706 THE BRAIN. 



suggest that the completeness of the loss of sensation in the hemiplegic man 

 is not a pure " deficiency " phenomenon, but is due to the lesion affecting the 

 chain of events in some way or other besides merely removing the link fur- 

 nished by means of the cortex. For, as we previously urged, the dog in 

 question, however curtailed its psychical life may have been, seemed to a 

 casual observer to feel and move much as usual. Neglecting visual and 

 auditory sensations with which we are not now dealing, it needed careful 

 observation to ascertain that some of the animal's movements fell short, the 

 failure being apparently due to the lack of adequately energetic coordinating 

 sensory impulses ; a stronger stimulus than usual had to be applied to the 

 skin in order to call forth the usual movements and other tokens that the 

 stimulus was " felt." As we have before urged, it is impossible to suppose 

 that the mere stump of cerebrum left in this case could have taken on all the 

 functions of the lost hemispheres ; and making, as we have previously done, 

 full allowance for the differentiation between man and dog, we must conclude 

 that in the more general sensations with which we are now dealing, as with 

 the more special visual sensations, the full development of a complete sensa- 

 tion is a complex act of more stages than one between the afferent impulse 

 along the afferent nerve and the affection of consciousness which we subject- 

 ively recognize as " the sensation ;" the cortical events are only some among 

 several. It follows that any analogy between the cortical events which play 

 their part in a sensation and the cortical events which immediately precede 

 the issue of impulses from the motor region along the fibres of the pyram- 

 idal tract is misleading; the highly differentiated motor localization does 

 not justify us in concluding that there exists a similar topographical distri- 

 bution of sensation. 



592. We may now attack the problem in a different way, and instead 

 of beginning with the cortex begin with afferent impulses started along affer- 

 ent nerves from their peripheral endings, and attempt to trace them central- 

 ward. And first we may call to mind what anatomical guidance we possess 

 (482). 



The fibres of posterior roots, the channels of afferent impulses, end in 

 the spinal cord in at least two main ways. One set is continued on, not 

 broken by any relays, as the median posterior tract, and by this tract repre- 

 sentatives of all the spinal nerves are connected with the gracile nucleus in 

 which ( 523) the median posterior column ends. The other fibres of a pos- 

 terior root appear to end in the gray matter not far from their entrance ; 

 but from the gray matter there starts the cerebellar tract, which, though not 

 conclusively proved to be, may be assumed to be, an efferent tract. We may 

 therefore probably suppose that afferent impulses along certain of the fibres 

 of the posterior root make their way upward along the cerebellar tract, and 

 there are some reasons for regarding the vesicular cylinder and the cells 

 which represent this where it is not conspicuous in the regions of the cord, 

 as a relay between the two systems of fibres. There are also the more scat- 

 tered fibres of the ascending antero-lateral tract ( 480), which probably is 

 also an afferent tract, and therefore probably also connected with the poste- 

 rior roots; but, as we have seen, our knowledge of this tract is imperfect, 

 though, if, as some urge, it ends in the restiform body, we may perhaps con- 

 sider it as similar at least to the cerebellar tract, and treat the two as one. 



Thus there seem to be at least two main recognized paths, in the form of 

 tracts of fibres, for afferent impulses along the cord ; one along the median 

 posterior column, the other along the lateral column in the cerebellar tract. 

 The latter passes straight up to the cerebellum by the restiform body, trav- 

 elling along the same side of the cord, and any crossing of impulses passing 

 along this tract must take place before they enter the tract ; we have, how- 



