672 TEXT-BOOK OF PHYSIOLOGY 



The delicacy of the sense of touch is measured by the slight increase or 

 decrease in the intensity of the stimulus that will produce an appreciable 

 change in the intensity of the sensation. Not all changes in the stimulus, 

 however, are attended by a change in the sensation. It has been deter- 

 mined that the latter will change only when the former changes in a definite 

 ratio, which for the volar surface of the third phalanx of the index-finger is as 

 29 is to 30. Thus, other things being equal, a sensation caused by a given 

 weight will only change with moderate stimulation when one-thirtieth of the 

 weight is either added or subtracted. The ratio of change, however, varies 

 in different regions of the body: thus, for the back of the hand the ratio 

 varies from one-tenth to one-twentieth; for the tongue, one- thirtieth to one- 

 fortieth. The difference of stimulus necessary to evoke a sensation is 

 known as the threshold difference. It seems to be a law not only for the 

 skin, but for other senses as well, that a change in the intensity of a sensa- 

 tion, to an appreciable extent, will occur only when the objective stimulus 

 changes in a definite ratio. This ratio, however, will vary not only in dif- 

 ferent regions of the skin, in different individuals, but with the sense-organ 

 investigated. 



Place Sense. The sensation evoked by stimulation of the skin is always, 

 under normal conditions, referred to the place stimulated. This holds true 

 not only for two or more points near or widely separated on the same side, 

 but also for corresponding points on opposite sides of the body, even when 

 the stimuli have the same intensity and are simultaneously applied. The 

 cause for this localizing power is to be found in a difference in the quality of 

 the sensation 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 

 sensor 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, there- 

 fore, 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 sensa- 

 tions 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 sensor 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 recognized: 



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 



