MUSCLES OF THE SAUEOPSIDA. 257 



aponeurosis. In the trunk, similar bundles arise from tlie 

 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 mus- 

 cular 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 magnMS, and passing from the first costal plate to 

 the scapula. The pectoralis major arises from the inner sur- 

 face of the plastron. The representative of the latissimus dor- 

 si 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 in- 

 tegument which extends between the trunk and the brachi- 

 um behind, and between the brachium and ante-brachium in 

 front. In correspondence with the slight mobility of the dor- 

 sal vertebrae, the episkeletal and hjrposkeletal 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 ab- 

 dominal muscles are usually small, and the internal oblique 

 may .be absent. A diaphragm, consisting of bundles of mus- 

 cular fibres, which pass from the ribs to the aponeurosis cover- 

 ing the ventral face of the lungs, is developed in all birds, but 

 attains the greatest degree of completeness in the JRatitce, and 

 especially in Apteryx. 



The muscles of the limbs are remarkably modified by the 

 excessive development of some of those found in other Verte- 

 bi'ata, and the suppression of others. 



Thus in all birds possessing the power of flight, the pec- 

 toralis 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. 



