THE NERVOUS SYSTEM 



521 



into the dorsal column of gray matter, and come into synaptic connexions 

 with dendrites of the same or of the opposite side. 



The fibers which enter the spinal cord by the dorsal root are somatic 

 afferent or visceral afferent, depending upon their peripheral connexions. 



Fig. 434. — Bifurcation of the dorsal root fibers within the spinal cord into ascending 

 and descending branches, which in turn give off collaterals; the termination of some of 

 these collaterals in synaptic relation to cells of the posterior gray column. (From 

 Ranson's "The Anatomy of the Nervous System," Courtesy of W. B. Saunders 

 Company.) 



The visceral motor fibers, which appear in the dorsal roots of lower 

 vertebrates, are not found in the dorsal spinal nerves of mammals. 



The neurites of the ventral roots are purely efferent. Of these there 

 are two kinds, somatic motor and visceral motor. The somatic innervate 

 skeletal muscles derived from the mesodermal somites, the visceral are 

 connected through the sympathetic ganglia with visceral involuntary 



