SEC. 3. ON PAINFUL AND SOME OTHER KINDS 



OF SENSATION. 



882. When excessive pressure is exerted on the skin, or 

 when the change of temperature passes certain limits, the sensa- 

 tion which is excited ceases to be recognized as one either of 

 touch or of temperature and takes on characters of its own ; we 

 then call it a sensation of pain. In this respect the skin as a 

 sensory organ appears at first sight to differ from the other organs 

 of sense which we have studied. We have no evidence that 

 simple stimulation of the retina, however excessive, will give rise 

 to pain, meaning by pain the kind of sensation we feel when the 

 skin is cut or burnt. We often speak it is true, especially in 

 cases of disease of the eye, of exposure to light causing pain, 

 but the pain in such cases is felt through the eyeball, not 

 through the retina and optic nerve ; the pain is not an excessive 

 development of visual sensations, it is a phase of that sensibility 

 which the subsidiary structures of the eye share, in common as 

 we shall see presently, with not only the skin but nearly all 

 other structures of the body. In like manner we have no evi- 

 dence that an auditory or an olfactory or a gustatory sensation 

 can, through mere intensity, become converted into a sensation 

 of pain, though we may, in the act of hearing, smelling or 

 tasting, receive sensations of pain from the ear, nose or mouth. 

 We often of course apply the word 'painful' to a sound, or a group 

 of visual sensations, or a smell or a taste ; but that is in the 

 sense of being exceedingly disagreeable, and has reference to 

 our classification of the complex psychical effects of all our sen- 

 sations into those which are pleasurable and those which are 

 painful. Without entering into any psychological analysis, we 

 may assume that the pain which we feel when the finger is cut 

 is a wholly different thing from the pain which is given to a 

 most delicately musical ear by even the most horrible discord ; 

 and in what follows we shall use the word pain in the first of 

 these two meanings. 



883. The above considerations suggest that in the case of 

 the skin as in the cases of the other organs of special sense, a 

 sensation of pain is not simply an exaggeration of a sensation 



