tots 



CHAPTER VIII 

 THE NERVOUS TISSUES 



THE nervous tissues include those tissue elements which are 

 peculiar to the nervous system. The essential unit of structure, 

 comparable to the cell of other tissues, is here the neurone. A 

 neurone is a nerve cell in the broadest sense of the term. It con- 

 sists of the cell body (nerve cell of the older writers, perikaryon), 

 together with all of its processes. These latter are divisible into 

 two varieties, the neuraxone and ihe.dendrites. 



The neurones 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 naked 

 eye. Their processes, usually of considerable number, vary in 

 length from a millimeter 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 circum- 

 stance renders it advisable to retain the term nerve fibre of the 

 older writers to designate, not as was the former conception, a his- 

 tological entity, but rather that portion of those long processes of 

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

 grey matter of the central nervous system. 



On this basis we may divide the neurone into the nerve cell 

 and the nerve fibre. The former term includes the cell body with 

 its dendrites and the proximal portion of its neuraxone, the distal 

 portion of the neuraxone forming the essential part of a long 

 nerve fibre. The nerve cells are found throughout the grey mat- 

 ter of the central nervous system and in the ganglia of the pe- 

 ripheral cerebro-spinal and sympathetic systems. Nerve fibres 

 occur in the white matter of the central nervous system and in 

 the nerve trunks and ganglia of the peripheral system. 



In the peripheral nervous system the nervous tissues are chiefly 

 supported by the connective tissues, but in the central nervous 



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