XI REFLEX ACTION 487 



impulse of an afferent nerve, or rather, perhaps, a group of 

 such independent nervous centres. 



In all these reflex actions of the spinal cord, the 

 structures necessary for their performance are, as already 

 pointed out (p. 340), a sensory surface, an afferent nerve, 

 a portion of the grey matter of the cord, an efferent 

 nerve, and a muscle or group of muscles. In the case of the 

 headless frog, the actions are of course quite involuntary, 

 and performed unconsciously, and the same remark holds 

 good in the case of a man whose spinal cord is so injured 

 as to be practically cut in two. But even in an uninjured 

 healthy man, similar reflex actions, although now under the 

 control of the will, are strikingly manifest, and play an 

 important part in his everj'day life. Thus the act of walk- 

 ing, though started by the will, is subsequently a reflex 

 action. When engaged in conversation or buried in 

 thought, a person walks with all his ordinary dexterity, 

 but in entire unconsciousness of the action. In this case 

 the afferent impulses are largely started from the stimula- 

 tion of the skin of the feet and legs which results from 

 the varying pressure and contact with the ground. Hence 

 the staggering gait in cases where, as a result of disease, the 

 chain of structures requisite for the liberation of the 

 reflexes is broken, as for instance by disease of the 

 posterior (afferent) roots of the spinal nerves. In such cases 

 walking is frequently possible only as the result of looh'uKj 

 at the ground (see p. 390) ; this accords with the fact that 

 even in health afferent impulses started in the sensory 

 surface (retina) of the eye play an important part in 

 giving rise to the reflexes of walking. But, on the other 

 hand, blind persons walk with no little dexterity. 



Again, the actions of micturition and defalcation are 

 really reflex actions carried out by the spinal cord, as 

 soon as tliey have been started by tlie will ; here the 

 sensory surfaces are the mucous membrane of the bladder 

 or rectum, the necessary stimulus being supplied as the re- 

 sult of their distension by the accumulated urine or faeces. 



