INNERVATION OF THE HUMAN SKEST 227 



end-organs entrusted specifically with carrying painful impressions 

 to a pain centre," but Professor Starling in his later work on Human 

 Physiology speaks of " a distinct sense of pain," probably subserved 

 by a distinct set of nerve fibres, but for the present purpose it is not 

 necessary that agreement on such a problem should be reached, for 

 it is alone with pain spots that we are concerned. He also points 

 out that on the one hand the cornea is sensitive to only one of 

 the four stimuli in question, that is, pain, and on the other 

 that the surface of the glans penis is sensitive to cold and 

 pain, but tactile sensation and warmth sensations are almost 

 entirely absent. 



Touch. — This form of stimulus and its response can only be 

 reckoned as useful to the organism, except that it may be, and often 

 must be indifferent. The great number of the touch spots can be 

 understood when it is declared by Professor Sherrington that 

 almost invariably there are one or more touch spots close to the 

 emergence of each hair, 1 and that they are very numerous also 

 on the palmar and plantar surfaces of the hand and foot. Of the 

 four forms of cutaneous stimuli those of touch are the only kind 

 that have so far been proved to have specialised corpuscles, the 

 other three having developed the physiological equivalent of cold, 

 pain and warmth spots. 



Warmth spots are decidedly the least numerous of the four, 

 those of pain being, as stated by Professor Sherrington, the most 

 numerous. It is obvious that unless thermal stimuli become 

 somewhat excessive they hardly can be described as " stimuli," 

 being more or less neutral in their action on a warm-blooded animal. 

 This cannot be entirely so, because it has been shown quite con- 

 clusively that warmth spots do exist, though much less numerous 

 than others. There is a significant fact as to thermal reaction 

 and that is that there are no pure heat spots like those of cold, for 

 the stimuli of about 49° C are so associated with those of pain 

 that warmth spots alone are distinguished, and among primitive 

 man no stimuli of heat could impinge on his skin, until he had 

 learned the use of fire, more powerful than those of solar heat. 



Such stimuli of heat as the rays of the sun would occasionally 

 discharge on the skin would resolve themselves into the general 

 stimulus of pain, and in this direction a far shorter initiation 

 occurred than with any of the four normal cutaneous stimuli. The 

 fact, at any rate, of there being no heat spots is to be noted. 



It remains now, having quoted three writers eminent in 

 physiology, psychology and zoology in support of the modest 



1 Scha/er, p. 922. 



Q2 



