THE NERVOUS SYSTEM 227 



to terminate all sensation and voluntary motion in the lower 

 half of the body, the patient seeming lifeless and powerless from 

 the waist downward. And yet, by tickling or pinching either 

 foot, the leg of the same side may be made to jerk, or even 

 to kick with considerable force ; but, unless the patient is ob- 

 serving 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 become diseased below the point of injury, 

 the muscles cease to contract. 



39. For the production of this form of nervous action, three 

 things are requisite (1) a nerve to conduct messages from 

 the surface of the body, one of that variety formerly described 

 as sensory, but which are now known to be incapable of awaken- 

 ing sensation; (2) a portion of uninjured spinal cord, which 

 shall reflect or convert impressions into impulses ; 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 posterior 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 sensation, we do not readily 

 recognize it in our own bodies. Reflex movements are best 

 studied in the cases of other persons, when the conditions en- 

 able us to distinguish between acts that are consciously, and 

 those that are unconsciously performed. For example, if the 

 foot of a person 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 be quickly withdrawn from the irritation. 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 obliterated by 

 disease, or by means of narcotic poisons. If the arm of a per- 

 son who has been rendered insensible by chloroform be raised, 



39. What are the requisites for the production of this form of nervous action ? 



40. Why do we not readily recognize the reflex activity of the cord in our own bodies ? 

 How best studied in others ? Example ? 



41. Similar movements ? Arm of a person ? Melted wax or heated coin on the hand ? 



