202 HISTORY OF THE HUMAN BODY 



with a powerful retractor system, lying on the ventral side 

 of the vertebral centra and employed in drawing back the 

 head and neck. This is undoubtedly homologous with the 

 prevertebral muscles of the cervical and thoracic region in 

 mammals (longus colli, etc.), and belongs with the ventral 

 division, innerved by ventral branches of the spinal nerves. 

 In birds, correlated with the lack of mobility of the trunk 

 vertebrae, the corresponding muscles are greatly reduced, but 

 as this loss of motion is not a complete one as in the turtle, so 

 also is the reduction of the muscles not as extreme. The 

 muscles of neck and tail, on the other hand, are extremely well 

 developed, thus emphasizing by contrast the almost rudimen- 

 tary condition of the muscles of the back. 



In taking up the differentiation of the axial muscles more 

 in detail, their division into dorsal and ventral masses must 

 be emphasized, for, although both are derived embryologically 

 from the epimeres, that is, the originally dorsal portions of 

 the mesodermic somites, yet the distinction is of fundamental 

 importance topographically and morphologically, because they 

 are innerved respectively by the dorsal and ventral branches of 

 the spinal nerves, a criterion which may always be relied upon 

 to help out the homology in doubtful cases, where a muscle 

 lies near the boundary between them or where an extreme de- 

 gree of differentiation has changed the primary location. 

 These divisions may be taken up in order, beginning with the 

 ventral, which are less complicated than the other. 



Throughout the course of these ventral axial muscles four 

 distinct regions may be distinguished, and the same layer, when 

 it is possible to trace it from one of these regions to the other, 

 becomes modified to partake of the features of the muscula- 

 ture characteristic of each. These are the' cervical, thoracic, 

 abdominal and caudal, the three former continuous, the lat- 

 ter more or less separated by the interposition of the hip- 

 girdle and the cloaca. In the abdominal region, even in fishes, 

 there is seen a tendency for the musculature to break up into 

 layers, each with a definite direction of fibers, distinct from 

 that of the others. In amphibians these layers consist pri- 



