THE NERVOUS SYSTEM. 167 



of the same side may be made to jerk, or even to kick with 

 considerable force ; but, unless the patient is observing his 

 limbs, he is wholly unconscious of these movements, which 

 are, therefore, performed independently of the brain. And 

 they are in nowise due to the muscles of the limb; for, 

 if the cord itself becomes diseased below the point of in- 

 jury, the muscles cease to contract. 



39. For the production of this form of nervous action 

 three things are requisite (1) a nerve to conduct messa- 

 ges from the surface of the body, one of that variety for- 

 merly described as sensory, but which are now incapable of 

 awakening sensation ; (2) a portion of uninjured spinal 

 cord which shall reflect or convert impressions into im- 

 pulses ; and (3) a motor nerve to conduct impulses outward 

 to the muscles. The power of the cord to enforce reflex 

 acts resides in the gray matter, into which the reflex nerves 

 enter and from which they depart, by means of their pos- 

 terior and anterior roots respectively. 



40. The Uses of the Reflex Action. The reflex 

 activity of the cord is exhibited in the healthy body in 

 many ways, but since it is never accompanied with sensa- 

 tion, we do not readily recognize it in our own bodies. 

 Reflex movements are best studied in the cases of other 

 persons, when the conditions enable us to distinguish 

 between acts that are consciously, and those that are un- 

 consciously performed. For example, if the foot of a per- 

 son soundly, asleep be tickled or pinched, it will be quickly 

 withdrawn from the irritation. 



41. Similar movements may be observed in cases where 

 the consciousness and sensation are temporarily obliter- 

 ated by disease, or by means of narcotic poisons. If the 

 arm of a person who has been rendered insensible by 



39. What arc lh" requisites for the production of this form of nervous action? 



40. Whv rlo we not readily recognize the reflex activity of the cord in our own 

 bodies ? How best studied in others ? Example ? 



4 1 . Similar movements ? Arm of a oersou ? Melted wax or heated coin on the 

 hand? 



