CHAPTER V 

 NERVOUS TISSUES 



GENERAL CONSIDERATIONS 



Nervous tissue comprehends those tissue elements which are peculiar 

 to the nervous system. In the protoplasm of nervous tissue proper 

 (neuroplasm) the fundamental properties of irritability and conductivity 

 have become predominant. The nervous system includes the cerebro- 

 spinal comprising the central (brain and spinal cord) and the periph- 

 eral (cerebral and spinal nerves) portions and the sympathetic divi- 

 sions. For convenience we may speak also of the central and peripheral 

 nervous systems, the latter including the sympathetic division. The 

 essential unit of structure, comparable to the cell of other tissues, is 

 here the neuron, or neurocyte. A neuron is a nerve cell in the broadest 

 sense of the term. It consists of the cell body (nerve cell of the older 

 writers, cyton), together with all of its processes. These latter are 

 divisible into two varieties, the axon and the dendrons (dendrites). 



The neurons are among the largest cells of the body. Their cell body 

 is of variable size, in some cases extremely minute, at other times 

 sufficiently large to be readily observed with the unaided eye. Their 

 processes, usually of considerable number, vary in length from a milli- 

 meter or less, up to half the height of man. It is therefore obviously 

 impossible to study microscopically at one time the entire course of 

 these longer processes. This circumstance renders it advisable to retain 

 the term nerve fiber of the older writers to designate, not as was the 

 former conception, a histological entity, but rather that portion of those 

 long processes of the nerve cell which pursues its course, as a rule, 

 outside of the gray matter of the central portion of the cerebrospinal 

 division. 



On this basis we may divide the neuron into the nerve cell and the 

 nerve fiber. The former term includes the cell body, or cyton, with its 

 dendrons and the proximal portion of its axon; the distal portion of 

 the axon forming the essential part of a long nerve fiber. The nerve cells 



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