S9 o TEXT-BOOK OF PHYSIOLOGY. 



related in some way to the part stimulated. Each cutaneous area 

 is supposed to give to the tactile sensation a quality or local sign 

 by virtue of which the mind is enabled to localize the point of contact. 



Each cutaneous area which has a local sign of its own is known 

 as a sensory circle, for the reason that the mind does not refer the 

 sensation to a point, but to an area more or less circular in outline. 

 The skin may therefore be regarded as composed of myriads of 

 such circles varying in size in different regions of the body. 



The delicacy of the localizing power in any part of the skin is 

 determined by testing the power which the part possesses, of 

 distinguishing the sensations produced by the contact of the 

 points of a pair of compasses placed close together. The distance 

 to which the points must be separated in order to evoke two separate 

 recognizable sensations is a measure of the diameter of the sensory 

 circle. Within this circle the two sensations become fused into one 

 sensation. The discriminative sensibility of different regions as 

 determined by compass points is shown in the following table; the 

 numbers represent the distances at which two sensations are recog- 

 nized : 



MM. 



Tip of tongue, * i.i 



Palmar surface of third phalanx of index-finger, 2.2 



Red surface of lips, 4.5 



Palmar surface of first phalanx of finger, 5.5 



Tip of nose, 6.8 



Palm of hand, 8.9 



Lower part of forehead, 22.6 



Dorsum of hand, 31.6 



Dorsum of foot, 40.6 



Middle of the back, 67.7 



The discriminative sensibility of any portion of the body is a 

 function of its mobility. This is shown by the fact that it increases 

 rapidly from the shoulders to the fingers and from the hips to the 

 toes. 



The Temperature Sense. The sensations of heat and cold 

 which are experienced from time to time are caused by changes in 

 the temperature of the skin produced in a variety of ways. As 

 these sensations are specifically different from those of touch, as 

 well as different from each other, it is highly probable that for each 

 sensation there are special nerve-endings distributed throughout the 

 skin^ Investigations have shown that all over the skin there are 

 innumerable spots of varying size which if stimulated evoke 

 sensations of heat or cold. Such points are termed heat and 

 cold spots. Each responds to but one kind of stimulus. A warm 

 object applied to a heat spot will evoke a sensation of warmth. 

 It will have no effect on the cold spot. The reverse is also true. 

 Between the cold and heat spots there are areas that are neutral, 



