686 TEXT-BOOK OF PHYSIOLOGY 



the afferent nerve is traced to the capsule, it is found to lose both its 

 neurilemma and its medulla, after which the naked fibril penetrates the 

 capsule, breaks up into a number of branches, and after pursuing a 

 more or less spiral course becomes connected with the tactile discs. 



4. Hair Wreaths. Just below the openings of the sebaceous glands the 



hair-follicles are surrounded by naked axis-cylinder fibrils in the form 

 of a wreath, which in all probability terminate in the cells of the external 

 root-sheath. These, too, are to be regarded as part of the touch 

 apparatus. 



5. Corpuscles of Vater or P acini. These are oval-shaped structures found 



along the nerves distributed to the palms of the hands and the soles 

 of the feet, on the nerves distributed to the external genital organs, to 

 joints and other structures. They consist of a thick capsule of lamel- 

 lated connective tissue in the interior of which is a bulb resembling 

 granular protoplasm. The axis-cylinder of the nerve-fiber enters the 

 capsule and becomes connected with the bulb. 



Other forms of peripheral organs are found in special regions of the skin 

 as well as in different animals. 



Touch Sense. The area, stimulation of which evokes sensations of 

 touch is coextensive with the skin and that limited portion of the mucous 

 membrane lining the mouth. Careful stimulation of the skin by means of a 

 fine stiff bristle has revealed the fact, however, that the touch area is not 

 continuous, but discrete, presenting itself under the form of small areas or 

 spots, separated by relatively large areas insensitive to the same agent. 

 Stimulation of these spots always calls forth a sensation of touch. For this 

 reason they are known as " touch spots." The number of such spots in any 

 given area of skin varies considerably. Thus, in the skin of the calf fifteen 

 such spots have been counted in a square centimeter. In the palm of the hand 

 from forty to fifty have been counted in an area of the same extent. They are 

 also especially abundant in the immediate neighborhood of the hair-follicles. 

 The peripheral end-organ associated with the touch spots in the neigh- 

 borhood of a hair-follicle is in all probability the wreath of nerve-fibrils 

 surrounding the follicle. In regions devoid of hairs the end-organ is the 

 Meissner corpuscle, for in the palmar surface of the distal phalanx of the 

 index-finger, where the touch sense is quite acute, about 20 corpuscles are 

 present in each square millimeter of surface. The specific stimulus neces- 

 sary to evoke the sensation of touch is a deformatio.n of the skin; and the 

 greater this is, within physiologic limits, the more pronounced is the sensation. 

 Pressure Sense. The contact of an external body is attended by a 

 certain amount of pressure, which, however, must attain a certain degree 

 before the sensation can be evoked. This is known as the threshold value, 

 or the degree of liminal intensity. Since the sensations are the result of 

 pressure, they are termed pressure sensations, and their intensity may be 

 expressed in terms of pressure. 



The sensitivity of the skin as determined by the pressure sense varies in 

 different regions of the body and in accordance with the size of the area 

 pressed. Thus, the liminal intensity of a stimulus for an area of nine square 

 millimeters for the skin of the forehead is 0.002 gram; for the flexor aspect 

 of the forearm, 0.003 gram; and for the hips, thigh, and abdomen, 0.005 

 gram; for the palmar surface of the finger, 0.019 gram; for the heel, i gram. 



