GENERAL ZOOLOGY 



Fig. 4.9. Different regions of 

 the central nervous system, in 

 cross section, to show the dis- 

 tribution of white (clear) and 

 gray (dotted) matter; dia- 

 grammatic. .4, spinal cord; 

 fi, medulla; C, cerebral 

 hemispheres. 



The cells of the nervous system that are directly concerned with its function 

 of coordination are known as neurons (p. 68). The neurons are arranged in 

 such a way that the parts containing the nuclei are located in groups which 

 constitute the gray matter (Fig. 4.9) of the central nervous system and the 

 ganglia of the peripheral nervous system. The processes which are always 

 continuous with the main part of the cytosome around the nucleus are called 

 nerve fibers, and bundles of them make up the white matter of the central 

 nervous system and the nerves of the peripheral system. 



Many neurons are arranged in what are known as reflex arcs; the reflex arc 

 is the cellular mechanism of reflex action. In a reflex arc the ends of the 

 terminal filaments of the axonic process of one neuron (see Fig. 4.22) come 

 in contact with a dendritic process or the cell body of another neuron, but 

 there is no structural continuity between the cells. Such places of contact 

 between different neurons are known as synapses and make possible the func- 

 tional continuity of the nervous system. In the simplest reflex arc there may 

 be only two neurons involved (Fig. 4.10.4). The stimulus is received by some 

 specialized group of cells constituting a receptor, which is a general term for 

 any type of sense organ. As a result of the reception of the stimulus, what 

 is known as a nerve impulse is established; this is conducted from the place of 

 stimulation toward the central nervous system along a nerve-cell process 

 (p. 68). In the simplest reflex arc the impulse will be conducted to the 



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