392 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



structures of various forms, which may be regarded as peripheral sense-organs 

 of the skin. 1 Some of these terminal organs are known respectively as end- 

 bulbs, touch-corpuscles, and Pacinian bodies (Figs. 197-200). Each organ 

 consists of a more or less conical body in which a nerve-fibre terminates. 

 The end-bulbs are found only on the dermis of the conjunctiva and the lips, 

 and in modified form on the sensitive surfaces of the genital organs ( Fig. 

 197). The touch-corpuscles, though apparently 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 eve- 

 lids 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. 198). 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 con- 

 nective 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. 200). " Ruffini's endings, found in the subcu- 

 taneous tissue of the finger, are formed by the branching and anastomosis of 

 terminal axis-cylinders inclosed within a special connective-tissue envelope. 

 Various other modifications of sensory nerve termination have been described. 



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 (^ 

 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. 2 



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, 



1 Cf. Barker : The Nervous System, 1899, pp. 361-421. 



2 Aubert und Kammler : Moleschotfs Untersvchungen, 1809, Bd. v. S. 145. 



