206 HISTORY OF THE HUMAN BODY 



triangular prismatic spaces located on either side of the mid- 

 dorsal line, between the spines of the vertebrae and the trans- 

 verse processes and ribs ; they are, however, far more complex 

 in structure, and are characterized by the presence of almost 

 numberless tendons that repeat one another metamerically and 

 are attached to corresponding parts of successive vertebrae or 

 ribs. The muscular elements are also less differentiated from 

 one another than in other parts of the system, and in many 

 cases the subdivision into separate muscles is an arbitrary one. 



But little has been done with the muscles of this group from 

 the standpoint of comparative anatomy, and thus their mor- 

 phological history cannot as yet be attempted. It must suf- 

 fice here to treat them to a rapid review as they exist in man, 

 with some attempt at a morphological arrangement of their 

 several elements, after which may be considered what is known 

 of their history. 



In man the back is covered superficially with several layers 

 of muscles which are dorsal in a topographical sense only, 

 having secondarily invaded this territory from other places 

 of origin. The most superficial of these, trapezius and latissi- 

 mus dorsi, form a thin covering over almost the entire back, 

 effectually concealing all beneath them. These belong to the 

 appendicular system and appear in the urodeles as small, fan- 

 shaped muscles which extend dorsally from shoulder and 

 humerus respectively, and cover but a small portion of the 

 trunk myotomes. With the gradual increase in the size and 

 importance of the limbs which they supply they have enlarged 

 to the extent found in man and in most of the mammals. The 

 rhomboidei, lying beneath these and of less extent, likewise 

 belong to the appendicular group. Still deeper, beneath all 

 of the appendicular muscles, are the two serrati posterior es, the 

 remains of a continuous sheet in certain primitive mammals. 

 Their innervation from ventral branches of the spinal nerves 

 shows that they, too, are originally strangers to the dorsal 

 region and belong rather with the ventral axial muscles, under 

 which head they have already received treatment. 



The removal of all the above exposes the genuine dorsal 

 axial muscles, the derivatives of the trunk myotomes, the 



