SENSATIONS OF TEMPERATURE. 543 



of the impression passes away and there no longer exists any 

 sensation with which the new stimulation can be compared. 



At best we can form but imperfect judgments of pressure by 

 the skin impressions alone. When we want to judge the weight 

 of a body we poise it in the free hand, which is moved up and 

 down so as to bring the muscles which elevate it into repeated 

 action. Hereby we call into action a totally different evidence, 

 namely, the amount of muscle power required to raise the weight 

 in question, and we find we can arrive at much more accurate 

 conclusions by this means. The peculiar recognition of how 

 much muscular effort is expended is commonly spoken of as 

 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 be- 

 cause of these variations the powers of judgment of high or low 

 temperature must be imperfect. The skin feels hot when its 

 bloodvessels 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, 



