SENSE OF TOUCH. 655 



the left side. 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 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 be successively tested with the same hand than 

 when two weights are placed, one on either hand. When the interval be- 

 tween 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 grammes), for example can not 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. 



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 great differences that have been 

 observed in the delicacy of the tactile sensibility in different parts differ- 

 ences which are very 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 (Sense of Local- 

 ity of Impressions). In certain parts of the cutaneous 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 peculiarly sensitive. The appreciation of tempera- 

 ture 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, also, by which the size, 

 form, character of the surface, consistence etc., of objects are appreciated, 

 is developed to a greater degree in some parts than in others. The tips of 

 the fingers generally 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 

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

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

 instances of this kind are on record. The blind have been known to become 

 proficients in conchology and botany, guided entirely by the touch. It is 

 related of a blind botanist, that he was able to distinguish ordinary plants 

 by the fingers and by the tip of the tongue. It is well known that the blind 

 learn to read with facility by passing the fingers over raised letters but little 

 larger than the letters in an ordinary folio Bible. 



An easy method of determining the relative delicacy of the tactile sen- 

 sibility of different portions of the cutaneous surface was devised a num- 



