THE SENSE OF TOUCH. 



589 



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 determined 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, how- 

 ever, varies in different regions of the body : 

 thus, for the back of the hand the ratio 

 varies from one-tenth to one- twentieth ; for 

 the tongue, one-thirtieth to one-fortieth. 

 The difference of stimulus necessary to 

 evoke a sensation is known as the threshold 

 difference. It seems to be a law not only 

 for the skin, but for other senses as well, 

 that a change in the intensity of a sensation, 



to an appreciable extent, will occur only when the objective stimulus 

 changes in a definite ratio. This ratio, however, will vary not only 

 in different regions of the skin, in different individuals, but with the 

 sense-organ investigated. 



Place Sense. The sensation evoked by stimulation of the skin 

 is always, under normal conditions, referred to the place stimulated. 

 This holds true not only for two or more points near or widely sepa- 

 rated on the same side, but also for corresponding points on 

 opposite sides of the body, even when the stimuli have the same 

 intensity and are simultaneously applied. The cause for this localiz- 

 ing power is to be found in a difference in the quality of the sensation 



FlG . 272. P ACINI AN 



CORPUSCLES, c. Cap- 

 sules, d. Endothelial 

 lining separating the 

 latter, n. Nerve. /. 

 Funicular sheath of 

 nerve. m. Central 

 mass. n'. Terminal 

 fiber; and o. Where 

 it splits up into finer 

 fibrils. 



