THE SENSE OF TOUCH. 133 



it ceases to dominate the medulla oblongata and the spinal cord. These 

 centres then act more vigorously, and diminish the calibre of the cerebral 

 blood vessels through the action of the vasomotor nerves, producing a con- 

 dition of physiological anaemia and sleep; during this state waste products 

 are removed, force is stored up, nutrition is restored, and waking finally 

 occurs. 



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 struc- 

 tures 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. t 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 

 enables 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 smooth- 

 ness^ is estimated by the impressions made upon the tactile papillae. 



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



