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AN AMERICAS TEXT-BOOK OF I'll Ysioijx ; V 



Fig. 201.— Cutaneous " cold " spots 

 (vertical shading) and "hot" spots 

 (horizontal shading), anterior sur- 

 face uf the thigh (from Waller, after 

 Goldscheider). 



Cold ami Warm Points. — The -kin 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 

 sin face. These spots appear to be the places of 

 termination of the temperature nerves in the epi- 

 dermis ( Fig. 201 ). 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. 201). 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- 

 sphere of carbon dioxide surrouuding an area of skin gives a sensation of 

 warmth. 1 



The specific difference of the two sets of temperature nerves is indicated by 

 the fact that when a warm and a cold body held close together are simulta- 

 neously brought near the skin, the sensation is either one of both warmth and 

 cold, or now one and now the other sensation predominates. 2 Any stimulation, 

 whether mechanical or electrical, applied to the sensitive points thus far de- 

 scribed in the skin, for the appreciation of either pressure, heat, or cold, pro- 

 vokes, when effective, only the proper sensation of that point; any irritation 

 of a cold, hot, or pressure point gives rise, respectively, to the sensation of 

 cold, heat, or pressure alone. 



As in other organs of special sense, the peripheral terminations of the 

 temperature nerves seem modified to be especially irritable toward their appro- 

 priate form of physical stimulus. Cold or heat directly applied to the nerve- 

 trnnk excites no temperature sensation. Thus, if the elbow be dipped into a 

 freezing mixture, as the lowered temperature penetrates to the ulnar nerve the 

 sensation will be one, not of cold, but of dull pain, and it will be referred to 



1 Goldscheider : Du Bois-ReymoncFa Archiv fur Physiologie, 1886, 1887 ; Blix: Zeitxchrift far 



:ie, 1884; Donaldson: Mind, 1885, vol. xxxix. 



2 Czermak : Sitzwngsbt richtt </. Wiener Akad., 1855,8.500; King: Arb. d. physiol. Anstcdi zu 

 Leipzig, 1876, S. 168. 



