736 



SPECIAL SOMATIC AND VISCERAL RECEPTORS 



is pressed upon the skin with a force sufficient to eHcit different in- 

 tensities of tactile impressions. This method permits of the deter- 

 mination of the acuity of these sen- 

 sations as well as of our ability to 

 localize them with accuracy. Esthes- 

 iometers are also in use which possess 

 two points of contact adjusted at 

 varying distances from one another. 

 This arrangement allows us to deter- 

 d mine how far these points must be 

 separated from one another in order 

 .J to give rise to two distinct sensations. 

 Our ability to tell whether a tactile 

 stimulus is single or double, is known 

 as tactile discrimination. 



Tactile Acuity, Localization and 

 Discrimination. — We have seen that 

 the adequate stimulus for sensations 

 of touch is a mechanical impact which 

 causes a deformity of the surface of 

 the skin and thus activates the sen- 

 sory nerve-endings contained therein. 

 This activation, however, is accom- 

 plished under normal condition with 

 the help of certain adjuncts consist- 

 ing in peculiar capsular investments 

 of the terminals of the sensory nerve 

 fiber. Thus, we find that the threshold value of a stimulus applied 

 to the tactile capsule, is very much lower than that required to 



363. — Herbst Corpuscles of 

 Dtjck. 



n, Medullated nerve-fibre; a,, its 

 axis-cylinder, terminating in an en- 

 largement at end of core; c, nuclei of 

 cells of core; t, nuclei of cells of outer 

 tunica; i' , inner tunica (Sobotta) X 

 380 diameters. 



Fig. 364. — Krause's Corpuscle. A and B, Genital Corpuscles from the Clitoris of 

 THE Rabbit (Izquierdo) ; C, from the Human Clitoris. (W. Krause.) 



ehcit a sensation from the nerve fiber itself. It is evident, there- 

 fore, that the skin is in possession of what might be termed tactile 



