THE MUSCULAR SYSTEM 215 



crnosus, etc., which, like certain of the pharyngeal muscles, are 

 derivations of the muscular layer of the intestinal wall, and 

 belong primarily to the involuntary system. 



Although, as previously stated, the morphological history of 

 the dorsal trunk muscles is almost unknown, there are yet a 

 few points of interest which may prove suggestive. Among 

 the amphibians there is a suggestion of a longitudinal sub- 

 division of this mass into a medial and a lateral portion, a 

 change which in the reptiles becomes complete and definite. In 

 the mammals this condition is still evident, with but a few sec- 

 ondary modifications which tend to obscure the plan somewhat. 

 Thus the spino-spinalis and transverso-spinalis systems, the 

 latter with all of its subdivisions, belong to the medial portion, 

 while the sacro-transverso-transversalis system, on the other 

 hand, with its two main subdivisions of ilio-costalis and longis- 

 simns, may be referred to the lateral portion. Of these two 

 systems it may be said in general that the separate slips of the 

 medial portion are inclined towards the median line and become 

 inserted into spinous processes, while those of the lateral 

 portion are inclined outwards (laterally) and become inserted 

 either into transverse processes or into ribs, the two being 

 genetically the same. The most important exception to this is 

 seen in the longissimns, which does not indeed violate the 

 principle just laid down, but which possesses a few origins 

 from spinous processes, and hence from the median line ; these, 

 however, have been proven to be of secondary origin, arising 

 through an association of a part of the longissimns with the 

 spinalis, a relation that is not established until during a fairly 

 late embryonic period. The only other doubtful case is that of 

 the splenius, which arises from spinous processes as though 

 belonging to the medial portion, but inserts in part laterally. 

 This muscle, however, is found only in mammals, and may 

 thus be considered to have developed long after the division 

 into the two portions had become established. A similar rela- 

 tionship in the obliqmts capitis, which, although belonging to 

 the medial portion, has its origin from a spinous process and its 

 insertion into a transverse process, has been explained above as 



