220 ANIMAL PHYSIOLOGY. 



ment for himself, lie will find that this is a test simply of the 

 power of localization of sensation, and that of the three parts 

 mentioned, the back is the most acutely affected by a given 

 amount of pressure; it is the part on which the pressure of a 

 fine point most quickly causes pain. The reason of this is 

 not very far to seek. The epidermis of the back is much 

 thinner than that of either the tongue or the ^fingers, and 

 therefore when an object touches it, the nerve-extremities 

 are less protected. On the other hand, the nerve-extremities 

 are far more numerous in the tongue and fingers, and a given 

 irritation produces a slighter impression on a greater number 

 of them. When the impressed condition of a nerve is carried 

 to a certain pitch of intensity, pain is the result ; but when a 

 number of nerves are impressed in a slighter degree, increased 

 information is obtained. Thus it happens that the back, 

 with few nerves and comparatively thin skin, is highly sensi- 

 tive, but a very poor tactile organ; while the fingers, with 

 many nerves and thick cuticle, bear rougher usage without 

 pain, at the same time that they are useful for touch. 



A very common experiment illustrates that a distinction 

 must be drawn between the localization of impressions on the 

 surface of a touching organ, and the sense of the position of 

 the organ, spoken of in a previous paragraph. If the middle 

 finger be crossed over the back of the forefinger, and a pencil 

 or a pea be placed between the tips of the two fingers, the 

 sensation produced is not that of a single round object, but 

 of two distinct objects; and if the eyes be shut, the illusion 

 will be complete. The reason is, that while each finger con- 

 veys a correct tactile sensation, the mind fails to recognise 

 the exact relative positions of the unusually placed tactile 

 surfaces. 



161. All the finer differences of touch disappear in pain. 

 Heat, cold, chemical and mechanical irritation, all produce 

 pain when applied in excess; and although the character of 

 the pain varies with the nature of the damage done to tex- 

 ture, in no case is there any resemblance between the pain 

 and the sensation caused by a minor degree of the stimulus. 



Pain, then, may be considered as a sensation distinct from 

 others, and resulting from an excess of the impressed condi- 

 tion in a sensory nerve; indeed, it may be felt in parts which 



