836 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



dermis of the conjunctiva and the lips, and in modified form on the sensitive 

 surfaces of the genital organs (Fig. 284). The touch-corpuscles, though appar- 

 ently absent from the greater part of the body, occur in great numbers in the 

 skin of the palmar surface of the hand and that of the fingers, especially at 

 their tips ; at the edge of the eyelids and the lips ; on the soles of the feet 

 and the toes ; and on the surface of the genital organs. The touch-corpuscle 

 often occupies a papilla of the dermis directly under the epidermis (Fig. 285). 

 The Pacinian bodies, which are oval corpuscles, larger than the foregoing, 

 and easily visible to the unaided eye, are found not in the skin proper, but 

 in the subcutaneous connective tissue beneath it. They are found in abundance 

 beneath the skin of the palm of the hand and the sole of the foot ; they are 

 also numerous along the nerves of the joints, and even among the sympathetic 

 nerves supplying the abdominal organs (Fig. 287). Sensory nerves also end 

 in tendons as somewhat arborescent expansions of axis-cylinder matter known 

 as the organs of Golgi, and in muscles near their tendinous attachments. 



1. Sense of Touch. The Relations between Sensation and Stimulus. 

 Many so-called " tactile sensations," such as wetness, hardness, roughness, etc., 

 are not simple sensations at all, but are complex judgments built up out of the 

 association of certain tactile, temperature, and muscular sensations, and con- 

 veying to us a knowledge of the surface, substance, and form of bodies. 



When analyzed, the sense of touch is nothing more than a sense of pressure 

 applied to the skin. To test the pressure sensibility of the skin the object 

 whose weight is to be estimated must not be lifted in the ordinary way, for 

 that would bring into play the muscular sensations. If the skin of the hand 

 is to be tested, the hand must be placed upon some firm support, such as a 

 table, and the weights be laid upon the skin. The smallest perceptible weight 

 that can thus be felt varies with the situation to which it is applied. Thus, 

 the greatest sensitiveness to pressure is found on the forehead, the temples, 

 the back of the hand, and the forearm, where a weight of .002 gram (-g^ 

 grain) can be perceived. The weight must be increased to .005 to .015 gram 

 to be felt by the fingers, and to 1.0 gram when laid on the finger-nail. 1 



The power of discriminating differences of pressure applied to the skin is 

 tested by finding the smallest increase that must be added to a weight in order 

 that it may be perceived as being heavier. This increment is not, as might 

 be supposed, the same for weights of different value, but it bears a distinct 

 proportion to them. Thus, a weight of 11 grains may just be perceptibly 

 heavier than one of 10 grains; but if we start with a weight of 100 grains, 

 a single grain added to it will arouse no difference of sensation, an increment 

 of 10 grains being necessary in order that one weight may appear heavier 

 than the other. This fact is the basis for Weber's law of the relation between 

 stimulus and sensation ; this law may be formulated as follows : The amount 

 of stimulus necessary to provoke a perceptible increase of sensation always bears 

 the same ratio to the amount of stimulus already applied. This law is found 

 to be only approximately correct, especially when very small and very large 

 1 Aubert and Kammler: Molescholt's Untersuchungen, 1859, vol. v. p. 145. 



