The most typical condition of the amphibian musculature is shown in Urodela. The dorsal lateral muscles retain most nearly the fish-like condition and lie in metameres. Mien 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 they 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 Annra differ from the Urodela chiefly in the absence of the transversal abdomi nis, and in the diminution or disappearance of myocomata in the adult.
In reptiles, as in Amphibia, the dorsal muscula ture retains more nearly the primitive condition. With the ossification of the skeletogenous and the consequent attachment of the muscles to the bone, has come about the differentiation of the so-called interspinales. semispinales. inter transversals, longissimus dorsi, and others. The last-named muscle is to he 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 vertebrr. The ventral musculature has become still more complicated. due. according to Wiedersheini, to the changed rhythmic method 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 muscles wide)] perform
this function have become more powerful in that the ribs have become buried in them. The muscu lature of successive myomeres, which projects outside or inside the ribs, becomes united into a single sheet. \Vali the disappearance of the ribs in the lumbar region of reptiles a modification of the intercostals must take place. A eontinuous muscle is developed between the last rib and the pelvis.
The phylogeny of many of the muscles of mam mals is not known. The more important muscles 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 fibres. The involuntary muscles are unstriated. The muscle fibre of vertebrate muscle. like that of inver tebrates, is funned by the differentiation 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 difTeren tiated portion of vertebrate muscle fibre is com posed of fibrillie and forms the main bulk of the cell. The fibrilla' are imbedded in the undiffer entiatcd part, which also surrounds them in the form of a sheath and contains the nueiei.
BIBLIOGRAPHY. Parker and Haswell, TextBibliography. Parker and Haswell, Text- Book of ZoOlogy (London, 1897) ; A. Lang. Text Book of Comparatire Anatomy, parts i. and ii.
(ib., 1891-96) ; Wiedersheim, Compara tire Anato my of Vertebrates, English adaptation, by W. N.
Parker (ib., 1S97) ; Gegenbaur; Harmer (ed itor), Cambridge Natural History, vols. i.-x. (ib., 1895-1902).