300 THE ANATOMY OF VEETEBRATED ANIMALS. 



correspond witli tlie 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 pyrami- 

 dales 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 

 foui'th 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 j^edoralis major arises from the inner 

 surface of the plastron. The representative of the latis- 

 simus 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 jpatagium, a fold of in- 

 tegument 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 vertebrse, 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 



