THE DELICACY OF TOUCH. 365 



devised for testing the relative sensibility of different parts of the skin, 

 in cases of more or less complete anaesthesia, or paralysis of sensation. 



There is no necessary relation between the delicacy of the tactile 

 sense in a part, and its common sensibility, for in some regions of the 

 skin, where the perception of tactile impressions is very perfect, com- 

 mon sensibility is much less marked than it is in other regions, where 

 the sense of touch is much less acute. In the soles of the feet, the 

 arm-pits, and the flanks, parts endowed with but little tactile sensi- 

 bility, the sensation of tickling can be most easily excited; but on the 

 ends of the fingers, where the delicacy of touch is highly developed, it 

 is difficult to produce tickling. 



Experiments have been made to determine the relative sensibility of 

 the skin, by placing weights on different portions of the body, and 

 ascertaining what is the minimum weight capable of exciting a tactile 

 impression. It was found, by Aubert and Kammler, that a body, 

 weighing two milligrammes, and covering one square millimetre, could 

 be distinguished upon the face; whereas, on the pulp of the fingers, a 

 body occupying the same area, must weigh from ten to fifteen milli- 

 grammes, in order to produce a distinct sensation of pressure. But 

 the sensibility of the face was considerably diminished, when the 

 minute hairs covering its surface were shaved off. The left half of 

 the body is said to be, as a rule, better able to appreciate weights by 

 their pressure on the skin, than the right half. The sensation of 

 pressure in different parts of the skin, does not exhibit such marked 

 differences as that of the sense of space. Increase of pressure is more 

 easily perceived than a diminution of the same. (Panum and Dohrn.) 

 It is easier to distinguish small differences with light weights, than 

 with heavy ones. (Weber and Fechner.) Slight differences in two 

 weights, can be more easily discriminated, when these are placed on 

 the same part of the skin a little time after each other, than when they 

 are simultaneously applied close together. 



Pressure only does not, however, convey to the mind a correct idea 

 of weight, for bodies appear heavier when their pressure is made to 

 act on a small surface of the skin than when it is extended over a 

 larger area. The sensation of the muscular effort required to resist 

 the pressure, is a much more important guide in judging of the weights 

 of bodies. Thus Weber has shown, that if the eyes are closed, and 

 two weights, one of which is somewhat heavier than the other, be 

 placed, one on either hand, we are unable to appreciate any difference 

 between them, so long as the hands are supported on cushions; but 

 the moment the hands are raised, the muscular effort thus made, im- 

 mediately informs us of a difference between the weights. The mus- 

 cular sense is here brought into play. 



As already mentioned, the mere contact of an object with the organ 

 of touch only conveys the notion of resistance; but a notion of the 

 extent of surface of an object is arrived at by alterations in the rela- 

 tive position of the organ of touch and the object touched, and in this 

 we are also assisted by the muscular sense, which gives us a knowledge 

 of the positions of the arm and hand. 



The knowledge of the dimensions, shape of surface, and distance o 



