398 THE SPINAL COKD. 



CHAPTER III. 



THE SPINAL CORD. 



WE have already seen that the spinal cord is a long ganglion, 

 covered with longitudinal bundles of nervous filaments, and occu- 

 pying the cavity of the spinal canal. It sends out nerves which 

 supply the muscles and integument of at least nine-tenths of the 

 whole body, viz., those of the neck, trunk, and extremities. All 

 these parts of the body are endowed with two very remarkable 

 properties, the exercise of which depends, directly or indirectly, 

 upon the integrity and activity of the spinal cord, viz., the power 

 of sensation and the power of motion. Both these properties are 

 said to reside in the nervous system, because they are so readily 

 influenced by its condition, and are so closely connected with its 

 physiological action. We shall therefore commence the study of 

 the spinal cord with an examination of these two functions, and of 

 the situation which they occupy in the nervous system. 



SENSATION. The power of sensation, or sensibility, is the power 

 by which we are enabled to receive impressions from external 

 objects. These impressions are usually of such a nature that we 

 can derive from them some information in regard to the qualities 

 of external objects and the effect which they may produce upon 

 our own systems. Thus, by bringing a foreign body into contact 

 with the skin, we feel that it is hard or soft, rough or smooth, cold 

 or warm. We can distinguish the separate impressions produced 

 by several bodies of a similar character, and we can perceive whe- 

 ther either one of them, while in contact with the skin, be at rest 

 or in motion. This power, which is generally distributed over the 

 external integument, is dependent on the nervous filaments rami- 

 fying in its tissue. For if the nerves distributed to any part of the 

 body be divided, the power of sensation in the corresponding region 

 is immediately lost. 



