SENSE OF TOUCH 627 



touch, as the fingers, enable one to appreciate differences in pressure 

 and weight with greatest accuracy. The sense of simple pressure, un- 

 aided by the estimation of weight by muscular effort, usually is more 

 acute on the left side. Differences in weight can be accurately distin- 

 guished when they amount to only one-sixteenth, by estimating the mus- 

 cular effort in lifting, in addition to the sense of pressure ; but the sense 

 of pressure alone enables most persons 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 if the weights are 

 successively tested with the same hand than when two weights are 

 placed, one in either hand. When the interval between the two trials 

 is more than forty seconds, slight differences in weight the difference 

 between fourteen and a half and fifteen ounces (411 and 425 grams), for 

 example can not be accurately appreciated. In such trials, it is neces- 

 sary to have the metals used of the same temperature, for cold metals 

 seem heavier than warm. 



SENSE OF TOUCH 



The different modes of termination of the sensory nerves have 

 already been described; and in many instances it is possible to explain, 

 by the anatomical characters of the nerves, the differences that have 

 been observed in the delicacy of the tactile sensibility and sense of con- 

 tact in different parts differences which are very important patho- 

 logically as well as physiologically. 



Variations in the Sense of Contact in Different Parts (Localization 

 of Impressions}. In certain parts of the cutaneous surface the general 

 sensibility is much more acute than in others. For example, a sharp 

 blow on the face is more painful than a similar injury to other parts; 

 and the eye, as is well known, is peculiarly sensitive. The appreciation 

 of temperature varies in different parts, this probably depending to a 

 great extent ' on habitual exposure. Some parts, as the soles of the 

 feet or the axilla, are peculiarly sensitive to titillation. The sense of 

 touch, also, by which the size, form, character of the surface, consistence 

 etc., of objects are appreciated, is developed in a greater degree in some 

 parts than in others. The tips of the fingers commonly are used to 

 ascertain those properties of objects revealed by the sense of touch. 

 This sense is capable of education and is almost always extraordinarily 

 developed in persons who are deprived of some other special sense, 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 physi- 

 ology, of the blind sculptor, Giovanni Gonelli, who was said to model 

 excellent likenesses, being guided by the sense of touch alone. Other 



