TOUCH, ETC. 17 



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 seconds, slight differences in weight, for example, the 

 difference between fourteen and a half and fifteen ounces, 

 cannot 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. 



These observations formularized some of the facts, suffi- 

 ciently 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 the volume on the ner- 

 vous system, the modes of termination of the sensory nerves ; 1 

 and, in many instances, it is possible to explain, by the ana- 

 tomical 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 impor- 

 tant, 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 cutaneous surface, the general sen- 

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

 sharp blow upon the face is more painful than a similar in- 

 jury to other parts ; and the eye, as is well known, is most 



1 See vol. iv., Nervous System, p. 36, et seq* 



