SENSATIONS OF TEMPERATURE. 545 



muscle sense, which may arise from a knowledge of how much 

 voluntary impulse is expended in exciting the muscles to action, 

 but more probably it depends upon afferent impulses arriving at 

 the sensorium from the muscles. By its means we aid the pres- 

 sure sense in arriving at accurate conclusions of the weight of 

 bodies, so that in the free hand we can distinguish between 39 

 grm. and 40 grm. 



TEMPERATURE SENSE. 



We are able to judge of the differences in temperature of bodies 

 which come in contact with our skin. Since our sensations have 

 no accurate standard for comparison, we are unable to form any 

 exact conception of the absolute temperature of the substances 

 we feel. The sensation of heat or cold, derived from the skin 

 itself, without its coming into contact with anything but air of 

 moderate temperature, varies with many circumstances, and 

 because of these variations the powers of judgment of high or 

 low temperature must be imperfect. The skin feels hot when 

 its blood vessels are full ; it feels cold when they are comparatively 

 empty. An object whose temperature is the same can thus give 

 the impression of being hot or cold, according as the skin itself is 

 full or empty of warm blood. But, independent of any very 

 material change in the blood supply of the cutaneous surface of 

 a part, any change in the temperature of its surroundings causes 

 a sensation of change of temperature, which is, however, a purely 

 relative judgment. Thus, if the hand be placed in cold water, 

 we have at first the sensation of cold ; to which, however, the 

 skin of the hand soon becomes accustomed so as no longer to 

 excite the sensation of cold ; if, now, the hand be placed in water 

 somewhat warmer but not higher in temperature than the 

 atmosphere we have a feeling of warmth. If the hand be now 

 placed in as hot water as the skin can bear, it feels at first un- 

 pleasantly hot, but this feeling soon passes away and the sensation 

 is comfortable. If, now, from this hot water it be placed again in 

 the water of the air temperature, this which before felt warm 

 now feels very cold. 



An important item in the estimation of the temperature of an 



