THE SENSES OF PRESSURE OR TOUCH 739 



It will be seen that the tongue, tips of the fingers and nose are 

 the most sensitive regions. Other areas are frequently beset with 

 hairs which tend to increase the intensity of the excitation in a per- 

 fectly mechanical way, because they act as levers upon the tactile 

 corpuscles lying in the immediate vicinity of their roots. Moreover, 

 the pressing down of their shafts tends to augment the displacement 

 of the surface layers. Hairs, therefore, tend to lower the threshold 

 value of the excitation and to impart to the latter a peculiar quality 

 which renders stroking movements and all laterally applied impacts 

 especially effective. Variations in tactile acuity may be produced 

 by increasing or decreasing the blood-supply, by the administration 

 of such drugs as morphin, strychnin and alcohol, and by training. 

 Thus, we find that the tactile sense is especially keen in blind persons 

 and in type-setters. 



Touch Illusions. — Weber conceived the skin as being subdivided 

 into a number of touch circles or, as Hermann has called them, touch 

 areas, within the boundaries of which the two points of an esthesiometer. 

 are perceived as one. The size of these fields differs, a fact which may 

 readily be deduced from the preceding table. It was assumed further 

 that every one of these touch points is represented in consciousness 

 by a local sign or quality which, however, does not retain a local 

 character but is projected outward to the area of the skin stimulated. 

 Furthermore, this sensation is not perceived as a rule in the form of a 

 simple deformation of the surface of the skin, but as an actual re- 

 production of the object. In many instances, this projection is even 

 extended to a point beyond the skin. Thus, we find that the peculiar 

 grating sensation produced when cutting into bone, is not referred 

 to the fingers, but to the knife itself. 



Our associations pertaining to tactile sensations, m^^y easily be 

 upset by subjecting them to unusual conditions. This fact is 

 typically illustrated by an experiment first described by Aristotle.^ 

 If the index and middle fingers of the right hand are crossed, a marble 

 rolled around between their tips in the palm Of the left hand will appear 

 as two. This illusion is due to the fact that the crossing of the fingers 

 brings two sets of tactile corpuscles together which are ordinarily 

 far removed from one another and are rarely called upon to act in 

 unison. Consequently, the corpuscles upon the radial side of the 

 index finger, as well as those upon the ulnar side of the middle finger, 

 give rise to separate sensations. Quite similarly, it has been observed 

 that the tactile sensations obtained from a flap of the skin of the fore- 

 head which has been turned downward to cover a defect of the nose, 

 are at first referred to the forehead. Later on, however, new judg- 

 ments are formed, which enable the individual finally to localize these 

 sensations correctly. If the tip of the nose is palpated between the 

 tips of two crossed fingers, it appears as two. 



1 Metaphysics, iii, Chapter 6. 



