THE PROPERTIES OF EACH PART OF THE REFLEX ARC 793 



hot bath may really depend on the fact that in the latter the cold spots 

 are stimulated as well as those of heat. It is at least interesting to note 

 that the physiological reflexes stimulated by either a cold or a very hot 

 bath are the same ; thus, a rise of blood pressure and a contraction of the 

 muscles of the skin occur in both cases. 



The Touch Sense. In order to investigate the touch sense accurately, 

 von Frey has devised a method of using hairs of different thickness each 

 mounted on a different handle. The hair which produces a sensation 

 of touch when pressed on the skin so that it just bends is then similarly 

 pressed on one scale pan of a balance, and the weight required in the 

 other scale pan to hold the beam horizontal when the hair just bends, is 

 ascertained. From the diameter of the hair one can then calculate how 

 many grams per square millimeter are necessary to elicit the sensation 

 of touch. The following quantitative results have been obtained by ap- 

 plying von Frey's method to different parts of the body: 



Gin. per sq. mm. 



Tongue and nose 2 



Lip 2.5 



Finger tip and forehead 3 



Back of finger 5 



Palm 7 



Forearm 8 



Back of hand 12 



Calf, shoulder 16 



Abdomen 26 



Outside of thigh 26 



Shin and sole 28 



Back of forearm 33 



Loin 48 



That the sense of touch is located in spots touch spots can best be 

 demonstrated on the calf of the leg. If this is shaved and then carefully 

 explored with a fairly stiff hair, it will be found that there are only 

 some twelve to fifteen spots in an area of a square centimeter at which 

 the hair can be felt. Between these spots there is no sensation of touch. 

 That these spots are composed of specialized receptors can be very clearly 

 shown by pressing a fine needle into one of them, when no pain will be 

 experienced but only a peculiar shotty sense of pressure. 



Careful examination of the position of the touch spots will further 

 show that they are grouped around hair follicles, particularly on the side 

 from which the hair extends the windward side, we may call it. This 

 fact explains the well-known experience that an object may be felt more 

 acutely on a hairy surface than after that surface has been shaved. The 

 hairs bend slightly 'when the object comes in contact with them, thus 



