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TEXT-BOOK OF PHYSIOLOGY. 



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 lamellated 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 con- 

 nected with the bulb (Fig. 272). 



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, pre- 

 senting 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. 



-in , i 



The peripheral end-organ associated 



with the touch spots in the neighborhood 

 f a hair follicle is in all probability the 

 touch- wreath of nerve-fibrils surrounding the 

 Cells of follicle. In regions devoid of hairs the 

 end-organ js the MeIs|ner^rpu S cIeV^r 

 in the palmar surface of the last 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 necessary to evoke the sensation of touch is a 

 deformation 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 



FIG. 271. TOUCH CORPUS- 

 CLE OF MEISSNER AND 

 WAGNER, b. Papilla of 



Nerve-fiber in 

 coruscle, g. 



