810 CUTANEOUS SENSATIONS. [Book hi. 



ing a motor end plate to the contraction in the muscular fibre 

 which that impulse brings about. The new changes started in 

 the first relay cell may be very different from those coursing 

 along the posterior root-fibre ; and these again in a similar way 

 may start still other changes in the next relay cell ; and so on. 

 We may therefore well hesitate to speak of or consider all the 

 events in the central nervous system as either motor or sensory 

 in nature. It is perhaps not an exaggeration to represent the 

 views of some observers as if they supposed that afferent impulses, 

 say tactile impulses, that is impulses eventually giving rise to 

 tactile sensations, travelled unchanged from the skin to the 

 cortex and there suddenly blossomed into sensations. If such a 

 view were true, undoubtedly the chief task of physiology, almost 

 the only one, would be to ascertain the tract along which these 

 impulses passed. But if on the other hand the views just now 

 urged have any real foundation, the question of tracts or paths 

 sinks into insignificance compared with the almost untouched 

 problems as to what are the several successive changes by which 

 simple impulses are developed into full sensations, and when and 

 how the changes are effected. 



§ 511. Seeing how unsatisfactory is our present knowledge 

 with regard to the tracts or paths of sensations in the relatively 

 simple spinal cord, it would be useless to attempt any discussion 

 as to their paths in the much more complex brain. If it be 

 probable that the passage is effected by relays of grey matter in 

 the former, the same method is much more probable in the latter ; 

 and if neither experiment nor clinical study throws much light 

 on the path up to the bulb, these cannot be expected to give 

 much help in the maze of grey matter and fibres by which the 

 bulb is joined to the cortex. The several defined areas or col- 

 lections of grey matter, and the several strands and tracts of 

 fibres must have of course a meaning ; but it may be doubted 

 whether we have even so much as a correct glimpse of that 

 meaning in any case, if we except those which are in immediate 

 connection with the cranial nerves and their nuclei. Seeing that 

 the thalamus appears on the one hand to be connected with all 

 or nearly all parts of the cortex, and on the other hand to serve 

 as the front of the tegmental system, it is tempting to suppose 

 that it plays an important part in sensations pertaining to the 

 body generally, as part of it, the pulvinar, certainly does with 

 reference to the special sense of sight ; but we have no decisive 

 indications as to what part it plays. And the part which it plays, 

 whatever that may be, is not an exclusively sensory one, since 

 both experimental and morbid lesions of the thalamus are apt to 

 produce disorders of movement as well as other efferent effects. 

 We ought perhaps to say the parts which it plays ; for it is a 

 complex body, having many ties and probably performing many 

 duties. 



