562 THE HUMAN BODY. 



sciousness, since the same brain regions are excited. The 

 more powerful the stimulus the wider the irradiation in 

 the cord, and hence the less accurate the discriminating 

 power. The more often an impulse has traveled, the more 

 does it tend to keep its own proper tract through the gray 

 matter of the cord, and get on to its own proper brain- 

 centre alone, hence the increase of tactile discrimination 

 with practice, for we cannot suppose it to be due to a growth 

 of more nerve-fibres down to the skin, and a rearrangement 

 of the old, with smaller areas of anatomical distribution. 

 As a general rule, more movable parts have smaller tactile 

 areas; this probably depends on practice, since they are the 

 parts which get the greatest number of different tactile- 

 stimulations. 



The Temperature Sense. By this we mean our faculty 

 of perceiving ^cold ancLgarjnth; ,and, with the help of these 

 sensations, of perceiving temperature differences in external 

 objects. Its organ is the whole skin, the mucous membrane 

 of mouth and fauces, pharynx and gullet, and the entry of 

 the nares. Direct heating or cooling of a sensory nerve may 

 stimulate it and cause pain, but not a true temperature 

 sensation; and the amount of heat or cold requisite is much 

 greater than that necessary when a temperature-perceiving 

 surface is acted upon; hence we must assume the presence 

 of temperature end-organs. 



In a comfortable room we feel at no part of the Body 

 either heat or cold, although different parts of its surface 

 are at different temperatures; the fingers and nose being- 

 cooler than the trunk which is covered by clothes, and this, 

 in turn, cooler than the interior of the mouth. The tem- 

 perature which a given region of the temperature organ 

 has (as measured by a thermometer) when it feels neither 

 heat nor cold is its temperature-sensation zero, and is not 

 associated with any one objective temperature; for hot only, 

 as we have just seen, does it vary in different parts of the 

 organ, but also on the same part from time to time. 

 Whenever a skin region has a temperature above its sensa- 

 sation zero we feel warmth; and vice versa: the sensation is 

 more marked the greater the difference, and the more 



