CUTANEOUS AND SOME OTHER SENSATIONS. 715 



limited to the recognition of the " currents of action " referred to in 570. 

 We are already going beyond our tether when we assume on the strength of 

 this that the processes started in the fibres of the pyramidal tract by artifi- 

 cial 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 out- 

 come 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 gray matter along the sensory tract, unless it be 

 that at each relay some transformation, some further elaboration of the im- 

 pulses take 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 ( 565) 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 sensations, 

 travelled unchanged from the skin to the cortex, and there suddenly blos- 

 somed 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 sensa- 

 tions, and when and how the changes are effected. 



598. 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 gray 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 gray matter and fibres by which the bulb is joined to the cortex. 

 The several defined areas or collections of gray matter, and the several 

 strands and tracts of fibres which we briefly described in a previous section, 

 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 one 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 con- 

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

 serve as the front of the tegrnental 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 



