SKIN SENSATIONS. 195 



peculiar sensation due to a touch spot. Having made certain 

 that the subject who should be blindfolded, can recognize this 

 sensation (and does not confuse it with one of pain, which is of 

 longer duration), proceed, systematically, to mark the touch 

 spots. Transfer the results to the chart, and note the relation- 

 ship of the touch spots to the hair follicles. By touching a hair 

 the sense of touch is accentuated. Repeat these observations 

 on the calf of the leg, where the hair follicles are less numerous. 

 Experiment 85. Select an aesthesiometer which can produce 

 a distinct sensation of touch when its hair is pressed to the 

 bending point on the end of one of the skin hairs. Apply it to a 

 hairless part of the skin and note that it elicits no sensation. 

 It is important to distinguish between the sensations of deep 

 pressure and touch. 



Experiment 86. -Touch the skin lightly with a blunt point; then 

 gradually increase the pressure and note the occurrence of the 

 deep pressure sensation. Note that this occurs only after there 

 has been decided deformation of the surface. 

 The threshold value for touch (touch acuity) varies in different 

 parts of the body. Several things, besides the presence of hairs, 

 determine this (thickness of skin, rate of application of stimulus, 

 previous friction of skin, etc.). In general, however, the order of 

 touch thresholds is as follows: lips, finger tips and forehead; dorsal 

 aspect of finger; palm, arm and thigh; forearm; while much less 

 sensitive are the extensor surfaces of the forearm and the loins. 

 Experiment 87. Using various hairs, determine the exact thres- 

 hold value for touch for the above regions by finding what thickness 

 of hair must be used to elicit the sensation of touch at the point 

 at which the hair just bends. Make a table of the results* 

 giving the "bending weight" of the hairs for each region. 

 Besides touch acuity it is important to study the LOCALIZATION 

 of touch sensations. This may be done either by asking the 

 (blindfolded) person to point with his finger to the exact place 

 where he was touched (absolute localization) or by ascertaining 

 the distance by which two points must be separated in order to 



*In many of these experiments on touch a fine camel's hair brush can also 

 be used. 



