562 THE NERVOUS SYSTEM 



cells with fibers running for short distances in the ground bundles are 

 numerous, and these short connectives are capable of conducting impulses 

 along the cord. All parts of the cord are not alike able to conduct all im- 

 pressions; and as there are separate nerve fibers for motor and for sensory 

 impressions, so in the cord separate and determinate tracts serve to conduct 

 always the same kind of impressions. The sensations 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 conduction of sensory and motor impressions through the 

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

 considerable reserve. 



Sensory Impulses. The sensory impressions of touch, pain, heat and 

 cold, and of the muscle sense are conducted to the spinal cord by the posterior 

 nerve roots. Certain sensory impressions are then carried directly into the 

 fasciculus gracilis 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 funiculi 

 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 363. From there the impulses pass to the direct 

 cerebellar tract on the same side, and thence up through the medulla to the 

 cerebellum, figure 388. The impressions of 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 cornu and median gray matter 

 of the apposite side. From this point, the impulses are taken up again by 

 intermediary neurones and conveyed through the anterior and lateral columns 

 of the cord to the brain in the ascending superficial antero-lateral tract, or 

 tract of Gowers. By reason of the great number of collaterals and the 

 interpolation in the course of the sensory path of many intermediary neu- 

 rones, it has been difficult to make out very sharply defined tracts in the 

 spinal cord for the conduction of the sensations of temperature, pain, and 

 touch. If one set of fibers is destroyed by disease, others seem able, through 

 the collaterals, to take up its functions. We can say that injury to the 

 lateral columns has resulted in loss of the sense of pain, heat, and cold, but 

 with only partial disturbance of touch sensations. 



It is probable, also, that pain and temperature sensations cross over at 

 once to a considerable extent and pass up in the opposite side of the cord to 

 which they enter. Touch and the muscle sense impressions, especially the 

 latter, pass up largely upon the same side until they reach the medulla or 

 cerebellum. 



