THE SENSE OF TEMPERATURE. 397 



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 someway 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 

 both surfaces. As in other sensations, fatigue and contrast play an important 

 part in the sense perceptions of temperature, and stimuli of rapidly-changing 

 intensity provoke the strongest sensations ; thus, when two fingers are both 

 dipped into hot or cold water, the fluid seems hotter or colder to that finger 

 which is alternately raised and lowered. 



In changing to a place of different temperature the skin for a time seems 

 warmer or cooler, but soon the temperature sensation declines, and on return- 

 ing to the original temperature the reverse feeling of cold or of warmth is 

 experienced. For every part of the skin, then, there is a degree of tempera- 

 ture, elevation above or depression below which arouses respectively the 

 feeling of warmth or of cold, and the temperature of the skin determining 

 the physiological null-point may vary within wide limits. 



The smallest differences of temperature that can be perceived fall, for most 

 parts of the skin, within 1° C. The skin of the temples gives perception of 

 differences of 0.4°-0.3° C. The surface of the arm discriminates 0.2°; the 

 hollow of the hand, 0.5°-0.4° ; the middle of the back, 1.2 . 1 



The size of the sensory surface affected modifies the intensity of temperature 

 sensation : if the whole of one hand and a single finger of the other hand be 

 dipped into warm or cold water, the temperature will seem higher or lower to 

 the member having the greatest surface immersed. 



1 Nothnagel : Deutsehes Archivfur klinische Medicin, L866, ii. S. 284. 



