FIG. 171. 



this practice ceases, the delicacy of the sense of touch will decrease 

 also." The sensory circles of the blind are also smaller than of other 

 people due to practice.* 



The sense of touch is liable to many illusions which have to be cor- 

 rected by other senses. But if the oher senses are not available for 

 the purpose, as in the case of the blind, touch becomes exceedingly in-, 



FIG. 171. Illusion of the Sense of Touch. 



In the unaccustomed position of the fin- 

 gers in E, a single ball gives the two sep- 

 arate stimulations which it requires tioo 

 balls to give when the fingers are in their 

 habitual position, as in A. 



telligent. The well known illu- 

 sion resulting from crossing the 

 fingers and placing a pea, or other 

 small, round object between the 

 ends, so that it touches the left 

 side of one finger and the right 

 side of the other, is due to the 

 want of habit in those parts of the 

 fingers of touching a single object 

 of that shape simultaneously. It " shows how firmly the representation 

 of the surface of our body is imprinted upon our brain," for the brain, 

 receiving its accustomed sensation from the parts, adopts also its accus- 

 tomed perception of the facts, although false, unless it is modified and 

 corrected by another sense. 



Muscular Sense. Besides the sense or senses belonging to the skin, 

 there is another, called the muscular sense, which appears to have its 

 seat and organ in the muscles. This is the sense of resistance to mus- 

 cular effort. Thus, if a weight is lifted, besides the sense of pressure, 

 which the skin conveys to us, is that sense of the amount of the weight 

 which we will have independently of the pressure sense ; for the press- 

 ure on the skin of a two-pound weight will be no greater than that of 

 one pound, if it be shaped to cover double the surface ; while the mus- 

 cle will easily feel the difference. The muscular strain may depend on 

 any other cause as well as gravity, but the principle is the same, and 

 the strain may be expressed in terms of gravity, as it generally is in 

 mechanics. Like everything else in organic structure, this sense is in- 

 creased in mobility by use ; in other words, improved by habit, and of- 

 ten becomes very delicate in persons accustomed to handle things and 

 sell by weight. This sense is not to be confounded with the feeling 

 of fatigue which follows exhaustive effort, and which is a subjective 

 sensation, because it furnishes us with no information of objects outside 

 of ourselves, but only of a state existing within us ; while a true sense- 

 is an organ used and operated by an external energy. The sense of 



