396 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



corresponding area of the skin of the opposite arm, but not by adjacent areas ; 

 in other words, the localizing power is central, not peripheral. Practice 

 aroused in both tactile areas a peculiar quality of sensation by which the 

 area was recognized. The improvement in localizing power is gradually lost 

 if unexercised. 



Pressure-points. — It has been found that if a light object, such as a lead- 

 pencil, be allowed to rest by a narrow extremity successively on different parts 

 of the skin, its weight will appear very different according to the part which 

 is touched. If the spots on which the weight appears greatest be marked with 

 ink, they will be found to have a constant position, and the skin may therefore 

 be mapped out in areas of pressure-point*, which are believed to indicate the 

 place of ending of pressure-nerve filaments. The pressure-points are rela- 

 tively few in number and are principally collected about the hair-follicles. 



The Importance of the End-organ. — The sense of touch or pressure is a 

 special sense ; that is, any irritation conveyed to the nerve-centres in which 

 the nerves of pressure terminate gives rise to a feeling of touch, just as dis- 

 turbance in the visual or the auditory centre is recognized in consciousness as 

 a sensation of sight or of sound. The complex anatomical structures known as 

 sense-organs may be considered as instruments each of which is differentiated 

 in a manner to make it particularly irritable toward some special form of 

 energy. Thus, the retina is most sensitive to the luminiferous ether; the organ 

 of Corti, to waves of endolymph, etc. To this differentiation of structure the 

 sensitiveness of the body to the forces of nature is chiefly due. The peripheral 

 ending of the pressure nerve, whether a naked axis-cylinder or a touch-corpus- 

 cle, is no doubt modified to be particularly irritable toward that form of energy 

 manifested in the molecular vibration of the tissue solids, brought about by 

 contact with foreign objects. Hairs, particularly those in certain localities of 

 some animals, as the whiskers of the cat, appear to have the function of trans- 

 mitting mechanical vibrations to the nerve-endings in greater intensity than 

 could be accomplished through the skin alone. 



No true sense of touch is aroused by direct irritation of a nerve-trunk or 

 exposed tissue, and touch-sensations do not arise from irritation of the internal 

 surfaces of the body. A fluid of the temperature of the body gives, when 

 swallowed, no sensation in the stomach ; when cooler or warmer than the 

 body, there is a sensation due, probably, to a transmission of temperature 

 change to the skin of the abdomen. 



Touch Illusions. — Certain peculiar errors in judgment may arise when 

 tactile sensations are associated in a manner unusual in experience. Thus, in 

 an experiment said to have been devised by Aristotle, if the forefinger and 

 the middle finger be crossed, a marble rolled between their tips will appear to 

 be two marbles; if the crossed finger-ends be applied to the tip of the nose, 

 there seems to be two noses. The illusion is due to the fact that under 

 ordinary circumstances simultaneous tactile sensations from the radial side of 

 the forefinger and the ulnar side of the middle finger are always caused by 



