Page:The New International Encyclopædia 1st ed. v. 14.djvu/192

* MXJSCULAK SYSTEM. 156 MUSCULAK SYSTEM. itself is sepaiatod into injomeres. The abdomen is bent upward, downward, or sidewise by the contraction of the nninde fibres in the corre- sponilinf; portion of the body. The appendajres are moved by muscles that pass out into them from the trunk. The muscles are either attached to the cuticular outer coverin;; or to inward pro- jecting parts of it. and often terminate in sinewy pieces that are wliite and are composed of bundles of cross-striated libres — the kind of fibre that characterizes all rapidly moving muscle. In Peripatus. however, the muscles are unstripped, save those which work the jaws. In insects, the thorax and metathorax, as in the case of the grassho])per, are crowded with leg and wing mus- cles. In the cockroach, where the wings are little used, the w ing muscles are poorly developed. The nuisculature of .Vmphioxus.likc that of the abdomen of Crustacea, is divided into metaniercs. Of these muscular segments or myomeres there are about si.xty, V-shaped on surface view, with the apex of the V extending forward. The myo- meres are composed of striated muscle fibres which extend longitudinally and are attached to the septsr immediately in front and behind them. The myomeres of the two sides alternate. The nuisculature of the dorsal wall is the thicker. In cydostomes, as well as in Amphioxus, the muscles of any myomere are attached to the anterior and posterior myocomata of that seg- ment. In selachians a significant change has occurred — the first step toward the complicated conditions of higher vertebrates. Certain of the ventral lateral muscles on either side of the median line become dillercntiated from the others, in that their fibres move out from the general level in correspondence with their greater functional activity. Thus wliilc the dorsal trunk muscles retain their primitive character, the ventral muscles have become sei)arated into two masses: a pair of strong mid-ventral ones which are known together as the rectus al)dominis, and the remaining muscular nuiss. We next find that in this latter mass the direction of the muscle fibres, which in .mphioxus and cydostomes is luiriznntal, has become in selachians oblitpu*: so that, from the mid-ventral line, they pass dorso- posteriorly. The remaining Ic'^s difierentiated portion of the ventral musculature is known as oblifpius abdominis, or oblicpu- abdominal muscle. Two kin<ls of changc-s have now occurred in the vertebrate nuisculature: (1) There is the func- tional difTcrentiation of parts of uuisdcs by which they become cut nlf from the remainder, and (2) changes in the direction of muscle fibres by which originally axial muscles have become oblique. In the lowest vertebrates the anterior and pos- terior ends of nniseles are attached to the ad- jacent myocomata. With the development of ribs from the fibrous tissue of the myocomata, muscular attachment has, in |)art. fallen upon them. The derma, indeed, is dillerentiated from the same tissue as the myocomata: hence it is not surprising that, in the migrations on the ends of muscles to adajit themselves to the new con- ditions, we find that some nniseles are attached to the derma. Finally, some miiMlcs. whose end* lome thus to lie outside of the myomeres, may extend alongside of several somites without signs of metamerism. Thus, by these simple modifica- tions, we gain the complex musculature of the higher vertebrates, where there are trunk mus- cles of different sizes and lengths, often without metamerism and attached to bony parts such as ribs, or to the skin. In Amphioxus the myomeres are arranged with reference to the sagittal plane frcnn which they extend obliquely outward and backward. In cydostomes the obliquity is so great that the myomeres overlap each other like the shingles on a roof, so that a cross-section of the body may cut three or four myocomata on each side. In many fishes the free edge of the myocomata, as it appears at the surface, is not merely boned, but is zigzag. The most typical condition of the amphibian musculature is shown in Urodela. The dorsal lateral muscles retain most nearly the fisli-like condition and lie in nietameres. When the ribs and transverse processes of the vertebra- become developed some of the muscles gain attachment to them, and thus give rise to the intercostal and, intertransversal muscles. The ventral lateral muscles of fishes are disposed in two regions. In Amphibia tbey comprise four regions,* derived from a migration and splitting of the two lateral muscles of fishes. In addition, lying under the internal oblique, there is a muscle that prob- ably has no representative in fishes. It is un- segmented and lies deeper than the skeleton and immediately within the peritoneal lining of the body cavity. The. ura diflTer from the Urodela chiefly in the absence of the transversal abdomi- nis, and in the diminution or disappearance of myocomata in the adult. i In reptiles, as in Amphibia, the dorsal muscula- ' ture retains more nearly the primitive condition. With the ossification of the skeletogenous tissue and the consequent attachment of the nuisdes to the bone, has come about the differentiation of the so-called interspinales. senns])inales, inter- transversals, longissimus dorsi. and others. The last-named muscle is to be regarded as a part of the dorsal musculature that has lost its meta- merism, and now passes from the pelvic girdle to several of the spines of the thoracic vertebrsB. The ventral musculature has become still more complicated, due. according to Wiedersheim, to the changed rhythmic metliod of respiration, con- trolled through the more and more greatly devel- oped lungs. To meet the needs of respiration the ribs must be moved. The nniseles which perform this function have become more powerful in that the ribs have become buried in them. The muscu- lature of successive myomeres, which ])roject» outside or inside the ribs, becomes united into a single sheet. With the disappearance of the ribs in the lumliar region of reptiles a modificatiim of the intercostals must take place. A continuous muscle is developed between the last rib and the pelvis. Die phylogeny of many of the muscles of mam- mals is not known. The more important musclea resemble those of reptiles. Two kinds of muscular fibres are found in the vertebrate body. All the quickly moving volun- tary muscles are composed of striated filues. The involuntary muscles are unstriated. The muscle fibre of vertebrate muscle, like that of inver- tebrates, is formed by the ditferentiatinn of a muscle cell. Only in the muscle cell of the lower invertebrates a much smaller portion of the cell is transformed into fibre structure than is the ease in vertebrate muscle fibre. The differen- tiated portion of vertebrate mus<'le fibri' is com- posed of fibrillip and forms the main bulk of the cell. The fibrilla' are imbedded in the umliffer-