432 



STUDIES IN ADVANCED PHYSIOLOGY. 



others reaching from the spinal cord to the ends of the 

 extremities. Anatomically neurons are classed on the basis 

 of the number of nerves issuing from the cell body. A 

 nervous cell with a single nerve fiber issuing from it is 

 spoken of as mono-neuric. These mono-neuric cells are, 

 however, physiologically really di-neuric, the single nerve 

 containing passages leading towards the cell and away from 



it, and so being practically 

 the same as two separate 

 nerves. The usual type is 

 the di-neuric neuron. 



More than two nerves, 

 however, may arise, in 

 which case the neuron is 

 spoken of as poly-neuric. 

 Illustrations of di - neuric 

 neurons may be found in 

 such ganglia as the spinal 

 root ganglia, in which a 

 nerve runs to each cell body 

 and a second nerve away 

 from it. Usually one of the 

 fibers or extensions of the 

 cell body becomes a typi- 

 cal nerve, while the other 

 branches sub-divide repeat- 

 edly and form a perfect 

 network of smaller fibrils 

 reaching out in various di- 

 rections. Such branched terminations are spoken of from 

 their resemblance to trees as dendrons. These dendrons do 

 not reach to distant points like the nerve does to skin, mus- 

 cles or sense-organs, but seem to terminate among the neigh- 

 boring cells and it is believed that impulses from one cell to 

 another are carried by dendrons of these contiguous nerves. 

 A neuron, therefore, consists essentially of two parts; 

 the cell body and its extensions into nerves and dendrons. 



Figf. 138. A POLY-NEURIC GANGLION CELL. 



(After Gegenbaur.) 



Dotted line indicates the main axis-cylin- 

 der. 



