THE NERVOUS SYSTEM OF MAN 1 55 



animals but nerve cells are the only structures that 

 scientists have ever found which are related to 

 nervous energy. These cells (Fig. 53) are so 

 unlike other cells that an amateur is often per- 

 plexed to recognize them. A little study, how- 

 ever, reveals that nearly all nerve cells have one or 

 more special outgrowths or processes, in some In- 

 stances of enormous length, that serve to connect 

 distant parts of the body with nervous centers. 

 Such cells grow and require nourishment in just 

 the same sense as the muscle cells or skin cells. 

 The technical term, neuron, is given to nerve cells 

 which are to be regarded as existing as morpho- 

 logical units, as being nourished as trophic units 

 and reacting as a chain of physiological units. 

 The outgrowths from the cell body of a neuron 

 are known either as an axon or as dendrites. It is 

 generally agreed by neurologists that a stimulus 

 enters a neuron over a dendrite and leaves over 

 the axon, so that the neuron can be said to have 

 polarity. The diameter of the axon is very 

 minute, but because of its frequent great length 

 the volume of the axon may be 187 times that of 

 the cell body of the neuron. Dendrites are wider, 

 especially close to the cell body, and branch ir- 

 regularly, while the axon maintains a uniform 

 diameter and branches in a regular manner. 



One or two other fundamental facts in regard 

 to nerve cells help us to understand something of 



