CHAPTER XXI 

 ANATOMICAL DESCRIPTION 



406. Composition of the Nervous System. The nervous 

 system is made up of nerve centers, nerves, and peripheral 

 end organs. These, though really constituting a single 

 system for the whole of man's organism, are commonly 

 described in two separate groups, or systems: the cerebro- 

 spinal or central and the sympathetic or ganglionic. 



The nerve centers are the brain and spinal cord (often 

 called the cerebro -spinal axis), and little knots of nervous 

 matter found in different parts of the body, along the 

 course of the nerves, called ganglia. All nerve fibers 

 arise in the nerve centers. 



407. Nervous Elements. We have learned that nerv- 

 ous tissue exists in two forms : gray matter, which is 

 almost wholly composed of nerve cells, and white matter, 

 which is almost wholly composed of nerve fibers. Nerve 

 fibers and nerve cells constitute the nervous elements. 



408. The Nerve Cell. Nerve cells are microscopic, irreg- 

 ular bits of protoplasm like other cells. Each contains 

 a large nucleus, within which is a nucleolus, and usually 

 each cell sends off one or more fine branches, or proc- 

 esses (Fig. 122). Sometimes these are so numerous 

 as to give the cell a stellate appearance. Nerve cells 

 are found only in the central nervous system, in the 



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