1056 ON CUTANEOUS AND [Book hi. 



in which the skin of a limb was sensitive to warmth, that is to 

 degrees of temperature above that of the limb, but insensitive 

 to cold. It may be remarked that in these cases, as in that of 

 the limb " gone to sleep, 1 ' the sensations of touch proper and of 

 cold seem to run together and sensations of pain and of heat 

 also to run together. 



It seems probable then from these considerations that we 

 possess three sets of terminal organs and three sets of fibres, 

 one for pressure, a second for heat and a third for cold. It 

 must be borne in mind however that the three sensations are 

 not wholly independent, since sensations of pressure are modi- 

 fied if changes in temperature be taking place at the same time 

 in the same spot of skin. Thus a penny cooled down nearly 

 to zero and placed on the forehead will be judged by most 

 people to be as heavy or even heavier than two pennies of the 

 temperature of the forehead itself, that is to say the sensation 

 of pressure is increased by a concomitant 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 sensa- 

 tion 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 proc- 

 esses through which the sensations are developed and does not 

 shew that the sensations have common terminal organs. 



§ 657. 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 

 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 tem- 

 perature 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 dis- 

 tinct sensation was largely dependent on the contrast as to tem- 

 perature 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 



