534 OF SENSATION. 



2. Of the Sense of Touch. 



939. By the sense of Touch is usually understood that modification 

 of the common sensibility of the body, of which the surface of the skin 

 is the especial seat, but which exists also in some of its internal reflexions. 

 In some animals, as in Man, nearly the whole exterior of the body is 

 endowed with it, in no inconsiderable degree ; whilst in others, as the 

 greater number of Mammalia, most Birds, Keptiles, and Pishes, and a 

 large proportion of the Invertebrata, the greater part of the body is so 

 covered with hairs, scales, bony or horny plates, shells of various kinds, 

 complete horny envelopes, &c., as to be nearly insensible ; and the 

 faculty is restricted to particular portions of the surface, or to organs 

 projecting from it, which often possess a peculiarly high degree of this 

 endowment. Even in Man, the acuteness of the sensibility of the 

 cutaneous surface varies greatly in different parts, being greatest at the 

 extremities of the fingers and in the lips, and least in the skin of the 

 trunk, arm, and thigh. Thus the two points of a pair of compasses 

 (rendered blunt by bits of cork) can be separately distinguished by the 

 point of the middle finger, when approximated so closely as l-3d of a 

 line ; whilst they require to be opened so widely as 30 lines from each 

 other, to be separately distinguished, when pressed upon the skin over 

 the spine, or upon that of the middle of the arm or thigh. 



940. The impressions that produce the sense of touch are received 

 through the sensory papilla?, with which the surface of the true Skin is 

 beset, more or less closely, according to the part of it that is examined. 

 These papillae are minute elevations, which enclose loops of capillary 

 vessels (Fig. 157), and branches of the sensory nerves. With regard 



Fig. 157. 



Capillary network at margin of lips. 



to the precise course of the latter, there is some uncertainty ; but it is 

 probable, from analogy, that the representation given of them by 

 Gerber (Fig. 158) is in the main correct ; and that each loop of the 

 Sensory nerve is connected with one or more vesicular foci, on some 

 change in which the formation of the sensory impression is immediately 

 dependent ( 382). It is peculiar to the sense of Touch, and to that of 

 Taste (which is closely related to it) that the impression must be made 

 by the contact of the object itself with the sensory surface, and not 

 through any intermediate agency. The only exception to this is in 

 regard to the sense of Temperature, which seems to be in many 

 respects different from ordinary touch ; here the proximity of the warm 



