462 THE SENSORY FUNCTIONS OF THE SKIN 



so-called motor sense (cf. Chapter XVIII). Since, however, these different 

 kinds of pressure sensations have not been sufficiently differentiated either 

 physiologically or psychologically, we shall limit the following discussion to 

 pressure sensations in their simplest form. 



Blix was the first to demonstrate by mechanical stimulation of the skin 

 with fine points, that the pressure sense like the temperature sense is not 

 continuously represented over the entire skin, but that the end organs of the 

 nerves mediating the sensation of pressure are separate from one another 

 and are not identical with the end organs of the temperature nerves (see 

 Fig. 182). 



Stimulating the pressure points as lightly as possible with a bristle pro- 

 duces a delicate but vivid and often somewhat tickling sensation, such as is 

 experienced when one of the fine hairs on the skin is moved. According to 

 Kiesow this shows that the tickling sensation is a sensation of pressure of a 

 peculiar shade occurring under special conditions (and in certain cases con- 

 nected with sensations of contraction). With a little stronger pressure the 

 sensation takes on a perfectly characteristic quality, as if it were produced 

 by a small, hard grain being pushed into the skin, hence the name " granu- 

 lar " used by Goldscheider. 



The pressure spots can be sought out also by means of induction shocks. 

 By monopolar stimulation of the spots a prickling sensation is experienced with 

 a strength of current which gives no sensation at all if applied in the intervals 

 between the spots, v. Frey, who has studied exhaustively the electric stimula- 

 tion of the pressure points, observes that induction shocks produced by as many 

 as 130 interruptions of the primary current per second are felt as independent 

 shocks, also that the constant current causes a discontinuous excitation. On 

 certain regions of the skin (fingers, tip of tongue, red edges of lips), Sergi has 

 found that mechanical shocks can still be appreciated as distinct if they occur 

 at intervals of 0.001-0.002 second. 



The pressure points are arranged with special reference to the hairs. Each 

 hair has a pressure point near its point of exit and directly above the deepest 

 part of the follicle (v. Frey). It cannot be said however that the number 

 of pressure points coincides exactly with the number of hairs. In the first 

 place the hairs often stand in twos and threes and then are so close together, 

 that it is not always possible to demonstrate the presence of pressure points 

 belonging to them. Besides, one finds here and there within the regions clothed 

 by hairs some pressure points standing quite apart and wholly unrelated to 

 any hair. 



The number of pressure points varies in different parts of the skin e. g., on 

 the flexor surface of the wrist 28 per sq. cm., on the anterior surface of the fore- 

 leg 5 per sq. cm. (Kiesow). According to v. Frey's estimate, the entire surface 

 of the adult body, with the exception of the head, would probably contain about 

 500,000 pressure points. 



Excitation of the pressure points appears to be accomplished by deforma- 

 tion of the skin. When a perfectly uniform pressure is applied to the skin 

 no sensation is produced, the best illustration of which is the fact that we 

 do not feel the pressure of the atmosphere. The following experiment also, 



