686 THE SENSES 



the temperature of our skin is the standard; and as this varies from hour to 

 hour according to the activity of the cutaneous circulation, our estimate of 

 the absolute temperature of any body must necessarily vary too. If we put 

 the left hand into water at 5 C. (40 F.) and the right into water at 45 C. 

 (110 F.), and then immerse both -in water at 27 C. (80 F.), it will feel warm 

 to the left hand, but cool to the right. Again, a piece of metal which has 

 really the same temperature as a given piece of wood will feel much colder, 

 since it conducts away the heat much more rapidly. For the same reason 

 air in motion feels very much cooler than air of the same temperature at rest. 



In some cases we are able to form a fairly accurate estimate of absolute 

 temperature. Thus, by plunging the elbow into a bath, a practiced bath- 

 attendant can tell the temperature sometimes within half a degree 

 centigrade. 



The temperatures which can be readily discriminated are between 10 and 

 45 C- (50 and 115 F.); very low and very high temperatures alike produce 

 a burning sensation. A temperature appears higher according to the extent 

 of cutaneous surface exposed to it. Thus, water of a temperature which 

 can be readily borne by the hand is quite intolerable if the whole body be 

 immersed. 



The delicacy of the sense of temperature coincides in the main with 

 that of touch, though at the elbow where the skin is thin, and the sense of 

 temperature is delicate, that of touch is not remarkably so. Weber has 

 further ascertained two points so near together on the skin that they pro- 

 duce but a single impression, at once give rise to two sensations when one 

 is hotter than the other. Moreover, of two bodies of equal weight, that 

 which is the colder feels heavier than the other. 



As every sensation is attended with a perception and leaves behind it an 

 idea in the mind which can be reproduced at will, we are enabled to compare 

 the idea of a past sensation with another sensation really present. Thus we 

 can compare the weight of one body with another which we had previously 

 felt, of which the idea is retained in our mind. Weber was indeed able to 

 distinguish in this manner between temperatures experienced one after 

 the other, better than between temperatures to which the two hands were 

 simultaneously subjected. This power of comparing present with past sensa- 

 tions diminishes, however, in proportion to the time which has elapsed between 

 them. After-sensations left by impressions on nerves of common sensibility 

 or touch are very vivid and durable. As long as the condition into which 

 the stimulus has thrown the organ endures, the sensation also remains, 

 though the exciting cause should have long ceased to act. Both painful and 

 pleasurable sensations afford many examples of this fact. 



Sense of Pain. As regards painful sensations, three views can be taken, 

 i, That it is a special sensation provided with a special conducting apparatus 

 in each part of the body; 2, that it is produced by an over-stimulation of the 



