CHAPTER XIV. 



THE ANATOMY AND PHYSIOLOGY OF THE NERVOUS 



SYSTEM. 



ANATOMY OF THE NERVOUS SYSTEM (EXCEPT THE 

 CEREBELLUM).i 



STRUCTURE OF NERVE=TISSUE. 



NERVE-TISSUES present themselves in two varieties: some as 

 white substance and some as gray substance. These two substances 

 are different, not only in color, but also in physical and chemical 

 properties and in anatomical arrangement. 



The gray substance contains as characteristic elements the nerve- 

 cells; the white substance, the nerve-fibers. These latter emerge from 

 the gray nervous substance to branch out toward the peripheral 

 organs. These two substances, gray and white, possess a common ele- 

 ment known as neuroglia; in addition, each contains blood-vessels. 



The Nerve-cell, or Neuron. The nerve-cell is the characteristic 

 fundamental element of the gray substance: it is an independent 

 unit of the nervous system. It is the element which gives to this 

 kind of nervous tissue its gray color. When these units are charged 

 with a strong portion of pigment, they are black, as in the locus niger 

 of the cerebral peduncles. When a little less pigmented they pre- 

 sent a grayish color: the color that is characteristic of <the brain 

 and the central portion of the spinal cord. They may be charged 

 with red pigment, then the cells are reddish; such cells constitute 

 the red nucleus of the head of the cerebral crura. 



STRUCTURE OF TPIE NERVE-CELL. The nerve-cell is composed 

 of (1) a mass of protoplasm inclosing a nucleus with its nucleolus; 

 (2) of simple or branched prolongations. The protoplasm of a 

 nerve-cell, like that of many other cells, is formed of a very delicate 

 network of bands whose meshes are filled with a clear or finely granu- 

 lar albuminoid substance. The network has been designated by the 

 name of spongioplasm and the intermediate substance is generally 

 termed hyaloplasm. As to these two components the protoplasm of 

 nerve-cells is like that of most other cells. 



Fibrils. One peculiarity is the presence in it of fibrils which 

 run through its substance. 

 (596) 



