The Muscles of Sauropsida

The most important deviations from the ordinary arrangement of the muscular system occur, as might be expected, in the Ophidia, in the Chelonia, and in Aves. In the first-mentioned group, the numerous muscles of the limbs are, of course, absent, and the mobility of the vertebrae, ribs, and jaws, is accompanied by a corresponding differentiation of the muscles of those parts. The episkeletal muscles form a continuous series (divisible into spinalis, semispinalis, longissimus dorsi, levatores costarum, and other muscles) from the end of the tail to the head; and, in the region of the back, constitute a thick mass which extends outward to the ends of the caudal ribs (the so-called transverse processes), and over the dorsal thirds of the other ribs. Beyond these points it is continued, as a thinner layer of muscular fibres, over the ventral half of the tail and trunk, passing from rib to rib in the latter region, where the more dorsal fibres are directed obliquely, only a longitudinal band running along the extremities of the ribs and representing a rectus abdominis. This muscle is continued forward to the hyoidean apparatus, and thence to the mandible. Superficial muscular bundles pass from the ribs to the scales. The hyposkeletal muscles are better developed than in most other Vertebrat, and also extend from the head to the end of the tail. A median dorsal set are connected with the subvertebral processes in the trunk, and with the bases of the representatives of the chevron-bones in the tail, and pass to the caudal and dorsal ribs. One set of these, in the trunk, act as retractors of the ribs. The muscles which correspond with the transversus abdominis commence in the tail by transversely-directed bundles of fibres, which arise from the roots of the caudal ribs (transverse processes), and meet in a median aponeurosis. In the trunk, similar bundles arise from the under surfaces of the ribs, and form two layers of oblique fibres, which also meet in the middle line.

In the Chelonia, the episkeletal muscles are always weakly developed, and may be altogether abortive in the dorsal region, while those of the abdominal walls are small. The recti are very weak, but muscles answering to the pyramidales extend from the pubes to the inner surface of the plastron. A muscular expansion analogous to a diaphragm may be attached to the bodies and ribs of the third and fourth dorsal vertebrae, whence it expands over the surface of the lungs. No muscles pass from the head to the shoulder-girdle. The pectoral arch is protracted, and the neck retracted, by a muscle attached to the cervical vertebrae and to the procoracoid. There is also a single retractor of the pectoral arch, apparently representing a serratus magnus, and passing from the first costal plate to the scapula. The pectoralis major arises from the inner surface of the plastron. The representative of the latissimus dorsi arises from the inner side of the first costal plate.

The cutaneous muscles of birds are well developed, and form broad expansions in various parts of the body. Special bundles of muscular fibres pass to the great quill-feathers of the tail and wings, and others to the patagium, a fold of integument which extends between the trunk and the brachium behind, and between the brachium and ante-brachium in front. In correspondence with the slight mobility of the dorsal vertebrae, the episkeletal and hyposkeletal muscles of the spine attain a considerable development only in the neck and in the tail. Owing to the great size of the sternum, the abdominal muscles are usually small, and the internal oblique may be absent. A diaphragm, consisting of bundles of muscular fibres, which pass from the ribs to the aponeurosis covering the ventral face of the lungs, is developed in all birds, but attains the greatest degree of completeness in the Ratitae, and especially in Apteryx.

The muscles of the limbs are remarkably modified by the excessive development of some of those found in other Vertebrata, and the suppression of others.

Thus in all birds possessing the power of flight, the pectoralis major, as the chief agent of the downward stroke of the wing, is very large and thick, taking its origin from the whole length, and a great part of the depth, of the keel of the sternum.

The elevation of the wing is chiefly effected by the pectoralis tertius, which arises beneath the foregoing muscle, and passes over the inner side of the scapulocoracoid articulation, as over a pulley, to reach the humerus. The muscles of the forearm and digits are reduced, in accordance with the peculiar modification of the skeleton of these parts. In the hindlimb of most birds there is a singular extensor muscle, which arises from the pubis, ends in a tendon which passes to the outer side of the knee-joint, and terminates in the leg by uniting with the flexor digitorum perforatus. The result of this arrangement is, that the toes are flexed whenever the leg is bent upon the thigh, and, consequently, the roosting bird is held fast upon his perch by the weight of his own body.