1106 CUTANEOUS SENSATIONS. [BOOK m. 



beyond our tether when we assume on the strength of this 

 that the processes started in the fibres of the pyramidal tract by 

 artificial stimulation are in all respects identical with those started 

 in the fibres of a motor nerve. We are going still more beyond 

 our tether when we assume that the processes started in the same 

 pyramidal fibres as the outcome of natural events in the motor 

 cortex are of the same kind. But these assumptions are trifles 

 compared with the assumption that the events taking place in the 

 fibres of the optic radiation, passing from the pulvinar to the 

 occipital cortex are identical with the events taking place in the 

 fibres of the optic tract on the way to the pulvinar, or that the 

 events travelling along the spinal cord to the brain as the result 

 of a prick of the little finger are identical with those which the 

 prick has started in the fibres of the ulnar nerve. Of the latter 

 events we know a little ; of the former events we know next to 

 nothing. And we may here ask the question what is the meaning 

 of these continual relays of grey matter along the sensory tract 

 unless it be that at each relay, some transformation, some further 

 elaboration of the impulses takes place, until what were the 

 relatively, but only relatively, simple impulses along the fibres of 

 the peripheral nerve are by successive steps changed in the 

 complex events which we call a conscious sensation ? This is 

 what we had in mind, when we gave (652) a note of warning 

 concerning the danger of considering 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 sen- 

 sations, 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 changes by which simple impulses are 

 developed into full sensations, and when and how the changes are 

 effected. 



686. 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 collections 



