284 ON CUTANEOUS AND [BOOK in. 



afferent impulses of the common sensibility of which we are now 

 speaking. 



If we suppose that the skin in common with the other tissues 

 of the body possesses this common sensibility, and if we further 

 suppose that in the skin as elsewhere, these afferent impulses 

 when developed, as is the case under normal circumstances, to a 

 slight extent only are not distinctly recognized by consciousness, 

 and that when they do assume such a magnitude or intensity as 

 to break in upon consciousness the change of consciousness which 

 they produce is of the kind which we call pain, we reach a conclu- 

 sion which is also supported by other considerations. On the one 

 hand such a view is in accord with the conclusion that cutaneous 

 sensations of pain are wholly distinct from and developed in a 

 wholly different way from sensations of touch and temperature ; 

 and, as we shall see, to this conclusion we are led by several 

 different arguments. On the other hand it relieves us from the 

 following difficulty. It may happen to a man to suffer pain in a 

 particular region or tissue of the body, once only in the course of 

 his lifetime or possibly not even once ; nay, we may suppose that 

 in this or that region or tissue pain is felt once only in one 

 individual among a large number of persons. If we suppose that 

 pain is not as suggested above an excessive phase of something 

 which is continually going on in a lower phase, but a something 

 by itself quite distinct from all other sensations, we are driven to 

 conclude, since such a sensation must have a special mechanism, 

 including special afferent nerve fibres to carry it out, that in the 

 case in question such a mechanism of pain has been preserved 

 intact but unused through whole generations in order that it may 

 once in a while come into use ; which is in the highest degree 

 improbable. This difficulty disappears if we suppose that the 

 constantly smouldering embers of common sensibility may be at 

 any moment fanned into the flame of pain. 



We may conclude then that the skin in common with other 

 tissues possesses common sensibility, and that when this is excited 

 in excess, so as to distinctly affect consciousness, we call it pain. 

 We thus experience through the skin three kinds of sensations, 

 those of touch, of temperature, and of common sensibility, but 

 the two former only are developed by further psychical processes 

 into perceptions ; it is by them alone that we obtain through the 

 skin knowledge of external objects. 



884. There is another consideration to be taken into view. 

 The agents which applied to the skin produce pain, act violently 

 on the skin, in many cases injuring the epidermis and affecting 

 the dermis. Moreover if the epidermis be removed, and the 

 stimulus, mechanical, thermal or chemical, be applied to the 

 dermis or to the nerves running in it, we still experience sensa- 

 tions of pain, though no longer those of touch and temperature 

 when a sharp or hot body is made to touch, not the intact skin 



