498 THE NERVOUS SYSTEM. 



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 shown, 

 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 

 that, rather, as there are separate nerve-fibres for motor 

 and for sensitive impressions, so in the cord, separate and 

 determinate parts serve to conduct always the same kind 

 of impression. 



The important and philosophical labours of Dr. Brown- 

 Sequard have cast much new light on all relating to the 

 functions of the spinal cord. It is not possible to do 

 justice to these investigations in any summary, however 

 lengthy and complete : the whole series, (delivered in 

 lectures at the College of Surgeons,) must be read and 

 studied. An attempt will be made here to point out only 

 the principal conclusions deducible from them. 



a. Sensitive impressions, conveyed to the spinal cord by 



