THE NERVOUS SYSTEM 13 



ently dependent entirely upon the direct stim- 

 ulation of muscle. 



In the adult human body there is at least 

 one muscle which, like the embryonic heart, 

 is open normally to a certain degree of direct 

 stimulation. This is the sphincter pupillae by 

 means of which the pupil of the eye is con- 

 stricted. This muscle is ordinarily under the 

 control of the oculomotor nerve, and it re- 

 sponds reflexly by contracting when the ret- 

 ina is brightly illuminated. But it can also 

 be made to contract by direct stimulation. If, 

 in a blind person, a strong beam of light is 

 thrown through the pupil on to the retina, 

 but without illuminating the iris, the muscle 

 does not shorten, thus showing that the or- 

 dinary reflex arc is inoperative. If, now, the 

 same beam is directed against the iris, in 

 which the muscle is embedded, a contraction 

 follows, thus giving evidence of the direct 

 stimulation of the muscle. Certain parts of N 

 the musculature of our bodies, then, exhibit 

 forms of response that are more primitive than 

 the reflex and that indicate something of the 

 probable nature of the beginnings of neuro- 

 muscular activity. 



If the embryonic vertebrate heart illustrates 



