CUTANEOUS SENSATIONS 243 



stimulus" i. e., the sensation increases in arithmetical, while 

 the stimulus increases in geometrical proportion. 



Different areas of the body vary in their power of discrimi- 

 nating pressure differences. The forehead, lips, and temples 

 appreciate an increase of ^ to ^j but the head, fingers, and 

 forearm can appreciate a stimulus only when increased from 

 21J- to T V of its previous intensity. When two equal weights 

 of different expanse press upon the skin, the larger appears the 

 heavier. A weight pressing upon the skin leaves an after- 

 sensation, so that the intervals between successive applications 

 of stimuli to touch endings cannot be less than g-y-o of a second 

 if they are to give separate sensations. If the impressions follow 

 at a more rapid rate, they will be fused together into a con- 

 tinuous sensation. 



Touch sensations are localized, i. e., they are referred to the 

 right portion of the body where the stimulus is applied. This 

 power is acquired early in life and the sensations of touch are 

 correlated with those of sight and those arising from muscles, 

 so that each area of the skin acquires a " local sign" The power 

 and fineness of localization differ greatly for different portions of 

 the skin. The following table is taken from Kirke's Handbook: 



TABLE OF VARIATIONS IN THE TACTILE SENSIBILITY OF THE 

 DIFFERENT PARTS 1 



Tip of tongue a 1 ? inch 



Palmar surface of third phalanx of forefinger . . . TJ 



Palmar surface of second phalanges of fingers 



Red surface of under lip 



Tip of the no'se I 



Middle of dorsum of tongue f 



Palm of hand -f% 



Centre of hard palate \ 



Dorsal surface of first phalanges of fingers .... -J. 2 



Back of hand 1 1 



Dorsum of foot near toes 1 \ 



Gluteal region 1* 



Sacral region H 



Upper and lower parts of forearm l 



Back of neck near occiput 2 



Upper dorsal and midlumbar regions . . . ... 2 



Middle part of forearm 2* 



Middle of thigh 2| 



Midcervical region 2| 



Middorsal region 2^ 



1 The measurement indicates the least distance at which the two blunted 

 points of a pair of compasses could be separately distinguished. E. H. 

 Weber. 



