MUSCULAR SYSTEM. 13! 



different groups. Thus the spinales connect the spinous processes, the 

 transversales the transverse processes of the successive vertebrae, 

 while the transverse -spinales extend from the transverse process of 

 one vertebra to the spinous process of the next. In the higher verte- 

 brates the anterior spinalis, connecting the first vertebra with the skull, 

 is divided into several rectus capitis muscles. The longissimus 

 dorsi group extends from the pelvis to the head, lying on either side in 

 the angle between spinous and transverse processes. It may be differen- 

 tiated into separate muscles a longissimus dorsi proper in the lumbar 

 region, an ileo-costalis inserted on the dorsal part of the ribs, and a 

 longissimus capitis along the side of the neck to the temporal region 

 of the skull. 



The muscles which move the appendages are divided into the 

 intrinsic, which are located in the limb itself and have their origin 

 either from the bones of the limb or from the supporting girdle, and the 

 extrinsic, which have their origin on the trunk and are inserted on the 

 girdle or the base of the limb. The latter move the limb as a whole, 



FIG. 139. Budding of muscles of appendage from myotomes in Pristiurus, after Rabl 



b, muscle buds; my, myotomes. 



while the intrinsic bend the limb on itself. As would be expected from 

 the motions of the fins, the intrinsic muscles are hardly noticeable in the 

 fishes, the various movements being accomplished by the extrinsic 

 group. These latter are divided into protractors which draw the 

 member forward; retractors which pull it back against the body; 

 levators which lift it and depressors which pull it down. 



In those vertebrates which are sufficiently known the first traces of the develop- 

 ment of the musculature of the appendages are the appearance of two buds (fig. 

 139) from the ventral border of a varying number of myotomes in the region of the 

 developing limb. These buds proliferate cords of cells which soon lose their 

 distinctness and form a blastema from which the intrinsic muscles arise, the 

 definitive muscles being innervated by as many spinal nerves as there are contribut- 

 ing myotomes. The extrinsic muscles arise directly from the myotomes. 



