THE LOCAL SIGN 463 



first made by Meissner, illustrates the same point. If the hand be dipped 

 into water or mercury at the temperature of the skin, no sensation is produced 

 in any part of the skin submerged so long as the hand is kept perfectly still 

 and contact with the vessel is avoided. But a sense of pressure is felt at the 

 boundary line between air and liquid. 



A weight allowed to rest upon the skin for a long time, is felt continuously, 

 but with less and less distinctness as time goes on. Should the weight be very 

 small, it may be felt only at the moment of its application. Removal of the 

 weight is not of course appreciated unless it is heavy enough to be felt all the. 

 time it is present. Often the sensation outlasts the stimulus, probably owing 

 to the persistence of the deformation in the skin. 



Kiesow gives the following data with regard to the sensitiveness of the dif- 

 ferent parts of the skin. The relative strength of the tetanizing induction cur- 

 rent necessary for the threshold stimulus is: on the tip of the tongue, 1; lips, 

 1.1; anterior half of the tongue, 5; tips of the fingers, 14-17; tip of the thumb, 

 19-21 ; edge of the kneepan, 21 ; styloid process of the ulna, 34-37. 



Stimulating the skin of the frog with pressure, Steinach was able to observe 

 an action current in the corresponding nerves, the strength of which was found 

 to depend upon that of the stimulus. 



3. THE LOCAL SIGN 



A person pricked on the skin with the point of a needle can tell with the 

 eyes closed exactly where the needle is applied. This ability to refer a cutane- 

 ous stimulus to the correct place is often described for brevity as the sense 

 of location,, but is better described as the power of localization. E. H. Weber, 

 who was the first to investigate this sense with any completeness,, applied the 

 two points of a draughtsman's compass to the skin and determined the least 

 distance from each other at which they could be distinguished by the subject 

 as two distinct points when applied to different parts of the skin. The less 

 this distance was found to be the greater was the ability of the skin to localize 

 the stimulus accurately. 



The following are some of Weber's results, given in millimeters: tip of the 

 tongue, 1; tips of the fingers, 2; lips, 4.5; dorsal side of the third joint of the 

 fingers, 7; side of the tongue, 9; outer surface of the eyelids, 11; dorsal side of 

 the first finger joint, 16; brow, 23; back of the hand, 31; sternum, 45; middle 

 of the back, 68. 



We see that the distance is greatest on the trunk and decreases more and 

 more as we pass toward the ends of the extremities. It is least on the tips of 

 the fingers (neglecting the tip of the tongue) i. e., just where the skin is 

 used for the most delicate touch and where the discernment of slight intervals 

 between objects is most necessary. 



Our ability to distinguish slight intervals of space with the skin is, how- 

 ever, not quite so limited as it might appear. In the first place as we know 

 from many experiences, it can be improved by practice:, in the second place 

 slight intervals are much more sharply distinguished, if the two places on 

 the skin are stimulated not simultaneously, but successively ( Judd, v. Frey) ; 

 and in the third place, this ability is much greater when two isolated pressure 



