36 PHYSIOLOGY CHAP. 



towards the deeper parts and in the surrounding areas (Fig. 21). 

 Both the positive and the negative alterations give rise to sensa- 

 tions of contact, and this with approximately equal strength of 

 stimulus shows the same characteristics on compression and on 

 traction. Accordingly the excitation of the sense-organ of pressure 

 is due to alteration in the intrinsic pressure of the organ, and the 

 intensity of the sensation depends on the amount and not on the 

 direction of the alteration. 



Since it is not possible to measure exactly the alteration in 

 pressure which, with different means of mechanical excitation of 

 the skin, gives rise to the sensation of pressure, it follows that in 

 order to obtain even an approximate valuation of the effective 

 threshold of stimulation we must take into account all the factors 

 that may raise or depress it. The investigations of v. Frey and 

 Kiesow show that the liminal value varies with the rate of 



Fit;. -21. Diagram of deformation of a cutaneous area (shown in section) by compression with a 

 weight (A), and by traction on a disc previously attached to the skin (B). The continuous 

 curved lines in A represent the positive change of pressure ; the dotted lines in B represent 

 its negative change. The variations of pressure in the skin are maximal at the edges of the 

 weight or disc, and become gradually less below the area compressed or pulled upon. 



stimulation and with the nature, size, and depth of the cutaneous 

 deformation. As regards the manner in which the mechanical 

 stimuli may at least be appreciated relatively if not measured 

 exactly, they concluded that : 



(a) The liminal mechanical stimulus cannot be estimated by 

 weight, because the effect of a given weight always depends on 

 the area of the surface of contact. 



(&) When the surface of contact remains constant, a given 

 weight produces a different effect on different parts of the skin, 

 because the number and the sensibility of the nerve-endings 

 excited varies in different cutaneous areas. It is consequently 

 only possible to compare liminal estimations when the experi- 

 ment is confined to the excitation of single nerve-endings, i.e. to 

 single tactile spots, by means of v. Frey's hairs. 



(c) If the same tactile spot is stimulated by a weight which 

 has a constant surface of contact, so as to produce near any such 

 point a deformation constant in depth and surface, the effect of 

 such an excitation varies with the rate at which the deformation 



