THE SENSE OF TOUCH. 151 



THE SENSE OF TOUCH. 



The Sense of Touch is a modification of general sensibility, and 

 located in the skin, which is especially adapted for this purpose, on account 

 of the number of nerves and papillary elevations it possesses. The structures 

 of the skin and the modes of termination of the sensory nerves have already 

 been considered. 



The Tactile Sensibility varies in acuteness in different portions of the 

 body ; being most marked in those regions in which the tactile corpuscles 

 are most abundant, e, g., the palmar surface of the third phalanges of the 

 fingers and thumb. 



The relative sensibility of different portions of the body has been ascer- 

 tained by means of a pair of compasses, the points of which are guarded 

 by cork, and then determining how closely they could be brought together, 

 and yet be felt at two distinct points. The following are some of the 

 measurements : 



Point of tongue, ]^ of a line. 



Palmar surface of third phalanx, I line. 



Red surface of lips, 2 lines. 



Palmar surface of metacarpus, 3 



Tip of the nose, 3 



Part of lips covered by skin, 4 



Palm of hand, 5 



Lower part of forehead, 10 



Back of hand, 14 



Dorsum of foot, 18 " 



Middle of the thigh, 30 " 



The sense of touch communicates to the mind the idea of resistance only, 

 and the varying degrees of resistance offered to the sensory nerves enable 

 us to estimate, with the aid of the muscular sense, the qualities of hardness 

 and softness of external objects. The idea of space or extension is obtained 

 when the sensory surface or the external object changes its place in regard 

 to the other, the character of the surface, its roughness or smoothness, is 

 estimated by the impressions made upon the tactile papillae. 



Appreciation of Temperature. The general surface of the body is more 

 or less sensitive to differences of temperature, though this sensation is 

 separate from that of touch ; whether there are nerves especially adapted 

 for the conduction of this sensation has not been fully determined. Under 

 pathological conditions, however, the sense of touch may be abolished, while 

 the appreciation of changes in temperature may remain normal. 



