730 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



Thus on lifting a cloth in which weights were concealed, Weber found that 

 ten people correctly indicated that a weight of 80 ounces was heavier than one 

 <of 78 ounces. Both Weber and Fechuer 1 appear to have satisfactorily shown 

 that with light weights smaller differences are perceived than with heavy 

 weights. Thus supposing that ounce weights are distinguishable in the ratio 

 of 29 to 30, i.e., that a weight of 29 ounces can be discriminated when at- 

 tentively examined from one of 30 ounces, a whole ounce requiring to be 

 added or subtracted before certainty is attained ; when drachms are em- 

 ployed the same ratio still subsists, so that in this instance the removal or 

 addition of one drachm to a weight of 30 drachms can be accurately appreci- 

 ated. Meissuer has drawn attention to the fact, which must be familiar to 

 most persons, that if the hand or foot be immersed in warm water or mer- 

 cury, the only sensation experienced is that of a ring surrounding the limb 

 at the surface of the fluid. This effect is, as he suggests, probably due to 

 the circumstance that the pressure is applied in such cases with perfect uni- 

 formity over the part immersed, and therefore causes no disturbance of the 

 nervous elements, the tactile sense being only excited at the line where the 

 pressure produced by the fluid is exchanged for that of the air. Valentin 2 

 has endeavored to ascertain the duration of a momentary excitation of the 

 sense of Touch by rolling a spiked wheel over the skin; when slowly rotated, 

 the contact of each spike was clearly discerned; but when only B ] () th of a 

 second intervened between the successive blows, they could no longer be dis- 

 tinguished. That the conditions under which certain of the modifications 

 of common sensation operate, are in some respects different from those of 

 ordinary Touch, is very easily shown. Thus, the feeling of tickling is ex- 

 cited most readily in parts which have but a low tactile sensibility, namely, 

 the armpits, flanks, and soles of the feet; whilst in the points of the fingers, 

 whose tactile sensibility is most acute, it cannot be excited. Moreover, the 

 nipple is very moderately endowed with ordinary sensibility; yet by a par- 

 ticular kind of irritation, a very strong feeling may be excited through it. 

 Again, in regard to Temperature, it is remarked by Weber that the left 

 hand is more sensitive than the right; although the sense of touch is un- 

 doubtedly the most acute in the latter. He states that if the two hands, 

 previously of the same temperature, be plunged into separate basins of warm 

 water, that in which the left hand is immersed will be felt as the warmer, 

 even though its temperature is somewhat lower than that of the other. In 

 regard to the sensations of heat and cold, he points out another curious fact, 

 that a weaker impression made on a large surface, seems more powerful 

 than a stronger impression made on a small surface; thus, if the forefinger 

 of one hand l)e immersed in water at 104, and the whole of the other hand 

 be plunged in water at 102, the cooler water will be thought the warmer; 

 whence the well-known fact, that water in which a finger can be held, will 

 scald the whole hand. Hence it also follows, that minute differences in 

 temperature, which are imperceptible to a single finger, are appreciated by 

 plunging the whole hand into water; in this manner a difference of one-third 

 of a degree may readily be detected, when the same hand is placed succes- 

 sively in two vessels. The judgment is more accurate, when the temperature 

 is not much above or much below the usual heat of the body; just as sounds 

 are best discriminated, when neither very acute nor very grave. 



597. Whether the channels for the conduction of sensations of pain, tick- 

 ling, and temperature are distinct from those of touch is still doubtful. The 



1 Elementc der Psychophysik, 1800. 



2 Uebor die Duuer dcr Tusteindriicke, A_rchiv f. Physiol. Heilk., Bd. xi, p. 438. 



