PHYSIOLOGY OF THE NERVOUS SYSTEM 309 



occur at the time, or even a little before the time, that he 

 realizes what has happened. If a feather is brought in 

 contact with the more sensitive parts of the face of a 

 sleeping person, there is a twitching of the skin and some- 

 times a movement of the hand to remove the offending 

 substance. Surgeons operating upon patients completely 

 under the influence of chloroform, and therefore com- 

 pletely unconscious, have observed strong reflex actions. 

 These and other similar cases indicate clearly that reflex 

 action occurs independently of the mind that the mind 

 neither causes nor controls it. If a further proof of this 

 fact were needed, it is supplied by experiments upon cer- 

 tain of the lower animals, 1 which live for a while after the 

 removal of the brain. These experiments show that the 

 nervous impulses that produce reflex action need only pass 

 through the spinal cord and do not reach the cerebrum, the 

 organ of the mind. 



The Reflex Action Pathway. By study of the impulses 

 that produce any reflex action, a rather definite pathway 

 may be made out, having the following divisions : 



I . From the surface of the body to the central nervous sys- 

 tem (usually the spinal cord). This, the afferent division, is 

 made up of di-axonic neurons, and these have (in the case of 

 the spinal nerves) their cell-bodies in the dorsal root ganglia 

 (page 295). They are acted upon by external stimuli, while 

 their impulses in turn act on the neurons in the spinal cord. 



l A frog from which the brain has been removed is suspended with its feet 

 downward and free to move. If a toe is pinched, the foot is drawn away, and 

 if dilute acid, or a strong solution of salt, is placed on the tender skin, the feet are 

 moved as if to take away the irritating substance. This of course shows that reflex 

 action can take place independently of the brain. 



Now if the spinal cord is also destroyed, there is no response when the irritation 

 of the skin is repeated. The animal remains perfectly quiet, because the destruc- 

 tion of the cord has interrupted the reflex action pathway. This shows that some 

 part of the central nervous system is necessary to reflex action. 



