526 



THE NERVOUS SYSTEM 



path from the cerebral cortex to the cell body of the peripheral 

 neurone. 



The peripheral or primary sensory neurones all follow a homol- 

 ogous path. Their cell body lies in the dorsal root ganglion of a 



spinal nerve or in the 

 cerebro-spinal ganglion 

 of a cranial nerve (e.g., 

 jugular, petrosal, and 

 cochlear ganglia). The 

 nerve cells of these 

 ganglia possess a single 

 process which imme- 

 diately divides in a Y- 

 or T-like manner into 

 a peripheral and a cen- 

 tral branch. 



The peripheral 

 process enters the dis- 

 tal part of the poste- 

 rior spinal nerve root 

 and follows the course 

 of the cerebro-spinal 

 nerve to its termina- 

 tion at the peripheral 

 end organ. The cen- 

 tral process enters the 

 proximal part of the 

 nerve root and thus 

 reaches the spinal cord 

 or brain. Once within the central nervous system it divides by a 

 Y or T branch into a short and a long process, the longer being 

 always directed toward the medulla oblongata. Hence in the spinal 

 nerves the long process passes cephalad, the short caudad. The 

 reverse occurs in the cranial nerves, the long process is caudad in 

 direction and the short is cephalad. 



Both long and short processes end according to the same plan, 

 yet their difference in length results in the appearance of long as- 

 cending and short descending peripheral neurone tracts in the 

 spinal cord while in the brain the corresponding descending tracts 

 are the longer. 



The peripheral neurones of the cranial nerves end by arboriza- 



-d.r. 



FlG. 405. A GANGLION OF A DORSAL NERVE ROOT OF 

 A MOUSE, AS SHOWN BY THE METHOD OF GOLGI. 



d. r, dorsal root ; n. per, peripheral spinal nerve ; v. r, 

 ventral root. (After van Gehuchten.) 



