THE NERVE FIBER 131 



According to the length of their axons, neurons are divided by Golgi 

 into two types. 



1. Golgi cells, Type I (Deiters' cells). 



2. Golgi cells, Type II (Golgi's cells). 



The cells of Type I possess a long axon which passes beyond the con- 

 fines of the gray matter in which it arises and usually becomes the axis 

 cylinder of a nerve fiber. 



The cells of Type II possess a short axon which forms its terminal 

 arborization in the vicinity of its parent cell-body. The cells of this type 

 are usually association and commissural neurons; they place in conduc- 

 tion relation other not very remote neurons. The cells of Type I, on the 

 other hand, are more frequently projection neurons ; they are distributed 

 from the nerve centers to other and perhaps very different tissues, their 

 courses lying in the long projection tracts and nerve trunks of the 

 nervous system. 



The cells of Type II are therefore most frequently intrinsic or endog- 

 enous neurons, their whole course lying in one division of the central 

 nervous system, e.g., the gray matter of the spinal cord. The cells of 

 Type I are more frequently extrinsic or exogenous; they arise in one 

 part of the nervous system to be distributed to a distant portion, e.g., 

 they arise in the peripheral ganglia and enter the spinal cord to terminate 

 in its gray matter, or vice versa. 



The size of a nerve cell is thought to bear a general relation to the 

 length of its axon, the larger cells possessing the longer axons. The 

 cells of Golgi's Type I are therefore larger than those of Type II. Like- 

 wise the cells of the motor tracts, whose axons are as a rule much longer 

 than those of the sensory tracts, are characterized by their large size 

 as compared witli the sensory cells. 



THE NERVE FIBER 



The origin of the nerve fiber and its relation to the other portions 

 of the neuron will be appreciated by tracing the course of the axou of a 

 motor nerve cell of the ventral horn of gray matter in the spinal cord. 

 This process, arising in the central gray matter, is at first a naked axon. 

 It soon leaves the gray matter to traverse the white matter and makes its 

 exit from the spinal cord as the axis cylinder of one of the fibers of 

 a ventral nerve root. On leaving the gray matter the axon acquires a 



