THE SENSE OF TEMPERATURE. 



841 



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

 part in the sense perceptions of temperature, aud 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. 



Cold and Warm Points. The skin is not uniformly sensitive to tem- 

 perature changes, but its appreciation of them seems to be limited to certain 

 points distributed more or less thickly over the 

 surface. These spots appear to be the places of 

 termination of the temperature nerves in the epi- 

 dermis (Fig. 288). There is little doubt that there 

 are two distinct varieties of temperature nerves, 

 one of which appreciates elevation of temperature, 

 or heat, and the other diminution of temperature, 

 or cold. Thus, if a blunt-pointed metal rod be 

 warmed and be touched in succession to various 

 parts of the skin, at certain spots it will be felt as 

 very warm, while at others it will not seem warm 

 at all. If, on the contrary, the rod be cooled, a 

 series of cold points may in the same way be made 

 out. The point of an ordinary lead-pencil may 

 be used with some success to pick' out the cold 

 spots. The " cold points " are more numerous 

 than the "hot," and those of each variety are 

 more or less distinctly grouped round centres, as 

 would be expected from the manner of nerve-distribution, though the groups 

 overlap to some extent (Fig. 288). Certain substances appear to act, prob- 

 ably by chemical means, as specific excitants of the two sets of nerves. 

 Thus, menthol applied to the skin gives a sensation of cold, while an atmo- 

 1 Nothnagel : Deutsche. Archivfur klinisehe Medicin, 1866, ii. S. 284. 



FIG. 288 Cutaneous " cold " spots 

 (vertical shading) and " hot " spots 

 (horizontal shading), anterior sur- 

 face of the thigh (from Waller, after 

 Goldscheider). 



