TOUCH. 



1171 



physical qualities, concerning which we re- 

 ceive information through the sense of touch. 

 Sense of Temperature. This sense is 

 called into action when there is a difference 

 between the temperature of the sensory organ 

 and that of the surrounding medium, or of 

 substances with which it is specially brought 

 into contact. It is one of which the intensity 

 is determined, more perhaps than that of any 

 other sensation, rather by the relative than by 

 the absolute condition of the body which 

 excites it. Thus, if one hand be immersed 

 for a time in hot water, and the other in cold, 

 and both then be plunged into tepid water, 

 this will seem cool to the former and warm to 

 the latter. So, again, a person coming out of 

 cold air into an atmosphere of moderate tem- 

 perature, derives from it the feeling of genial 

 warmth, whilst another, coming into the very 

 same atmosphere from one much hotter, com- 

 plains of its chillness. Again, when the tem- 

 perature of different substances is compared 

 by the hand, the sense is not so much influ- 

 enced by the absolute amount of caloric 

 possessed by each, as by their power of 

 imparting cold or heat to the sensory organ. 

 Hence substances which are good conductors 

 (such as metals or marble) are felt to be 

 colder than those which conduct heat badly 

 (such as wood), although really of the same 

 temperature, because they draw off the heat 

 of the sensory surface more rapidly ; whilst, 

 on the other hand, it" both be warmer than 

 the sensory surface, the best conductors will 

 seem to be the hottest, because their caloric 

 is most readily imparted. Further, the sense 

 of temperature is influenced in a remarkable 

 degree by the extent of surface on which the 

 impression is made. Every one is familiar 

 with the fact that hot water in which a single 

 finger may be held without inconvenience, 

 will be felt intolerably scalding when the 

 whole hand is immersed in it. And it has 

 been shown by Professor Weber, that if one 

 vessel of water be heated to 98 and another 

 to 104 , and the whole of the hand be im- 

 mersed in the former, while the finger alone 

 is immersed in the latter, a wrong judgment 

 of their relative temperatures will be probably 

 given, that which is really the cooler being 

 pronounced the hotter, on account of the 

 larger extent of surface on which it acts. 

 This mistake was made in some of his experi- 

 ments, when the difference was as much as 

 eight degrees ; the cooler water being at 98, 

 and the hotter at 106, and yet the former 

 being esteemed the hotter. So, again, the 

 immersion of the entire hand enables minute 

 differences of temperature to be detected, 

 which could not be recognised by the immer- 

 sion of a single finger. By the former method, 

 a difference of only one-third of a degree may 

 be distinguished ; the entire hand being im- 

 mersed, repeatedly and successively, in two 

 vessels of water, differing only that much in 

 their relative warmth. But it is remarked by 

 Professor Weber, that these minute differ- 

 ences are best detected when the medium 

 examined does not fall short of, or exceed 



very considerably, the usual temperature of 

 the body ; just as the ear can best perceive a 

 difference of tone in sounds which are neither 

 very acute or very grave.* 



It is a remarkable fact, discovered by Pro- 

 fessor Weber, that the left hand is in most 

 persons more sensible to variations of tem- 

 perature than the right. Thus, when the 

 hands of a person lying in bed, and having 

 exactby the same temperature, are plunged 

 each in a separate vessel of hot water, the 

 left hand is believed to be in the hotter 

 medium, although the water in which it is 

 immersed is really one or two degrees colder 

 than the other. This difference is the more 

 remarkable, as the power of tactile discrimina- 

 tion is usually greater in the right hand ; and 

 it is attributed by Professor Weber to a 

 difference in the thickness of the epidermis, 

 the left hand usually having a thinner epi- 

 dermis than the right, especially in the palm, 

 because it is less used. But this will only 

 apply to the hand ; and since (as will be 

 presently shown) we possess a greater power 

 of discriminating pressures through the entire 

 surface of the left side than through that 

 of the right, it would seem much more pro- 

 bable that there is an original difference in 

 the tactile endowments of the two sides 

 respectively. There is certainly a strongly 

 marked difference between different parts of 

 the trunk in regard to their sensibility to tem- 

 perature, as is experienced by those who 

 sponge themselves over with cold water im- 

 mediately on leaving their bed in the morning. 

 In the writer's case, the parts most sensitive 

 to the cold are in the centre of the dorsal 

 region behind ; in front, between the lower 

 end of the sternum and the umbilicus ; and 

 the corresponding portions of the flanks. 

 These spots are among the parts of the in- 

 tegument least possessed of tactile discrimi- 

 nation ; and yet the cold sponge passing over 

 them seems to be much lower in temperature 

 than when it is applied to other parts. 



Some further experiments have recently 

 been made by Professor Weber, to determine 

 whether the sense of temperature is received 

 through any other channel than the sensory 

 apparatus contained in the integuments.f 

 The first means of which he availed himself 

 for deciding this question, was that afforded by 

 the results of accident or surgical operations, 

 in which a portion of skin has been left defi- 

 cient. Thus, in three cases in which a large 

 portion of the skin had been destroyed by a 

 burn, and in which healing had not advanced 

 so far as to renew the organ of touch, it was 

 found that no correct discrimination could be 

 made between two spatulas, one of them at a 

 temperature of from 48 to 54, the other of 

 from 113 to 122, which were brought into 



* He further remarks, that the comparison be- 

 tween two temperatures can be best made when the 

 impressions are not simultaneously made upon two 

 different parts, but are made in quick succession upon 

 the same part ; as mentioned hereafter to be the case 

 in regard to weights. 



t MUller's Archiv. 18-19. Heft, iv. s. 273283. 



4F 2 



