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 pupillaB 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 

 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- 

 mu scalar activity. 



If the embryonic vertebrate heart illustrates 



