CHAPTER XXII.

THE MUSCULAR SYSTEM.

In all the Coelenterata, except the Ctenophora, the contractile elements of the body wall consist of filiform processes of ectodermal or entodermal epithelial cells (figs. 375 and 376 B). The elements provided with these processes, which were first discovered by Kleinenberg, are known as myoepithelial cells. Their contractile parts may either be striated (fig. 376) or non-striated (fig. 375). In some instances the epithelial part of the cell may nearly abort, its nucleus alone remaining (fig. 376 A); and in this way a layer of muscles lying completely below the surface may be established.

[FIG. 375. MYOEPITHELIAL CELLS OF HYDRA. (From Gegenbaur; after Kleinenberg.)

_m._ contractile fibres.]

There is embryological evidence of the derivation of the voluntary muscular system of a large number of types from myoepithelial cells of this kind. The more important of these groups are the Chætopoda, the Gephyrea, the Chætognatha, the Nematoda, and the Vertebrata[242].

[242] If recent statements of Metschnikoff are to be trusted, the Echinodermata must be added to these groups. The amoeboid cells stated in the first volume of this treatise to form the muscles in this group, on the authority of Selenka, give rise, according to Metschnikoff, only to the cutis, while the same naturalist states the epithelial cells of the vaso-peritoneal vesicles are provided with muscular tails.

While there is clear evidence that the muscular system of a large number of types is composed of cells which had their origin in myoepithelial cells, the mode of evolution of the muscular system of other types is still very obscure. The muscles may arise in the embryo from amoeboid or indifferent cells, and the Hertwigs[243] hold that in many of these instances the muscles have also phylogenetically taken their origin from indifferent connective-tissue cells. The subject is however beset with very serious difficulties, and to discuss it here would carry me too far into the region of pure histology.

[243] O. and R. Hertwig, _Die Coelomtheorie._ Jena, 1881.

_The voluntary muscular system of the Chordata._

The muscular fibres. The muscular elements of the Chordata undoubtedly belong to the myoepithelial type. The embryonic muscle-cells are at first simple epithelial cells, but soon become spindle-shaped: part of their protoplasm becomes differentiated into longitudinally placed striated muscular fibrils, while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular fibrils are either placed at one side of the epithelial part of the cell, or in other instances (the Lamprey, the Newt, the Sturgeon, the Rabbit) surround it. The latter arrangement is shewn for the Sturgeon in fig. 57.

[FIG. 376. MUSCLE-CELLS OF LIZZIA KÖLLIKERI. (From Lankester; after O. and R. Hertwig.)

A. Muscle-cell from the circular fibres of the subumbrella. B. Myoepithelial cells from the base of a tentacle.]

The number of the fibrils of each cell gradually increases, and the protoplasm diminishes, so that eventually only the nucleus, or nuclei resulting from its division, are left. The products of each cell probably give rise, in conjunction with a further division of the nucleus, to a primitive bundle, which, except in Amphioxus, Petromyzon, etc., is surrounded by a special investment of sarcolemma.

The voluntary muscular system. For the purposes of description the muscular system of the Vertebrata may conveniently be divided into two sections, viz. that of the head and that of the trunk. The main part, if not the whole, of the muscular system of the trunk is derived from certain structures, known as the muscle-plates, which take their origin from part of the primitive mesoblastic somites.

[FIG. 377. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E.

_nc._ neural canal; _pr._ posterior root of spinal nerve; _x._ subnotochordal rod; _ao._ aorta; _sc._ somatic mesoblast; _sp._ splanchnic mesoblast; _mp._ muscle-plate; _mp´._ portion of muscle-plate converted into muscle; _Vr._ portion of the vertebral plate which will give rise to the vertebral bodies; _al._ alimentary tract.]

It has already been stated (pp. 292-296) that the mesoblastic somites are derived from the dorsal segmented part of the primitive mesoblastic plates. Since the history of these bodies is presented in its simplest form in Elasmobranchii it will be convenient to commence with this group. Each somite is composed of two layers--a somatic and a splanchnic--both formed of a single row of columnar cells. Between these two layers is a cavity, which is at first directly continuous with the general body cavity, of which indeed it merely forms a specialised part (fig. 377). Before long the cavity becomes however completely constricted off from the permanent body cavity.

Very early (fig. 377) the inner or splanchnic wall of the somites loses its simple constitution, owing to the middle part of it undergoing peculiar changes. The meaning of the changes is at once shewn by longitudinal horizontal sections, which prove (fig. 378) that the cells in this situation (_mp´_) have become extended in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a somite. The inner layer of each somite, immediately within the muscle-band just described, begins to proliferate, and produce a mass of cells, placed between the muscles and the notochord (_Vr_). These cells form the commencing vertebral bodies, and have at first (fig. 378) the same segmentation as the somites from which they sprang.

After the separation of the vertebral bodies from the somites the remaining parts of the somites may be called muscle-plates; since they become directly converted into the whole voluntary muscular system of the trunk (fig. 379, _mp_).

According to the statements of Bambeke and Götte, the Amphibians present some noticeable peculiarities in the development of their muscular system, in that such distinct muscle-plates as those of other vertebrate types are not developed. Each side-plate of mesoblast is divided into a somatic and a splanchnic layer, continuous throughout the vertebral and parietal portions of the plate. The vertebral portions (somites) of the plates soon become separated from the parietal, and form independent masses of cells constituted of two layers, which were originally continuous with the somatic and splanchnic layers of the parietal plates (fig. 79). The outer or somatic layer of the vertebral plates is formed of a single row of cells, but the inner or splanchnic layer is made up of a kernel of cells on the side of the somatic layer and an inner layer. The kernel of the splanchnic layer and the outer or somatic layer together correspond to a muscle-plate of other Vertebrata, and exhibit a similar segmentation.

Osseous Fishes are stated to agree with Amphibians in the development of their somites and muscular systems[244], but further observations on this point are required.

[244] Ehrlich, "Ueber den peripher. Theil d. Urwirbel." _Archiv f. mikr. Anat._, Vol. XI.

[FIG. 378. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF SCYLLIUM CONSIDERABLY YOUNGER THAN 28 F.

_ch._ notochord; _ep._ epiblast; _Vr._ rudiment of vertebral body; _mp._ muscle-plate; _mp´._ portion of muscle-plate already differentiated into longitudinal muscles.]

In Birds the horizontal splitting of the mesoblast extends at first to the dorsal summit of the mesoblastic plates, but after the isolation of the somites the split between the somatic and splanchnic layers becomes to a large extent obliterated, though in the anterior somites it appears in part to persist. The somites on the second day, as seen in a transverse section (fig. 115, _P.v._), are somewhat quadrilateral in form but broader than they are deep.

Each at that time consists of a somewhat thick cortex of radiating rather granular columnar cells, enclosing a small kernel of spherical cells. They are not, as may be seen in the above figure, completely separated from the ventral (or lateral as they are at this period) parts of the mesoblastic plate, and the dorsal and outer layer of the cortex of the somites is continuous with the somatic layer of mesoblast, the remainder of the cortex, with the central kernel, being continuous with the splanchnic layer. Towards the end of the second and beginning of the third day the upper and outer layer of the cortex, together probably with some of the central cells of the kernel, becomes separated off as a muscle-plate (fig. 116). The muscle-plate when formed (fig. 117) is found to consist of two layers, an inner and an outer, which enclose between them an almost obliterated central cavity; and no sooner is the muscle-plate formed than the middle portion of the inner layer becomes converted into longitudinal muscles. The avian muscle-plates have, in fact, precisely the same constitution as those of Elasmobranchii. The central space is clearly a remnant of the _vertebral portion of the body cavity_, which, though it wholly or partially disappears in a previous stage, reappears again on the formation of the muscle-plate.

The remainder of the somite, after the formation of the muscle-plate, is of very considerable bulk; the cells of the cortex belonging to it lose their distinctive characters, and the major part of it becomes the vertebral rudiment.

In Mammalia the history appears to be generally the same as in Elasmobranchii. The split which gives rise to the body cavity is continued to the dorsal summit of the mesoblastic plates, and the dorsal portions of the plates with their contained cavities become divided into somites, and are then separated off from the ventral. The later development of the somites has not been worked out with the requisite care, but it would seem that they form somewhat cubical bodies in which all trace of the primitive slit is lost. The further development resembles that in Birds.

The first changes of the mesoblastic somites and the formation of the muscle-plates do not, according to existing statements, take place on quite the same type throughout the Vertebrata, yet the comparison which has been instituted between Elasmobranchs and other Vertebrates appears to prove that there are important common features in their development, which may be regarded as primitive, and as having been inherited from the ancestors of Vertebrates. These features are (1) the extension of the body cavity into the vertebral plates, and subsequent enclosure of this cavity between the two layers of the muscle-plates; (2) the primitive division of the vertebral plate into an outer (somatic) and an inner (splanchnic) layer, and the formation of a large part of the voluntary muscular system out of the inner layer, which in all cases is converted into muscles earlier than the outer layer.

The conversion of the muscle-plates into muscles. It will be convenient to commence this subject with a description of the changes which take place in such a simple type as that of the Elasmobranchii.

[FIG. 379. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F.

_sp.c._ spinal canal; _W._ white matter of spinal cord; _pr._ posterior nerve-roots; _ch._ notochord; _x._ subnotochordal rod; _ao._ aorta; _mp._ muscle-plate; _mp´._ inner layer of muscle-plate already converted into muscles; _Vr._ rudiment of vertebral body; _st._ segmental tube; _sd._ segmental duct; _sp.v._ spiral valve; _v._ subintestinal vein; _p.o._ primitive generative cells.]

At the time when the muscle-plates have become independent structures they form flat two-layered oblong bodies enclosing a slit-like central cavity (fig. 379, _mp_). The outer or somatic wall is formed of simple epithelial-like cells. The inner or splanchnic wall has however a somewhat complicated structure. It is composed dorsally and ventrally of a columnar epithelium, but in its middle portion of the muscle-cells previously spoken of. Between these and the central cavity of the plates the epithelium forming the remainder of the layer commences to insert itself; so that between the first-formed muscle and the cavity of the muscle-plate there appears a thin layer of cells, not however continuous throughout.

When first formed the muscle-plates, as viewed from the exterior, have nearly straight edges; soon however they become bent in the middle, so that the edges have an obtusely angular form, the apex of the angle being directed forwards. They are so arranged that the anterior edge of the one plate fits into the posterior edge of the one in front. In the lines of junction between the plates layers of connective-tissue cells appear, which form the commencements of the intermuscular septa.

The growth of the plates is very rapid, and their upper ends soon extend to the summit of the neural canal, and their lower ones nearly meet in the median ventral line. The original band of muscles, whose growth at first is very slow, now increases with great rapidity, and forms the nucleus of the whole voluntary muscular system (fig. 380, _mp´_). It extends upwards and downwards by the continuous conversion of fresh cells of the splanchnic layer into muscle-cells. At the same time it grows rapidly in thickness by the addition of fresh spindle-shaped muscle-cells from the _somatic layer_ as well as by the division of the already existing cells.

_Thus both layers of the muscle plate are concerned in forming the great longitudinal lateral muscles, though the splanchnic layer is converted into muscles very much sooner than the somatic_[245].

[245] The brothers Hertwig have recently maintained that only the inner layer of the muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demonstrate the incorrectness of this view, and in Acipenser (_vide_ fig. 57, _mp_) the two layers of the muscle-plate retain their original relations after the cells of both of them have become converted into muscles.

Each muscle-plate is at first a continuous structure, extending from the dorsal to the ventral surface, but after a time it becomes divided by a layer of connective tissue, which becomes developed nearly on a level with the lateral line, into a dorso-lateral and a ventro-lateral section. The ends of the muscle-plates continue for a long time to be formed of undifferentiated columnar cells. The complicated outlines of the intermuscular septa become gradually established during the later stages of development, causing the well-known appearances of the muscles in transverse sections, which require no special notice here.

The muscles of the limbs. The limb muscles are formed in Elasmobranchii, coincidently with the cartilaginous skeleton, as two bands of longitudinal fibres on the dorsal and ventral surfaces of the limbs (fig. 346). The cells, from which these muscles originate, are derived from the muscle-plates. When the ends of the muscle-plates reach the level of the limbs they bend outwards and enter the tissue of the limbs (fig. 380). Small portions of several muscle-plates (_m.pl_) come in this way to be situated within the limbs, and are very soon segmented off from the remainder of the muscle-plates. The portions of the muscle-plates thus introduced soon lose their original distinctness. There can however be but little doubt that they supply the tissue for the muscles of the limbs. The muscle-plates themselves, after giving off buds to the limbs, grow downwards, and soon cease to shew any trace of having given off these buds.

[FIG. 380. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.

The section is diagrammatic in so far that the anterior nerve-roots have been inserted for the whole length; whereas they join the spinal cord halfway between two posterior roots.

_sp.c._ spinal cord; _sp.g._ ganglion of posterior root; _ar._ anterior root; _dn._ dorsally directed nerve springing from posterior root; _mp._ muscle-plate; _mp´._ part of muscle-plate already converted into muscles; _m.pl._ part of muscle-plate which gives rise to the muscles of the limbs; _nl._ nervus lateralis; _ao._ aorta; _ch._ notochord; _sy.g._ sympathetic ganglion; _ca.v._ cardinal vein; _sp.n._ spinal nerve; _sd._ segmental (archinephric) duct; _st._ segmental tube; _du._ duodenum; _pan._ pancreas; _hp.d._ point of junction of hepatic duct with duodenum; _umc._ umbilical canal.]

In addition to the longitudinal muscles of the trunk just described, which are generally characteristic of Fishes, there is found in Amphioxus a peculiar transverse abdominal muscle, extending from the mouth to the abdominal pore, the origin of which has not been made out.

It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish-like stage. The middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. That these muscle-plates give rise to the main muscular system of the trunk, at any rate to the episkeletal muscles of Huxley, is practically certain, but the details of the process have not been made out.

In the Perennibranchiata the fish-like arrangement of muscles is retained through life in the tail and in the dorso-lateral parts of the trunk. In the tail of the Amniotic Vertebrata the primitive arrangement is also more or less retained, and the same holds good for the dorso-lateral trunk muscles of the Lacertilia. In the other Amniota and the Anura the dorso-lateral muscles have become divided up into a series of separate muscles, which are arranged in two main layers. It is probable that the intercostal muscles belong to the same group as the dorso-lateral muscles.

The abdominal muscles of the trunk, even in the lowest Amphibia, exhibit a division into several layers. The recti abdominis are the least altered part of this system, and usually retain indications of the primitive intermuscular septa, which in many Amphibia and Lacertilia are also to some extent preserved in the other abdominal muscles.

In the Amniotic Vertebrates there is formed underneath the vertebral column and the transverse processes a system of muscles, forming part of the hyposkeletal system of Huxley, and called by Gegenbaur the subvertebral muscles. The development of this system has not been worked out, but on the whole I am inclined to believe that it is derived from the muscle-plates. Kölliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin.

Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out.

It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kölliker however believes that they have an independent origin.

The limb-muscles, both extrinsic and intrinsic, as may be concluded from their development in Elasmobranchii, are derived from the muscle-plates. Kleinenberg found in Lacertilia a growth of the muscle-plates into the limbs, and in Amphibia Götte finds that the outer layer of the muscle-plates gives rise to the muscles of the limbs.

In the higher Vertebrata on the other hand the entrance of the muscle-plates into the limbs has not been made out (Kölliker). It seems therefore probable that by an embryological modification, of which instances are so frequent, the cells which give rise to the muscles of the limbs in the higher Vertebrata can no longer be traced into a direct connection with the muscle-plates.

_The Somites and muscular system of the head._

The extension of the somites to the anterior end of the body in Amphioxus clearly proves that somites, similar to those of the trunk, were originally present in a region, which in the higher Vertebrata has become differentiated into the head. In the adult condition no true Vertebrate exhibits indications of such somites, but in the embryos of several of the lower Vertebrata structures have been found, which are probably equivalent to the somites of the trunk: they have been frequently alluded to in the previous chapters of this volume. These structures have been most fully worked out in Elasmobranchii.

The mesoblast in Elasmobranch embryos becomes first split into somatic and splanchnic layers in the region of the head; and between these layers there are formed two cavities, one on each side, which end in front opposite the blind anterior extremity of the alimentary canal; and are continuous behind with the general body-cavity (fig. 20 A, _vp_). I propose calling them the head-cavities. The cavities of the two sides have no communication with each other.

Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the cleft; and at a later period it becomes, owing to the formation of a second cleft, divided into three sections: (1) a section in front of the first or hyomandibular cleft; (2) a section in the hyoid arch between the hyomandibular cleft and the hyobranchial or first branchial cleft; (3) a section behind the first branchial cleft.

The front section of the head-cavity grows forward, and soon becomes divided, without the intervention of a visceral cleft, into an anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth involution. The posterior part lies completely within the mandibular arch.

As the rudiments of the successive visceral clefts are formed, the posterior part of the head-cavity becomes divided into successive sections, there being one section for each arch. Thus the whole head-cavity becomes on each side divided into (1) a premandibular section; (2) a mandibular section (_vide_ fig. 29 A, _pp_); (3) a hyoid section; (4) sections in each of the branchial arches.

[FIG. 381. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO.

The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the premandibular and mandibular head-cavities 1_pp_ and 2_pp_, etc. The section is moreover somewhat oblique from side to side.

_fb._ fore-brain; _l._ lens of eye; _m._ mouth; _pt._ upper end of mouth, forming pituitary involution; 1_ao._ mandibular aortic arch; 1_pp._ and 2_pp._ first and second head-cavities; 1_vc._ first visceral cleft; _V._ fifth nerve; _aun._ auditory nerve; _VII._ seventh nerve; _aa._ dorsal aorta; _acv._ anterior cardinal vein; _ch._ notochord.]

The first of these divisions forms a space of a considerable size, with epithelial walls of somewhat short columnar cells (fig. 381, 1_pp_). It is situated close to the eye, and presents a rounded or sometimes a triangular figure in section. The two halves of the cavity are prolonged ventralwards, and meet below the base of the fore-brain. The connection between them appears to last for a considerable time. These two cavities are the only parts of the body-cavity within the head which unite ventrally. The section of the head-cavity just described is so similar to the remaining sections that it must be considered as serially homologous with them.

The next division of the head-cavity, which from its position may be called the mandibular cavity, presents a spatulate shape, being dilated dorsally, and produced ventrally into a long thin process parallel to the hyomandibular gill-cleft (fig. 20, _pp_). Like the previous space it is lined by a short columnar epithelium.

[FIG. 382. HORIZONTAL SECTION THROUGH THE PENULTIMATE VISCERAL ARCH OF AN EMBRYO OF PRISTIURUS.

_ep._ epiblast; _vc._ pouch of hypoblast which will form the walls of a visceral cleft; _pp._ segment of body-cavity in visceral arch; _aa._ aortic arch.]

The mandibular aortic arch is situated close to its inner side (fig. 381, 2_pp_). After becoming separated from the lower part (Marshall), the upper part of the cavity atrophies about the time of the appearance of the external gills. Its lower part also becomes much narrowed, but its walls of columnar cells persist. The outer or somatic wall becomes very thin indeed, the splanchnic wall, on the other hand, thickens and forms a layer of several rows of elongated cells. In each of the remaining arches there is a segment of the original body-cavity fundamentally similar to that in the mandibular arch (fig. 382). A dorsal dilated portion appears, however, to be present in the third or hyoid section alone (fig. 20), and even there disappears very soon, after being segmented off from the lower part (Marshall). The cavities in the posterior parts of the head become much reduced like those in its anterior part, though at rather a later period.

It has been shewn that the divisions of the body-cavity in the head, with the exception of the anterior, early become atrophied, _not so however their walls_. The cells forming the walls both of the dorsal and ventral sections of these cavities become elongated, and finally become converted into muscles. Their exact history has not been followed in its details, but they almost unquestionably become the musculus constrictor superficialis and musculus interbranchialis[246]; and probably also musculus levator mandibuli and other muscles of the front part of the head.

[246] _Vide_ Vetter, "Die Kiemen und Kiefermusculatur d. Fische." _Jenaische Zeitschrift_, Vol. VII.

The anterior cavity close to the eye remains unaltered much longer than the remaining cavities.

Its further history is very interesting. In my original account of this cavity (No. 292, p. 208) I stated my belief that its walls gave rise to the eye-muscles, and the history of this process has been to some extent worked out by Marshall in his important memoir (No. 509).

Marshall finds that the ventral portion of this cavity, where its two halves meet, becomes separated from the remainder. The eventual fate of this part has not however been followed. Each dorsal section acquires a cup-like form, investing the posterior and inner surface of the eye. The cells of its outer wall subsequently give rise to three sets of muscles. The middle of these, partly also derived from the inner walls of the cup, becomes the rectus internus of the eye, the dorsal set forms the rectus superior, and the ventral the rectus inferior. The obliquus inferior appears also to be in part developed from the walls of this cavity.

Marshall brings evidence to shew that the rectus externus (as might be anticipated from its nerve supply) has no connection with the walls of the premandibular head-cavity, and finds that it arises close to the position originally occupied by the second and third cavities. Marshall has not satisfactorily made out the mode of development of the obliquus superior.

The walls of the cavities, whose history has just been recorded, have definite relations with the cranial nerves, an account of which has already been given at p. 461.

Head-cavities, in the main similar to those of Elasmobranchii, have been found in the embryo of Petromyzon (fig. 45, _hc_), the Newt (Osborn and Scott), and various Reptilia (Parker).

BIBLIOGRAPHY.

(507) G. M. Humphry. "Muscles in Vertebrate Animals." _Journ. of Anat. and Phys._, Vol. VI. 1872.

(508) J. Müller. "Vergleichende Anatomie d. Myxinoiden. Part I. Osteologie u. Myologie." _Akad. Wiss._, Berlin, 1834.

(509) A. M. Marshall. "On the head cavities and associated nerves of Elasmobranchs." _Quart. J. of Micr. Science_, Vol. XXI. 1881.

(510) A. Schneider. "Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere." _Sitz. d. Oberhessischen Gesellschaft_, 1873.

(511) A. Schneider. _Beiträge z. vergleich. Anat. u. Entwick. d. Wirbelthiere._ Berlin, 1879.

_Vide_ also Götte (No. 296), Kölliker (No. 298), Balfour (No. 292), Huxley, etc.