SENSORY IMPULSES 



529 



by none better than by the common paraplegia, or loss of sensation and volun- 

 tary motion in the lower part of the body, in consequence of destructive 

 disease or injury of a section 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 formerly 

 supposed, through the gray substance of the cord, 

 i.e., from cell to cell through the short filaments 

 lying wholly within the gray substance. But 

 cells with fibers running for short distances in 

 the ground bundles are numerous, and these 

 short connectives are capable of conducting im- 

 pulses along the cord. All parts of the cord 

 are not alike able to conduct all impressions; 

 and as there are separate nerve fibers for motor 

 and for sensory impressions, so in the cord sepa- 

 rate and determinate tracts serve to conduct 

 always the same kind of impressions. The sen- 

 sations of touch, and perhaps of temperature and 

 pain, do not appear to have such sharply limited 

 tracts as do the motor impulses. 



Experimental and other observations point 

 to the following conclusions regarding the con- 

 duction of sensory and motor impressions through 

 the spinal cord. Many of these conclusions must, 

 however, be received with considerable reserve. 



Sensory Impulses. The sensory impres- 

 sions of touch, pain, heat and cold, and of the 

 muscular sense are conducted to the spinal cord 

 by the posterior nerve roots. Certain sensory 

 impressions 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 the muscle sense and of the 

 sense of touch that take this course through the cord, though the sense of 

 touch is not wholly interrupted upon injury to the posterior columns. In 

 lower animals it is scarcely interfered with at all. The posterior columns 

 unquestionably are the primary muscle sensory paths. Visceral sensations 

 are carried by the posterior root fibers to the cells of the column of Clarke 

 in the posterior horn, figure 361. From there the impulses pass to the direct 

 34 



FIG. 369. Diagram to Show 

 the Manner in Which the Fibers 

 of the Posterior Nerve Ro9ts 

 Enter and Ascend the Posterior 

 Columns of the Cord. (Edin- 

 ger.) 



