496 THE NERVOUS SYSTEM. 



nervous 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 voluntarily 

 exciting the motor nerves proceeding from the portion of 

 cord isolated from the brain. 



Illustrations of this are furnished by various examples. 

 of paralysis, but by none better than by the common para- 

 plegia, or loss of sensation and voluntary motion in the 

 lower part of the body, in consequence of destructive 

 disease or injury of a portion, including the whole thick- 

 ness, of the spinal cord. Such lesions destroy the com- 

 munication between the brain and all parts of the spinal 

 cord below the seat of injury, and consequently cut off 

 from their connection with the mind the various organs 

 supplied with nerves issuing from those parts of the cord. 

 But if this lower portion of the cord preserves its integrity, 

 the various parts of the body supplied with nerves from it, 

 though cut off from the brain, will nevertheless be subject 

 to the influence of the cord, and, as presently to be showD, 

 will indicate its other powers as a nervous centre. 



From what has been already said, it will appear probable 

 that the conduction of impressions along the cord is effected 

 (at least, for the most part) through the grey substance, 

 i, e., through the nerve-corpuscles and filaments connecting 

 them. But there is reason to believe that all parts of the 

 cord are not alike able to conduct all impressions; and 



