au i ii iT 



CHAPTER XXXVI. 



THE CUTANEOUS SENSES. 



The subject keeps his eyes closed and indicates 

 to the observer the nature and intensity of the 

 sensation produced. It is sufficient in the following 

 exercises to detect a marked difference in any one 

 sensation ; the exact evaluation of the difference 

 need not be attempted. 



Tactile and pressure senses. Explore the lips, 

 skin of the face, dorsum of the hand and fore- 

 arm with von Frey's hair sesthesiometer. 



For the least perceptible stimulus the human 

 hair, and for less sensitive parts the horse hair, 

 instrument is used. 



A given length of each hair is capable of 

 exerting a certain maximal pressure, which can be 

 evaluated in grammes per square centimeter. For 

 convenience the hair is placed within a sheath, 

 by means of which the exposed length of hair 

 can be varied, and with it the pressure which 

 can be exerted. 



The after sensation is frequently very marked, 

 the sensation lasting some time after the stimulus 

 is removed. 



Minimal stimuli applied to the face produces 

 tickling instead of a markedly tactile impression. 



The tactile sensation passes into a feeling of 

 pressure with an increase of the stimulus. The 

 horse hair may even induce pain. 



29 



H 



FIG. 55. 



Jisthesio- 

 meter. 



H Human 

 or horsehair 



S Movable 

 sheath by 

 means of 

 which the 

 length of 

 hair expos- 

 ed may be 

 varied. 



