840 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 

 two different objects. It is a not uncommon surgical operation to replace a 

 loss of skin on the nose by cutting a flap in the skin of the forehead, without 

 injury to the nerves, and sliding the flap round upon the nose. Touching 

 the piece of transplanted skin gives the patient the sensation of being touched, 

 not upon the nose, but upon the forehead ; after a time, however, a new fund 

 of experience is accumulated, and the sensation of contact with the transplanted 

 flap is rightly referred to the nose. Persons who have suffered amputation of 

 a lower limb often complain of cramps and other sensations in the lost toes. 

 The illusion no doubt comes from irritation, in the nerve-stump, of fibres 

 which previously bore irritations from the toes. 



2. Temperature Sense. The skin is also an organ for the detection of 

 changes of temperature in the outer world. Such temperature differences prob- 

 ably make themselves manifest by raising or lowering the temperature of the 

 skin itself, and thus in some way irritating the terminal parts of certain sensory 

 nerves, the temperature nerves. The sensitiveness of the skin to temperature 

 variations is not the same in all parts ; thus, it is more acute in the skin of the 

 face than in that of the hand ; in the legs and the trunk the sensibility is least. 

 We refer temperature sensations, somewhat like those of touch, to the periphery 

 of the body, and localize them on the surface. The skin over various parts 

 of the body may have different temperatures without exciting corresponding 

 local differences of sensation. Thus, the forehead and the hand usually seem 

 to be of the same temperature, but if the palm be laid upon the temples, 

 there is commonly felt a decided sensation of temperature change in one or 



