BRAIN AND NERVOUS SYSTEM. 333 



762. It is not to be presumed that the -pressure upon the 

 spinal marrow, from a curvature or distortion of the spine, 

 will produce so sudden or perceptible injury as Mr. J. suf- 

 fered ; but his case illustrates the connection between the con- 

 dition of the spinal marrow and the health and power of the 

 organs and systems which derive their nerves from it. These 

 cases ( 759, p. 329) show also how the organs may suffer 

 from an injury to the spine, or interruption of the action of 

 the great nerve which connects them with the brain. What- 

 ever may be the cause of this injury, the consequence of im- 

 paired life and diminished power must follow, in those parts 

 or organs which receive their nerves from the spinal cord 

 below the point of pressure. 



CHAPTER III. 



If Nerve be injured or diseased in its Trunk, Pain is felt at its outer 

 Terminations. Injury of Optic Nerve excites Sensation of Light. 

 Arrangement of the Brain and Nerves like that of Bells and Servant 

 in Hotel. 



763. THE impression being made on the outer extremity of 

 the sensory nerve, and the sensation being in the brain at the 

 other end, the nerve is a mere channel, or highway, through 

 which the impression is carried inward. The healthy nerve 

 receives no impressions and originates no sensations in any 

 part of its course ; it only carries those which it receives at 

 its end ; and the brain recognizes and understands no other 

 power or function in the nerve than that of receiving im- 

 pressions at its outer extremity. It therefore refers all feel- 

 ings arid impressions, which come to it through the nerve, to 

 those extremities. 



764. If, by accident or disease, any impression is made 

 upon the trunk of the nerve, if we touch or irritate it, in any 

 part of its course between the outer and inner ends, this im- 

 pression is conveyed to the brain , but that organ refers it. 

 not to the point where the impression is actually received, 



