392 THE NERVOUS SYSTEM. 



these roots elicit signs of intense suffering. Occasionally, under 

 this last irritation, muscular movements also ensue; but these 

 are either voluntary, or the result of the irritation being re- 

 flected from the sensitive to the motor fibres. Occasionally, 

 too, irritation of the distal ends of divided anterior roots elicits 

 signs of pain, as well as producing muscular movements: the 

 pain thus excited is probably the result of cramp (Brown-Se- 

 quard). 



As an example of the experiments of which the preceding 

 paragraph gives a summary account, this may be mentioned : 

 If in a frog the three posterior roots of the nerves going to the 

 hinder extremity be divided on the left side, and the three an- 

 terior roots of the corresponding nerves on the right side, the 

 left extremity will be deprived of sensation, the right of motion. 

 If the foot of the right leg, which is still endowed with sensa- 

 tion but not with the power of motion, be cut off, the frog will 

 give evidence of feeling pain by movements of all parts of the 

 body except the right leg itself, in which he feels the pain. If, 

 on the contrary, the foot of the left leg, which has the power 

 of motion, but is deprived of sensation, is cut off, the frog does 

 not feel it, and no movement follows, except the twitching of 

 the muscles irritated by cutting them or their tendons. 



Functions of the Spinal Cord. 



The spinal cord manifests all the properties already assigned 

 to nerve centres (see p. 382). 



1. It is capable of conducting impressions, or states of ner- 

 vous excitement. Through it, the impressions made upon the 

 peripheral extremities or other parts of the spinal sensitive 

 nerves are conducted to the brain, where alone they can be 

 perceived by the mind. Through it, also, the stimulus of the 

 will, applied to the brain, is capable of exciting the action of 

 the muscles supplied from it with motor nerves. And for all 

 these conductions of impressions to and fro between the brain 

 and the spinal nerves, the perfect state of the cord is necessary ; 

 for when any part of it is destroyed, and its communication 

 with the brain is interrupted, impressions on the sensitive 

 nerves given off from it below the seat of injury, cease to be 

 propagated to the brain, and the mind loses the power of vol- 

 untarily exciting the motor nerves proceeding from the por- 

 tion of cord isolated from the brain. 



Illustrations of this are furnished by various examples of 

 paralysis, but by none better than by the common paraplegia, 

 or loss of sensation and voluntary motion in the lower part of 

 the body, in consequence of destructive disease or injury of a 



