NERVOUS SYSTEM— GENERAL CONSIDERATIONS. 2H 



{BRAIN ABOVE MEDULLA | 

 6EN80RV CENTRE'"V f" ") tv':/ /-" INHIBITORY CENTRE 



SENSORY CELL AND 

 AFFERENT NERVE 



MOTOR CELL AND 

 EFFERENT NERVE 



Fig. 187. — Diagram intended to illustrate nervous mechanism of— 1, automatism; 2, 

 refiex action; and 3, how nervous impulses in the latter case may pass into the 

 higher parts of brain and become part of consciousness, or be wholly inhibited. 

 A reflex or automatic center may, for the sake of simplicity, be reduced to a sin- 

 gle cell, as above on the left. 



pending chiefly on the condition of the central nervous sys- 

 tem. 2. The movements of the muscles harmonize (are co-ordi- 

 nated), and tend to accomplish some end — are purposive. If 

 the nerve alone and not the skin be stimulated, there may be a 

 spasm only and not adaptive movement. 3. Nervous impulses, 

 when very abundant, may pass along unaccustomed or less ac- 

 customed paths (experiments 4 and 5). This is sometimes spoken 

 of as the radiation of nervous impulses. 



The sixth experiment is very important, for it shows that 

 the result varies far more with the condition of the nervous 

 centers (cells) than the stimulus, the part excited, or any other 

 factor. 



Aiitomatisin. — But, seeing that these central cells have such 

 independence and controlling power, the question arises, Are 



