various myotomes by determining their segmental innervation. It is known, for example, that the latissimus dorsi arises from the seventh and eighth* cervical myotomes, but later undergoes a migration, becoming attached to the lower thoracic and lumbar vertebrae and to the crest of the ilium, far away from its place of origin (Mall), and yet it retains its nerve-supply from the seventh and eighth cervical nerves with which it was originally associated, its nerve-supply consequently indicating the extent of its migration.

By following the indications thus afforded, it may be seen that the changes which occur in the myotomes may be referred to one or more of the following processes: 1. A longitudinal splitting into two or more portions, a process well illustrated by the trapezius and sternomastoid, which have differentiated by the longitudinal splitting of a single sheet and contain therefore portions of the same myotomes. The sternohyoid and omohyoid have also differentiated by the same process, and, indeed, it is of frequent occurrence.

2. A tangential splitting into two or more layers. Examples of this are also abundant and are afforded by the muscles of the fourth, fifth, and sixth layers of the back, as recognized in English text-books

This enumeration is based on convenience in associating the myotomes with the nerves which supply them. The myotomes mentioned are those which correspond to the sixth and seventh cervical vertebrae.

of anatomy, by the two oblique and the transverse layers of the abdominal walls, and by the intercostal muscles and the transversus of the thorax.

3. A fusion of portions of successive myotomes to form a single muscle, again a process of frequent occurrence, and well illustrated by the rectus abdominis (which is formed by the fusion of the ventral portions of the last six or seven thoracic myotomes) or by the superficial portions of the sacro-spinalis.

4. A migration of parts of one or more myotomes over others. An example of this process is to be found in the latissimus dorsi, whose history has already been referred to, and it is also beautifully shown by the serratus anterior and the trapezius, both of which have extended far beyond the limits of the segments from which they are derived.

5. A degeneration of portions or the whole of a myotome. This process has played a very considerable part in the evolution of the muscular system in the vertebrates. When a muscle normally degenerates, it becomes converted into connective tissue, and many of the strong aponeurotic sheets which occur in the body owe their origin to this process. Thus, for example, the aponeurosis connecting the occipital and frontal portions of the occipito-frontalis is formed in this process and is muscular in such forms as the lower monkeys, and a good example is also to be found in the aponeurosis which occupies the interval between the superior and inferior serrati postici, these two muscles being continuous in lower forms. The strong lumbar aponeurosis and the aponeuroses of the oblique and transverse muscles of the abdomen are also good examples.

Indeed, in comparing one of the mammals with a member of one of the lower classes of vertebrates, the greater amount of connective tissue compared with the amount of muscular tissue in the former is very striking, the inference being that these connectivetissue structures (fasciae, aponeuroses, ligaments) represent portions of the muscular tissue of the lower form (Bardeleben). Many of the accessory ligaments occurring in connection with diarthrodial joints apparently owe their origin to a degeneration of muscle tissue, the fibular lateral ligament of the knee-joint, for instance, being probably a degenerated portion of the peroneus longus, while the sacrotuberous ligament appears to stand in a similar relation to the long head of the biceps femoris (Sutton).

6. Finally, there may be associated with any of the first four processes a change in the direction of the muscle-fibers. The original antero-posterior direction of the fibers is retained in comparatively few of the adult muscles and excellent examples of the process here referred to are to be found in the intercostal muscles and the muscles of the abdominal walls. In the musculature associated with the branchial arches the alteration in the direction of the fibers occurs even in the fishes, in which the original direction of the muscle-fibers is very perfectly retained in other myotomes, the branchial muscles, however, being arranged parallel with the branchial cartilages or even passing dorso-ventrally between the upper and lower portions of an arch, and so forming what may be regarded as a constrictor of the arch. This alteration of direction dates back so far that the constrictor arrangement may well be taken as the primary condition in studying the changes which the branchial musculature has undergone in the mammalia.

It would occupy too much space 'in a work of this kind to consider in detail the history of each one of the skeletal muscles of the human body, but a statement of the general plan of their development will not be out of place. For convenience the entire system may be divided into three portions - the cranial, trunk and limb musculature; and of these, the trunk musculature may first be considered.

The Trunk Musculature

It has already been seen (p. 82) that the myotomes at first occupy a dorsal position, becoming prolonged ventrally as development proceeds so as to overlap the somatic mesoderm, until those of opposite sides come into contact in the mid-ventral line. Before this is accomplished, however, a longitudinal splitting of each myotome occurs, whereby there is separated off a dorsal portion which gives rise to a segment of the dorsal musculature of the trunk and is supplied by the ramus dorsalis

of its corresponding spinal nerve. In the lower vertebrates this separation of each of the trunk myotomes into a dorsal and ventral portion is much more distinct in the adult than it is in man, the two portions being separated by a horizontal plate of connective tissue extending the entire length of the trunk and being attached by its inner edge to the transverse processes of the vertebrae, while peripherally it becomes continuous with the connective tissue of the

Fig. 120. - Embryo of 13 mm. showing the Formation of the Rectus Muscle. - (Mall.)

dermis along a line known as the lateral line. In man the dorsal portion is also much smaller in proportion to the ventral portion than in the lower vertebrates. From this dorsal portion of the myotomes are derived the muscles belonging to the three deepest layers of the dorsal musculature, the more superficial layers being composed of muscles belonging to the limb system. Further longitudinal and tangential divisions and a fusion of successive myotomes bring about the conditions which obtain in the adult dorsal musculature.

While the myotomes are still some distance from the mid-ventral line another longitudinal division affects their ventral edges (Fig. 120), portions being thus separated which later fuse more or less perfectly to form longitudinal bands of muscle, those of opposite sides being brought into apposition along the mid-ventral line by the continued growth ventrally of the myotomes. In this way are formed the rectus and pyramidalis muscles of the abdomen and the depressors of the hyoid bone, the genio-hyoid and genio-glossus* in the neck region. In the thoracic region this rectus set of muscles, as it may be termed, is not represented except as an anomaly, its absence being probably correlated with the development of the sternum in this region.

The lateral portions of the myotomes which intervene between the dorsal and rectus muscles divide tangentially, producing from their dorsal portions in the cervical and lumbar regions muscles, such as the longus capitis and colli and the psoas, which lie beneath the vertebral column and hence have been termed hyposkeletal muscles (Huxley). More ventrally three sheets of muscles, lying one above the other, are formed, the fibers of each sheet being arranged in a definite direction differing from that found in the other sheets. In the abdomen there are thus formed the two oblique and the transverse muscles, in the thorax the intercostals and the transversa thoracis, while in the neck these portions of some of the myotomes disappear, those of the remainder giving rise to the scaleni muscles, portions of the trapezius and sternomastoid (Bolk), and possibly the hyoglossus and styloglossus. In the abdominal region, and to a considerable extent in the neck also, the various portions of myotomes fuse together, but in the thorax they retain in the intercostals their original distinctness, being separated by the ribs.

This muscle is supplied by the hypoglossal nerve, but for the present purpose it is convenient to regard this as a spinal nerve, as indeed it primarily is.

The table on page 203 will show the relation of the various trunk muscles to the portions of the myotomes.

The intimate association between the pelvic girdle and the axial skeleton brings about extensive modifications of the posterior trunk myotomes. So far as their dorsal portions are concerned probably all these myotomes as far back as the fifth sacral are represented in the sacro-spinalis, but the ventral portions from the first lumbar myotome onward are greatly modified. The last myotome taking part in the formation of the rectus abdominis is the twelfth thoracic and the last to be represented in the lateral musculature of the

Fig. 121. - Perineal Region of Embryos of (A) Two Months and (25) Four to Five Months, showing the Development of the Perineal Muscles.

dc, Nervus dorsalis clitoridis; p, pudendal nerve; sa, sphincter ani; sc sphincter cloacae; sv, sphincter vaginse. - (Popowsky.)

abdomen is the first lumbar, the ventral portions of the remaining lumbar and of the first and second sacral myotomes either having disappeared or being devoted to the formation of the musculature of the lower limb.

The ventral portions of the third and fourth sacral myotomes are represented, however, by the levator ani and coccygeus, and are the last myotomes which persist as muscles in the human body, although traces of still more posterior myotomes are to be found in muscles such as the curvator coccygis sometimes developed in connection with the coccygeal vertebrae.

The perineal muscles and the external sphincter ani are also developments of the third and fourth (and second) sacral myotomes. At a time when the cloaca (see p. 280) is still present, a sheet of muscles lying close beneath the integument forms a sphincter around its opening (Fig. 121). On the development of the partition which divides the cloaca into rectal and urinogenital portions, the sphincter is also diyided, its more posterior portion persisting as the external sphincter ani, while the anterior part gradually differentiates into the various perineal muscles (Popowsky).

The Cranial Musculature

As was pointed out in an earlier chapter, the existence of distinct mesodermic somites has not yet been completely demonstrated in the head of the human embryo, but in lower forms, such as the elasmobranch fishes, they are clearly distinguishable, and it may be supposed that their indistinctness in man is a secondary condition. Exactly how many of these somites are represented in the mammalian head it is impossible to say, but it seems probable, from comparison with lower forms, that there is a considerable number. The majority of them, however, early undergo degeneration, and in the adult condition only three are recognizable, two of which are prseoral in position and one postoral. The myotomes of the anterior praeoral segment give rise to the muscles of the eye supplied by the third cranial nerve, those of the posterior one furnish the superior oblique muscles innervated by the fourth nerve, while from the postoral myotomes the lateral recti, supplied by the sixth nerve, are developed. The muscles supplied by the hypoglossal nerve are also derived from myotomes, but they have already been considered in connection with the trunk musculature.

The remaining muscles of the head differ from all other voluntary muscles of the body in the fact that they are derived from the branchiomeres formed by the segmentation of the cephalic ventral mesoderm. These muscles, therefore, are not to be regarded as equivalent to the myotomic muscles if their embryological origin is to be taken as a criterion of equivalency, and in their case it would seem, from the fact that they are innervated by nerves fundamentally distinct from those which supply the myotomic muscles, that this criterion is a good one. They must be regarded, therefore, as belonging to a special category, and may be termed branchiomeric muscles to distinguish them from the myotomic set.

If their embryological origin be taken as a basis for homology, it is clear that they should be regarded as equivalent to the muscles derived from the ventral mesoderm of the trunk, and these, as has been seen, are the non-striated muscles associated with the viscera, among which may be included the striated heart muscle. At first sight this homology seems decidedly strained, chiefly because long-continued custom has regarded the histological and physiological peculiarities of striated and non-striated muscle tissue as fundamental. It may be pointed out, however, that the branchiomeric muscles are, strictly speaking, visceral muscles, and indeed give rise to muscle sheets (the constrictors of the pharynx) which surround the upper or pharyngeal portion of the digestive tract. It is possible, then, that the homology is not so strained as might appear, but further discussion of it may profitably be deferred until the cranial nerves are under consideration.

The skeleton of the first branchial arch becomes converted partly into the jaw apparatus and partly into auditory ossicles, and the muscles derived from the corresponding branchiomere become the muscles of mastication (the temporal, masseter, and pterygoids), the mylohyoid, anterior belly of the digastric, the tensor veli palatini and the tensor tympani. The nerve which corresponds to the first branchial arch is the trigeminus or fifth, and consequently these various muscles are supplied by it.

The second arch has corresponding to it the seventh nerve, and its musculature is partly represented by the stylohyoid and posterior belly of the digastric and by the stapedius muscle of the middle ear. From the more superficial portions of the branchiomere, however, a sheet of tissue arises which gradually extends upward and downward to form a thin covering for the entire head and neck, its lower portion giving rise to the platysma and the nuchal fascia which extends backward from the dorsal border of this muscle, while its upper parts become the occipito-frontalis and the superficial muscles of the face (the muscles of expression), together with the fascia? which unite the various muscles of this group. The extension of the platysma sheet of muscles over the face is well shown by the development of the branches of the facial nerve which supply it (Fig. 122).

Fig. 122. - Head of Embryos (.4) of Two Months and (B) of Three Months showing the Extension of the Seventh Nerve upon the Face. - (Popowsky.)

The degeneration of the upper part of the third arch produces a shifting forward of one of the muscles derived from its branchiomere, the stylopharyngeus arising from the base of the styloid process. The innervation of this muscle by the ninth nerve indicates, however, its true significance, and since fibers of this nerve of the third arch also pass to the constrictor muscles of the pharynx, a portion of these must also be regarded as having arisen from the third branchiomere.

The cartilages of the fourth and fifth arches enter into the formation of the larynx and the muscles of the corresponding branchiomeres constitute the muscles of the larynx, together with the remaining portions of the constrictors of the pharynx and the muscles of the soft palate, with the exception of the tensor. Both these arches have branches of the tenth nerve associated with them and hence this nerve supplies the muscles named. In addition, two of the extrinsic muscles of the tongue, the glosso-palatinus and chondroglossus, belong to the fourth or fifth branchiomere, although the remaining muscles of this physiological set are myotomic in origin.

Finally, portions of two other muscles should probably be included in the list of branchiomeric muscles, these muscles being the trapezius and sternomastoid. It has already been seen that they are partly derived from the cervical myotomes, but they are also innervated in part by the spinal accessory, and since the motor fibers of this nerve are serially homologous with those of the vagus it would seem that the muscles which they supply are probably branchiomeric in origin. Observations on the development of these muscles, determining their relations to the branchiomeres, are necessary, however, before their morphological significance can be regarded as definitely settled.

The table on page 209 shows the relations of the various cranial muscles to the myotomes and branchiomeres, as well as to the motor cranial nerves.

Eleventh

Trapezius. Sternomastoid.

Tenth

Constrictors of pharynx (in part). Pharyngopalatinus. Levator veli palatini. Musculus uvulae. Muscles of the larynx. Glosso-pal atinus.

Chrondro glossus.

â– 5 .S

Stylo-pha ryngeus.

Constrictors of pharynx (in part).

6 > <U CO

Stylohyoid.

Digastric (posterior belly).

Stapedius. Platysma. Occipitofrontalis.

Muscles of expression.

CO

a) 3 h-1 M

3

Temporal. . Masseter.

Pterygoids.

Mylohyoid.

Digastric (anterior belly).

Tensor veli palatini.

Tensor tympani.

3 3

O u CO O

â€¢A M V

1 O

Â«5 <u "0 s

'in U % I u pq

I 3 1

The Limb Muscles.

It has been customary to regard the limb muscles as derivatives of certain of the myotomes, these structures in their growth vent rally in the trunk walls being supposed to pass out upon the postaxial surface of the limb buds and loop back again to the trunk along the praeaxial surface, each myotome thus giving rise to a portion of both the dorsal and the ventral musculature of the limb. This view has not, however, been verified by direct observation of an actual looping of the myotomes over the axis of the limb buds; indeed, on the contrary, the limb muscles have been found to develop from the cores of mesenchyme which form the axes of the limb buds and from which the limb skeleton is also developed. This may be explained by supposing that the limb muscles are primarily derivatives of the myotomes and that an extensive concentration of their developmental history has taken place, so that the axial mesenchyme actually represents myotomic material even though no direct connection between it and the myotomes can be discovered. Condensations of the developmental history certainly occur and the fact that the muscles of the human limbs, as they differentiate from the axial cores, present essentially the same arrangement as in the adult seems to indicate that there is actually an extensive condensation of the phylogenetic history of the individual muscles, since comparative anatomy shows the arrangement of the muscles of the higher mammalian limbs to be the result of a long series of progressive modifications from a primitive condition. However, even though this be the case, there is yet the possibility that the limb musculature, like the limb skeleton, may take its origin from the ventral mesoderm and consequently belong to a different embryological category from the axial myotomic muscles.

The strongest evidence in favor of the myotomic origin of the limb muscles is that furnished by their nerve supply, this presenting a distinctly segmental arrangement. This does not necessarily imply, however, a corresponding primarily metameric arrangement of the muscles, any more than the pronouncedly segmental arrangement of the cutaneous nerves implies a primary metamerism of the dermis (see p. 143). It may mean only that the nerves, being segmental, retain their segmental relations to one another even in their distribution to non-metameric structures, and that, consequently, the limb musculature is supplied in succession from one border of the limb bud to the other from succeeding nerve roots.

But whether further observation may prove or disprove the myotomic origin of the limb musculature, the fact remains that it possesses a segmentally arranged innervation, and it is possible,

Fig. 123. - Diagram of a Segment of the Body and Limb. bl, Axial blastema; dm, dorsal musculature of trunk; rl, nerve to limb; s, septum between dorsal and ventral trunk musculature; str.d, dorsal layer of limb musculature; tr.d and tr.v, dorsal and ventral divisions of a spinal nerve; vm, ventral musculature of the trunk. - (Kollmann.) therefore, to recognize in the limb buds a series of parallel bands of muscle tissue, extending longitudinally along the bud and each supplied by a definite segmental nerve. And furthermore, corresponding to each band upon the ventral (praeaxial) surface of the limb bud, there is a band similarly innervated upon the dorsal (postaxial) surface, since the fibers which pass to the limb from each nerve root sooner or later arrange themselves in praeaxial and postaxial groups as is shown in the diagram Fig. 123. The first nerve which enters the limb bud lies along its anterior border, and consequently the muscle bands which are supplied by it will, in the adult, lie along

Fig. 124. - External Surface of the Os Innominatum showing the Attachment of Muscles and the Zones Supplied by the Various Nerves.

12, Twelfth thoracic nerve; I to V, lumbar nerves; 1 and 2, sacral nerves. - (Bolk.) the outer side of the arm and along the inner side of the leg, in consequence of the rotation in opposite directions which the limbs undergo during development (see p. 101).

The first nerve which supplies the muscles attached to the dorsum of the ilium is the second lumbar, and following that there come successively from before backward the remaining lumbar and the

Fig. 125. - Sections through (A) the Thigh and (B) the Calf showing the Zones Supplied by the Nerves. The Nerves are Numbered in Continuation with the Thoracic Series. - (A, after Bolk.)

first and second sacral nerves. The arrangement of the muscle bands supplied by these nerves and the muscles of the adult to which they contribute may be seen from Fig. 124. What is shown there is only the upper portions of the postaxial bands, their lower portions

extending downward on the anterior surface of the leg. Only the sacral bands, however, extend throughout the entire length of the limb into the foot, the second lumbar band passing down only to about the middle of the thigh, the third to about the knee, the fourth to about the middle of the crus and the fifth as far as the base of the fifth metatarsal bone, and the same is true of the corresponding praeaxial bands, which descend from the ventral surface of the os coxae (innominatum) along the inner and posterior surfaces of the leg to the same points. The first and second sacral bands can be traced into the foot, the first giving rise to the musculature of its

Fig. 126. - Section through the Upper Part of the Arm showing the Zones Supplied by the Nerves.

$v to jv, Ventral branches; 5J to Sd, dorsal branches of the cervical nerves. - (Bolk.)

inner side and the second to that of its outer side, the praeaxial bands forming the plantar musculature, while the postaxial ones are upon the dorsum of the foot as a result of the rotation which the limb has undergone.

In a transverse section through a limb at any level all the muscle bands, both praeaxial and postaxial, which descend to that level will be cut and will lie in a definite succession from one border of the limb to the other, as is seen in Fig. 125. In the differentiation of the individual muscles which proceeds as the nerves extend from the trunk into the axial mesenchyme of the limb, the muscle bands undergo modifications similar to those already described as occurring in the trunk myotomes. Thus, each of the muscles represented in Fig. 125, B, is formed by the fusion of elements derived from two or more bands; the soleus and gastrocnemius represent deep and superficial layers formed from the same bands by a horizontal (tangential) splitting, these same muscles contain a portion of the second sacral band which overlaps muscles composed only of higher myotomes, and the intermuscular septum between the peroneus brevis and the flexor hallucis longus represents a portion of the third sacral band which has degenerated into connective tissue.

A similar arrangement occurs in the bands which are to be recognized in the musculature of the upper limb. These are supplied by the fourth, fifth, sixth, seventh and eighth cervical and the first thoracic nerves, and only those supplied by the eighth cervical and the first thoracic nerves extend as far as the tips of the fingers. The arrangement of the bands in the upper part of the brachium may be seen from Fig. 126, in connection with which it must be noted that the fourth cervical band does not extend down to the level at which the section is taken and that the praeaxial band of the eighth cervical nerve and both the praeaxial and postaxial bands of the first thoracic are represented only by connective tissue in this region.

In another sense than the longitudinal one there is a division of the limb musculature into more or less definite areas, namely, in a transverse direction in accordance with the jointing of the skeleton. Thus, there may be recognized a group of muscles which pass from the axial skeleton to the pectoral girdle, another from the limb girdle to the brachium or thigh, another from the brachium or thigh to the antibrachium or crus, another from the antibrachium or crus to the carpus or tarsus, and another from the carpus or tarsus to the digits. This transverse segmentation, if it may be so termed, is not, however, perfectly definite, many muscles, even in the lower vertebrates, passing over more than one joint, and in the mammalia, especially, it is further obscured by secondary migrations, by the partial degeneration of muscles and by an end to end union of primarily distinct muscles.

The latissimus dorsi, serratus anterior and pectoral muscles are all examples of a process of migration as is shown by their innervation from cervical nerves, as well as by the actual migration which has been traced in the developing embryo (Mall, Lewis). In the lower limb evidences of migration may be seen in the femoral head of the biceps, comparative anatomy showing this to be a derivative of the gluteal set of muscles which has secondarily become attached to the femur and has associated itself with a praeaxial muscle to form a compound structure. An appearance of migration may also be produced by a muscle making a secondary attachment below its original origin or above the insertion and the upper or lower part, as the case may be, then degenerating into connective tissue. This has been the case with the peroneus longus, which, in the lower mammals, has a femoral origin, but has in man a new origin from the fibula, its upper portion being represented by the fibular lateral ligament of the knee-joint. So too the pectoralis minor is primarily inserted into the humerus, but it has made a secondary attachment to the coracoid process, its distal portion forming a coraco-humeral ligament.

The comparative study of the flexor muscles of the antibrachial and crural regions has yielded abundant evidence of extensive modifications in the differentiation of the limb muscles. In the tailed amphibia these muscles are represented by a series of superposed layers, the most superficial of which arises from the humerus or femur, while the remaining ones take their origin from the ulna or fibula and are directed distally and laterally to be inserted either into the palmar or plantar aponeurosis, or, in the case of the deeper layers, into the radius (tibia) or carpus (tarsus). In the arm of the lower mammalia the deepest layer becomes the pronator quadratus, the lateral portions of the superficial layer are the flexor carpi ulnaris and the flexor carpi radialis, while the intervening layers, together with the median portion of the superficial one, assuming a more directly longitudinal direction, fuse to form a common flexor mass which acts on the digits through the palmar aponeurosis. From this latter structure and from the carpal and metacarpal bones five layers of palmar muscles take origin. The radial and ulnar portions of the most superficial of these become the flexor pollicis brevis and abductor pollicis brevis and the abductor quinti digiti, while the rest of the layer degenerates into connective tissue, forming tendons

Fig. 127. - Transverse sections through (A) the forearm and (B) the hand showing the arrangement of the layers of the flexor muscles. The superficial layer is shaded horizontally, the second layer vertically, the third obliquely to the left, the fourth vertically, and the fifth obliquely to the right. AbM, abductor digiti quinti; AdP, adductor pollicis; BR, brachio-radialis; ECD, extensor digitorum communis; ECU, extensor carpi ulnaris;Â£Z, extensor indicis; EMD, extensor digiti quinti; EMP, abductor pollicis longus; ERB, extensor carpi radialis brevis; FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; FLP, flexor pollicis longus; FM, flexor digiti quinti brevis; FP, flexor digitorum profundus; FS, flexor digitorum sublimis; ID, interossei dorsales; IV, interossei volares; L, lumbricales; OM, opponens digiti quinti; PL, palmaris longus; PT, pronator teres; R, radius; U, ulna; II-V, second to fifth metacarpal.

which pass to the four ulnar digits. Gradually superficial portions of the antibrachial flexor mass separate off, carrying with them the layers of the palmar aponeurosis from which the tendons representing the superficial layer of the palmar muscles arise, and they form with these the flexor digitorum sublimis. The deeper layers of the antibrachial flexor mass become the flexor digitorum profundus and the flexor pollicis longus (Fig. 127, A), and retain their connection with the deeper layers of the palmar aponeurosis which form their tendons; and since the second layer of the palmar muscles takes origin from this portion of the aponeurosis it becomes the lumbrical muscles, arising from the profundus tendons (Fig. 127,

Fig. 128. - Transverse sections through (A) the crus and (B) the foot, showing the arrangement of the layers of the flexor muscles. The shading has the same significance as in the preceding figure. AbH, abductor hallucis; AbM, abductor minimi digiti; AdH, adductor hallucis; ELD, extensor longus digitorum; F, fibula; FBD, flexor brevis digitorium; FBH, flexor brevis hallucis; FBM, flexor brevis minimi digiti; FLD, flexor longus digitorum; G, gastrocnemius; ID, interossei dorsalis; IV, interossei ventrales; L, lumbricales; P, plantaris; Pe, peroneus longus; Po, popliteus; S, soleus; T, tibia; TA, tibialis anticus; TP, tibialis posticus; I-V, first to fifth metatarsal.

B). The third layer of palmar muscles becomes the adductors of the digits, reduced in man to the adductor pollicis, while from the two deepest layers the interossei are developed. Of these the fourth layer consists primarily of a pair of slips corresponding to each digit, while the fifth is represented by a series of muscles which extend obliquely across between adjacent metacarpals. With these last muscles certain of the fourth layer slips unite to form the dorsal interossei, while the rest become the volar interossei. j The modifications of the almost identical primary arrangement in the crus and foot are somewhat different. The superficial layer of the crural flexors becomes the gastrocnemius and plantaris (Fig. 128, A) and the deepest layer becomes the popliteus and the interosseous membrane. The second and third layers unite to form a common mass which is inserted into the deeper layers of the plantar aponeurosis and later differentiates into the soleus and the long digital flexor, the former shifting its insertion from the plantar aponeurosis to the os calcis, while the flexor retains its connection with the deeper layers of the aponeurosis, these separating from the superficial layer to form the long flexor tendons. The fourth layer partly assumes a longitudinal direction and becomes the tibialis posterior and the flexor hallucis longus and partly retains its original cblique direction and its connection with the deep layers of the plantar aponeurosis, becoming the quadratus plantse. In the foot (Fig. 128, B) the superficial layer persists as muscular tissue, forming the abductors, the flexor digitorum brevis and the medial head of the flexor hallucis brevis, the second layer becomes the lumbricales, and the third the lateral head of the flexor hallucis brevis and the adductor hallucis, while the fourth and fifth layers together form the ioterossei, as in the hand, the flexor quinti digiti brevis really belonging to that group of muscles.

Literature

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L. Bolk: " Rekonstruktion der Segmentirung der Gliedmassenmuskulatur dargelegt an den Muskeln des Oberschenkels und des Schultergurtels," Morphol. Jahrbuch, xxii, 1895.

L. Bolk: "Die Sklerozonie des Humerus," Morphol. Jahrbuch, xxill, 1S96.

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W. H. Lewis: " The Development of the Arm in Man," Amer. Jour, of Anat., 1, 1902

J. B. MacCallum: "On the Histology and Histogenesis of the Heart Muscle-cell," Anat. Anzeiger, xiil, 1897.

J. B. MacCallum: "On the Histogenesis of the Striated Muscle-fiber and the Growth of the Human Sartorius Muscle," Johns Hopkins Hospital Bulletin, 1898

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Caroline McGill: "The Histogenesis of Smooth Muscle in the Alimentary Canal and Respiratory Tract of the Pig," Internat. Monatschr. Anat. und Phys., xxiv, 1907.

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J. B. Sutton: "Ligaments, their Nature and Morphology," London, 1897.

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