THE SENSE OF TOUCH. 233 



Mastication is performed with difficulty, from inability to retain 

 the food between the teeth until it is completely triturated. 



Function. Governs all the movements of the tongue and influences 

 the functions of mastication, deglutition and articulation. 



THE SENSE OF TOUCH. 



The sense of touch is a modification of general sensibility, and is 

 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 

 ascertained by means of a pair of compasses : the points of the 

 instrument being guarded by cork, it was then determined how 

 closely they could be brought together, and yet be felt at two different 

 points. The following are some of the measurements : 



Point of tongue l / 2 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 or softness of external objects. The idea of 



