THE SOMATIC MOTOR DIVISION. igi 



opposite the middle of the myotomes, pass through the membranous 

 skeleton and divide into dorsal and ventral rami. The rami of 

 each nerve run upon the inner face of the myotome to which 

 they belong and twigs from them penetrate the myotome to end 

 in relation with the muscle fibers. These ventral nerves make 

 no connection with the dorsal nerves, which lie hi the spaces 

 between each two myotomes. In all higher classes each ventral 

 nerve unites with an adjacent dorsal nerve to form a composite 

 structure called a spinal nerve. The union takes place at or near 

 the distal end of the ganglion of the dorsal nerve and the composite 

 nerve immediately divides into dorsal and ventral rami. At 



R. 



ven. arcuate fibers 



FIG. 102. A diagrammatic representation of the somatic motor components of 

 a trunk segment. 



about the same point, or from the ventral ramus, the ramus com- 

 municans is given off to the sympathetic ganglion. (See p. 200.) 

 In urodeles (Bardeen) where the trunk muscles have the same 

 simple segmental arrangement as in fishes, the spinal nerve lies 

 in a myoseptum between two muscle segments. The sensory 

 fibers are distributed to the skin both before and behind this 

 septum, and the motor fibers enter both the adjacent muscle 

 segments and innervate the muscle fibers at their ends. Each 

 nerve therefore helps to innervate two muscle segments and any 

 muscle fiber may be innervated from two spinal nerves. These 

 facts seem to have an important bearing on the question of met- 

 amerism in the vertebrate body and also upon the problems of 



