THE CEREBRO-SPINAL NERVOUS SYSTEM. 481 



Functions of the Ganglia on Posterior Roots. The ganglia act as 

 centres for the nutrition of the nerves, since when the nerves are severed 

 from connection with the ganglia, the parts of the nerves so severed 

 degenerate, whilst the parts which remain in connection with them do 

 not. 



Functions of the Spinal Cord. 



The power of the spinal cord, as a nerve-centre, may be arranged 

 under the heads of (1) Conduction; (2) Transference; (3) Keflex action. 



(1) Conduction. The functions of the spinal cord in relation to con- 

 duction, may be best remembered by considering its anatomical connec- 

 tions with other parts of the body. From these it is evident that, with 

 the exception of some few filaments of the sympathetic, there is no way 

 by which nerve-impulses can be conveyed from the trunk and extremi- 

 ties to the brain, or vice versa, other than that formed by the spinal 

 cord. Through it, the impressions made upon the peripheral extremi- 

 ties or other parts of the spinal sensory nerves are conducted to the 

 brain, where alone they can be perceived. Through it, also, the stimulus 

 of the will, conducted from 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 brains 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, im- 

 pressions on the sensory nerves given off from it below the seat of injury, 

 c ease to be propagated to the brain, and the brain loses the power of 

 voluntarily exciting the motor-nerves proceeding from the portion of 

 cord isolated from it. 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 portion, including 

 the whole thickness, 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 

 brain the various organs supplied with nerves issuing from those parts of 

 the cord. 



It is not probable that the conduction of impressions along the cord 

 is effected (to any great extent), as was formerly supposed, through the 

 gray substance, i. e., through the nerve-corpuscles and filaments connect- 

 ing them. All parts of the cord are not alike able to conduct all im- 

 pressions; and as there are separate nerve-fibres for motor and for sensory 

 impressions, so in the cord, separate and determinate tracts serve to con. 

 duct always the same kind of impression. 



Experimental and other observations point to the following conclu- 



31 



