SENSE OF TOUCH. 751 



Habit and education enable us to appreciate with great nicety differences in weight ; 

 but this is chiefly due to the sense of resistance to muscular effort and has little depend- 

 ence upon the sense of touch. In the elaborate and classical experiments of Weber, this 

 point was very strikingly illustrated. The observations of this physiologist upon the 

 sense of touch and general sensibility were very varied and extensive ; and, among the 

 most important of the results with regard to the appreciation of pressure and weight, 

 are the following : 



In general, those parts which are most sensitive to the impressions of touch, as the 

 fingers, enable us to appreciate differences in pressure and weight with the greatest accu- 

 racy. The sense of simple pressure, unaided by the estimation of weight by muscular 

 effort, is generally more acute upon the left side, probably because the integument of the 

 left hand is thinner than that of the right hand. Differences in weight can be accurately 

 distinguished, when they amount to only one-sixteenth, by employing muscular effort in 

 lifting, as well as the sense of pressure ; but the sensa of pressure alone enables us to 

 appreciate a difference of not less than one-eighth. When weights are tested by lifting 

 with the hand, the appreciation of slight differences is more delicate when the weights 

 are successively tested with the same hand than when two weights are placed, one on 

 either hand. When the interval between the two trials amounts to more than forty sec- 

 onds, slight differences in weight the difference between fourteen and a half and fifteen 

 ounces, for example cannot be accurately appreciated. In such trials, it is necessary 

 to have the metals used of the same temperature, for cold metals seem heavier than 

 warm. 



These observations formularized some of the facts, sufficiently evident to every one, 

 relating to the appreciation of slight differences in weight. It is well known that experts 

 acquire, in this regard, wonderful delicacy and accuracy. Those who are in the daily 

 habit of handling coins not only count with astonishing rapidity, but are able to detect 

 and throw out a light piece instantly and with unerring certainty. 



Sense of Touch. 



We have already considered, in connection with the nervous system, the modes of 

 termination of the sensory nerves ; and, in many instances, it is possible to explain, by 

 the anatomical characters of the nerves, the great differences that have been observed in 

 the delicacy of the tactile sensibility in different parts differences which are exceedingly 

 important, pathologically as well as physiologically, and which have been studied by 

 Weber, Valentin, and others, with great minuteness. 



Variations in the Tactile Sensibility in Different Parts. In certain parts of the cuta- 

 neous surface, the general sensibility is much more acute than in others. For example, 

 a sharp blow upon the face is more painful than a similar injury to other parts ; and the 

 eye, as is well known, is most exquisitely sensitive. The appreciation of temperature also 

 varies in different parts, this probably depending to a great extent upon habitual exposure. 

 Some parts, as the soles of the feet or the axilla, are peculiarly sensitive to titillation. 

 The sense of touch, however, by which we appreciate the size, form, character of the 

 surface, consistence, etc., of objects, is developed to a greater degree in some parts than 

 in others; a fact which can be very readily explained, in some instances, by the ana- 

 tomical arrangements of the peripheral sensory nerves. When we wish to ascertain 

 those properties of objects revealed by the sense of touch, we generally employ the fin- 

 gers. This sense is capable of education and is almost always extraordinarily developed 

 in persons who are deprived of other special senses, as sight or hearing. The blind learn 

 to recognize individuals by feeling of the face. A remarkable instance of this is quoted 

 in works on physiology, of the blind sculptor, Giovanni Gonelli, who was said to model 

 the most striking likenesses entirely by the sense of touch. Other instances of this kind 



