SEC. 2. ON PAINFUL AND SOME OTHER KINDS OF 



SENSATION. 



§ 650. 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, espe- 

 cially 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 

 sensations 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. 



§ 651. The above considerations suggest that in the case of 

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



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