138 SENSE OF TOUCH. 



able, may produce an unpleasant impression of cold. Under opposite 

 circumstances, a feeling of heat exists. 



In regard to the mode in which the temperature of bodies is appre- 

 ciated, there are peculiarities, which would favour the idea of the sense 

 of heat being distinct from that of tact or touch. Professor Weber, 

 for example, found that the left hand is more sensitive than the right, 

 although the sense of touch is more acute in the latter ; and that if the 

 two hands, at the time of like temperature, be plunged into separate 

 basins of water, the one in which the left hand is, will appear to be 

 the warmer, even although its temperature may be somewhat lower than 

 that of the other. It would seem, too, from Weber's experiments, that 

 in regard to sensations of heat and cold, a weaker impression made 

 upon a large surface appears more powerful than a stronger made upon 

 a small surface; and, accordingly, to judge of nice shades of difference 

 in the temperature of a fluid, the whole hand will enable a variation to 

 be detected, that would be inappreciable to the finger. A difference 

 of one-third of a degree it is affirmed, may be easily detected, when 

 the same hand is placed successively in two vessels of water, or any 

 other fluid. 1 



These and other phenomena of an analogous kind have led to the 

 suggestion, that every nerve of sensation is composed of several 

 nerves, each of which may have its special function ; and that the 

 nerves of touch comprise some which appreciate temperature, others, 

 which perceive the resistance of bodies, and others which effect 

 touch properly so called. In proof of this a recent writer urges that 

 either of these faculties may be lost, without the other being so. Thus, 

 when the arm has been " asleep," and sensibility is returning to it, 

 the hand first perceives temperature, then the resistance of bodies, 

 and it is not until some time afterwards that the faculty of touch, pro- 

 perly so called, is exercised. In the lower extremities the contrary 

 takes place; the sense of touch first returns; then a sensation of 

 pricking is experienced, followed by the perception of temperature, and 

 the power of appreciating resistance returns last. It may be added, 

 that many cases are recorded, in which the sense of temperature has 

 been lost, whilst the ordinary sense of tact remained; and, as remarked 

 by Dr. Carpenter, 2 it is an additional evidence in favour of the distinct- 

 ness of nervous fibres to convey the impressions of temperature, that 

 these are frequently affected, a person being sensible of heat or of 

 chilliness in some part of the body, without any real alteration of its 

 temperature, whilst there is no corresponding affection of the tactile 

 sensations. 



By tact we are likewise capable of forming a judgment of many of 

 the qualities of bodies, such as their size, consistence, weight, distance, 

 and motion. This faculty, however, is not possessed exclusively by the 

 sense in question. We can judge of the size of bodies by the sight ; of 

 distance, to a certain extent, by the ear, &c. To appreciate these cha- 

 racters, it is necessary, that the sense should be used actively; that we 

 should call into exercise the admirable instrument with which we are 



1 E. H. Weber, Art. Tastsirm und das Gemeingefiihl in Wagner's Handworterbuch der 

 Physiologie, 22ste Lieferung, s. 549. Braunschweig, 1849. 



a Principles of Physiology, 2d Amer. edit., p. 229. Philad., 1845. 



