THE NERVOUS SYSTEM. 573 



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 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 communication 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 motor or sensory impulses is 

 effected under ordinary circumstances (to any great extent), as was for- 

 merly supposed, through the gray substance, i.e., through tne nerve- 

 corpuscles and filaments connecting them. All parts of the cord are not 

 alike able to conduct all impressions; and as there are separate nerve- 

 fibres for motor and for sensory impressions, so in the cord, separate and 

 determinate tracts serve to conduct always the same kind of impres- 

 sion. The sensations of touch, temperature, and pain, however, do not 

 appear to have such sharply limited tracts as the motor impulses. 



Experimental and other observations point to the following conclu- 

 sions regarding the conduction of sensory and motor impressions through 

 the spinal cord. Many of these conclusions must, however, be received 

 with considerable reserve. 



a. Sensory Impressions. By sensory impressions are here meant 

 the sensations of touch and pain, of heat and cold, and of muscular sense. 

 These impressions are conveyed to the spinal cord by the posterior nerve- 

 roots. Part of them are then carried directly into the postero-median 

 column on the same side, and thence up to the nucleus of this column 

 in the medulla. It is mainly the impulses of muscle sense that are thus 

 carried. Other sensations are carried by the posterior root-fibres to the 

 cells of the column of Clarke. From there the impulses are conveyed to- 

 the direct cerebellar tract on the same side, and thence up to the cere- 

 bellum. These are mainly sensations that subserve the sense of equili- 

 brium, and are closely connected in function with those which pass up 

 the column of Goll to its nucleus. The impressions of touch and pain, 

 and of heat and cold, are conveyed to the nerve-cells in the posterior 

 cornua of the same side in part, and in part to the nerve-cells in the 

 posterior cornua and median gray of the opposite side. From this point, 

 the impulse is taken up again by intermediary neurons and conveyed 

 through the anterior and lateral columns of the cord, in the ascending 

 tract of Gowers and Tooth, to the brain. By reason of the great number 

 of collaterals and the interpolation in the course of the sensory impulse 

 of many intermediary neurons, no very sharply defined tract has yet 

 been satisfactorily made out in the spinal cord for the conduction of 



