REFLEX ACTIONS. 109 



No questions are more difficult of solution; but 

 while protesting against a tendency to undervalue 

 all life below that of man, we must remember we 

 have in our bodies processes going on which are 

 not the result of volition, as when the blood circu- 

 lates, and its particles arrange themselves in the 

 pattern required to form our tissues and organs, 

 and also that many of our actions belong to the 

 class termed by physiologists, "reflex," that is, 

 he result of external impressions upon the nervous 

 system, in which the sentient brain takes no part. 

 Thus when a strong light stimulates the optic 

 nerve, the portion of brain with which it is connected 

 in its turn stimulates the iris to contract the pupil; 

 and it is supposed that after a man has begun to 

 walk, through the exercise of his will, he may continue 

 to walk, by a reflex action; as his feet press the 

 ground they transmit an impression to the spinal cord, 

 and the legs receive a fresh impulse to locomotion, al- 

 though the mind is completely occupied with other 

 business, and pays no attention to their proceedings."^ 

 The ordinary movements of insects appear to be 

 of this character, and to be excited by the ganglia 

 belonging to the segment to which the moving limbs 

 are attached. Thus a centipede will run, after its 

 head has been cut off, and a water-beetle (Dytiscus) 

 swam energetically when thrown into water after 



* See Carpenter's "Manual of Physiology." 



