6 3 4 



TEXT-BOOK OF PHYSIOLOGY. 



Touch Sense. The area, stimulation of which evokes sensations of 

 touch is coextensive with the skin and that limited portion of the mucous 

 membrane lining the mouth. Careful stimulation of the skin by means of a 

 fine stiff bristle has revealed the fact, however, that the touch area is not 

 continuous, but discrete, presenting itself under the form of small areas or 

 spots, separated by relatively large areas insensitive to the same agent. 

 Stimulation of these spots always calls forth a sensation of touch. For this 

 reason they are known as " touch spots." The number of such spots in any 

 given area of skin varies considerably. Thus, in the skin of the calf fifteen 

 such spots have been counted in a square centimeter. In the palm of the 



hand from forty to fifty have been counted in an 

 area of the same extent. They are also especi- 

 ally abundant in the immediate neighborhood of 

 the hair-follicles. 



The peripheral end-organ associated with the 

 touch spots in the neighborhood of a hair- follicle is 

 in all probability the wreath of nerve-fibrils sur- 

 rounding the follicle. In regions devoid of hairs 

 the end-organ is the Meissner corpuscle, for in the 

 palmar surface of the distal phalanx of the index- 

 finger, where the touch sense is quite acute, about 

 20 corpuscles are present in each square millimeter 

 of surface. The specific stimulus necessary to 

 evoke the sensation of touch is a deformation of the 

 skin; and the greater this is, within physiologic 

 limits, the more pronounced is the sensation. 



Pressure Sense. The contact of an external 

 body is attended by a certain amount of pressure, 

 which, however, must attain a certain degree before 

 the sensation can be evoked. This is known as the 

 threshold value, or the degree of liminal intensity. 

 Since the sensations are the result of pressure, they are termed pressure 

 sensations, and their intensity may be expressed in terms of pressure. 



The sensitivity of the skin as determined by the pressure sense varies in 

 different regions of the body and in accordance with the size of the area 

 pressed. Thus, the liminal intensity of a stimulus for an area of nine square 

 millimeters for the skin of the forehead is 0.002 gram; for the flexor aspect 

 of the forearm, 0.003 gram; and for the hips, thigh, and abdomen, 0.005 

 gram; for the palmar surface of the finger, 0.019 gram; for the heel, i gram. 

 The delicacy of the sense of touch is measured by the slight increase or 

 decrease in the intensity of the stimulus that will produce an appreciable 

 change in the intensity of the sensation. Not all changes in the stimulus, 

 however, are attended by a change in the sensation. It has been deter- 

 mined that the latter will change only when the former changes in a definite 

 ratio, which for the volar surface of the third phalanx of the index-finger is as 

 29 is to 30. Thus, other things being equal, a sensation caused by a given 

 weight will only change with moderate stimulation when one-thirtieth of the 

 weight is either added or subtracted. The ratio of change, however, varies 

 in different regions of the body: thus, for the back of the hand the ratio 



FIG. 297. P ACINI AN 



CORPUSCLES, c. Capsules. 



d. Endotheiial lining sepa- 

 rating the latter, n. Nerve. 



minai fiber; and a. Where 

 ^splits up into finer fibrils. 



