THE PHYSIOLOGY OF THE SPECIAL SENSES. 143 



So in our bodies. If the central nerve-cells are themselves 

 diseased and irritated, the impression on the brain is projected on 

 to some external surface change ; and in such a way people who 

 are insane see, hear, and feel things around them which do not 

 exist except to themselves, but they see and hear them very really. 



Now, this perfect localisation of sensations is not equally 

 present in all of us at all periods of our lives. It is partly the 

 result of past impressions, memory, and partly the combined 

 result of using all our senses together. By education or training, 

 the senses can be made much more acute ; that is, the power of 

 localisation can be made more accurate, and not only that, but a 

 slight stimulus applied to the surface can be much more fully 

 interpreted by comparing it with previous stimuli like it. 



Let us take, for example, the neuro-epithelial mechanism of 

 the skin, which, we say, is endowed with the sense of Touch. 

 This sense of Touch is in reality at least threefold. There is the 

 Tactile sense, or sense of Touch proper, and the sense of Tem- 

 perature, together with something quite different — namely, the 

 sense of Pain, though this sense of pain is unfortunately not 

 Hmited to the surface of the skin. That these three are quite 

 distinct from one another, and travel by different nerve-fibres to 

 the brain, is undoubted, for in certain not uncommon diseases of 

 the nervous system the sense of pain may be altogether absent, 

 whilst the senses of touch and temperature remain. Thus, the 

 person can feel anything touching him, can tell a warm body from 

 a cold ; but if pinched or pricked with a pin, feels no pain what- 

 ever, but merely feels the contact of the pin touching him. So 

 also in other cases, the person can tell hot from cold, but cannot 

 feel the contact of the hot or cold body. 



The different parts of the surface of the body differ very 

 greatly in their power of dehcately appreciating touch and 

 temperature, and those parts which are most sensitive to touch are 

 not the most sensitive to temperature. There is a simple instru- 

 ment called an yEsthesiometer, by which the tactile sensibility of 

 different parts of the surface may be readily estimated. It consists 

 of two fine points, which can be brought close together or sepa- 

 rated by a sliding scale, so that the distance between them can 

 easily be read off. The points are then laid on the skin and 



