ARRANGEMENT OF THE NERVOUS SYSTEM 563 



The Internal Characteristics of the Neuron. ^ — The maturing of 

 the nerve-cell necessitates several changes. First, we have the 

 estabhshment of the polarity of the cell, i.e., the neuroblast sends 

 out an axon, which is soon followed by the formation of dendrites. 

 In some cases, these processes then become medullated or are en- 

 veloped solely b}^ neurolemma; or both. While these alterations in 



'" i: 





f 



Ai 



Fig. 279. — Cell from the Anterior Horn of the Spinal Cord of a Rabbit, 



Showing Nissl's Bodies. 



Ax, Axon; D, dendrite; K, nucleus; N, nucleolus. (Klopsch.) 



the configuration of the neuron are being completed, the cytoplasm 

 of the cell-body becomes more highly differentiated, presenting finally 

 the following details: 



(1) A well marked nucleus and nucleolus surrounded by a relatively thick layer 

 of cytoplasm; (2) flake-like masses of a complex protein substance chemically 



^ Nerve-cells were first recognized by Ehrenberg, in 1833, in the spinal ganglia 

 of the frog. In 1838, Remak established the fact that nerve fibers are prolongations 

 of the cell-bodies. This observation was made upon the sympathetic fibers of 

 mvertebrates. It was found to hold true in mammals by Helmholtz and Hanover 

 (1842). In 1863 the observations of Deiters were published which showed that 

 the cells of the central nervous system possess two kinds of processes, namely, 

 protoplasmic prolongations and a real fiber process. Gerlach (1871), Golgi (1873) 

 and Ramon Y. Cajal (1888), furnished additional data regarding the structure 

 of the neuron. 



