1430 ON CUTANEOUS AND [BOOK in. 



that is to say the sensation of pressure is increased by a con- 

 comitant sensation of cold; and a similar modification of the 

 sensation of pressure is also often observed when the object 

 pressing is not colder but warmer than the skin pressed on. A 

 similar effect seems to be shewn in certain cases of disease of the 

 central nervous system in which it has been recorded that a hot 

 body such as a heated spoon was felt when brought in contact 

 with the skin, though the same spoon applied at the temperature 

 of the skin itself produced no sensation at all, and the heated spoon 

 was recognized not as a hot body, but simply as something touching 

 the skin. The exact explanation of these facts is not very clear, 

 but it may perhaps be argued that the effect is brought about 

 amid the central processes through which the sensations are 

 developed and does not shew that the sensations have common 

 terminal organs. 



889. In attempting to understand the nature of the periphe- 

 ral events through which the sensory impulses giving rise to 

 sensations of pressure of heat and of cold are developed two or 

 three matters must be borne in mind. In the first place, as we 

 have already said, though the skin has a temperature of its own, 

 we are not directly . conscious of that, or at all events are not 

 distinctly conscious of it in the same way that we become conscious 

 of any sudden change in that temperature; nor indeed are we, 

 except in extreme cases, distinctly conscious that the temperature 

 of one region differs from that of another, or that the temperature 

 of the same region gradually varies from time to time. It would 

 seem as if the development of a clear and distinct sensation was 

 largely dependent on the contrast as to temperature between an 

 area of the skin and surrounding areas ; and indeed we have 

 already pointed out the marked effects of contrast. The same 

 applies to pressure ; we are not, at least distinctly and directly, 

 conscious of the uniform pressure of the atmosphere over the 

 whole surface of the body, when we stand naked in still air. 

 We are not however justified in assuming that under the above 

 circumstances nothing whatever is taking place in the sensory 

 nerves of the skin, that when we feel a sensation the change in 

 the sensory apparatus (using that phrase in its widest sense to 

 include both peripheral and central parts) is one from absolute 

 quiescence to activity ; it is not impossible, and some facts indeed 

 seem to suggest, that even when we feel no distinct cutaneous 

 sensations, afferent impulses still continue to stream onwards 

 from the peiiphery to the central nervous system, supplying as 

 it were a groundwork of nervous events which enter largely in 

 various ways into the conduct of the whole body, but which do 

 not distinctly affect consciousness. If this be so, we may infer 

 that the affection of consciousness which we call a sensation is the 

 immediate effect of an adequately large change in this ground- 

 work, rather than of a set of quite new isolated impulses passing 



