

THE NERVOUS SYSTEM. 97 



tated as before, tin general signs of pain still remain, although the con- 

 traction of the muscles does not occur. The signs of pain disappear when 

 the posterior root is divided. From these experiments it is believed that 

 the stimulus passes down the anterior root to the mixed nerve and returns 

 to the central nervous system through the posterior root by means of cer- 

 tain sensory fibres from the posterior root, which loop back into the ante- 

 rior root, before cont'nuing their course into the mixed nerve-trunk. 



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 COED. 



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

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



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

 duction may be best remembered by considering its anatomical connections 

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

 ception of some few filaments of the sympathetic, there is no way by 

 which nerve-impulses can be conveyed from the trunk and extremities to 

 the brain or vice versd, other than that formed by the spinal cord. 

 Through it, the impressions made upon the peripheral extremities 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, con- 

 ducted 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 brain and the spinal nerves, the per- 

 fect state of the cord is necessary; for when any part of it 4s destroyed, 

 and its communication with the brain is interrupted, impressions on the 

 sensory nerves given off from it below the seat of injury, cease 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 paraly- 

 sis, but by none better than by the common paraplegia, or loss of sensa- 

 tion 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 communication be- 

 tween 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 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 all parts 

 VOL. II. 7. 



