208 



THE NERVOUS SYSTEM 



nervous activities which will 

 be discussed later. Nerve cells 

 (gray matter) are found on the 

 inside of the cord and nerve 

 fibers (white matter) are found 

 on the outside. The spinal 

 cord is called the first level of 

 the central nervous svstem. 



Cellular structure of the 

 central nervous system. The 

 unit of structure of nervous 

 tissue is a highly specialized 

 cell called a neuron. It con- 

 tains the cell body proper, the 

 cyton, from which fine proto- 

 plasmic processes (dendrites) extend. One of the processes may 

 extend a great length and is known as the axon. The dendrites 

 divide and with branches from other nerve cells form a mass of 

 extremely fine fibers. The gray matter of the brain and spinal 

 cord consists of nerve cells or neurons. Collections of these are 



A photomicrograph of the spinal cord shows 

 the white matter on the outside, and the gray 

 matter in the form of anterior and posterior 

 horns, on the inside. 



cLenSlrit^e^ 



nerve centers. A 

 number of axons 

 banded together 

 constitute a nerve. 

 Nerves transmit 

 messages from sense 

 organs such as the 

 eve, to the cell 

 bodies of the neu- 

 rons, or from the 

 cell bodies of the 



neurons to muscles when the dendrites of two neurons come into contact with 



, J _. . each other, they form synapses. Nerve impulses travel from 



or glands. in order axon to dendrite across one of the many junctions or synapses. 



