582 THE HUMAN BODY. 



it touches many distinct nerve areas ; if we could discriminate 

 the excitations of each of these from that of its immediate 

 neighbors we would get the sensation of a series of points 

 touching us, one for each nerve region excited ; but in the 

 absence of intervening unexcited nerve areas the sensations are 

 fused together. 



The ultimate differentiation of tactile areas takes place in 

 the central organs, as will be more fully pointed out in the 

 next chapter. Afferent nerve impulses reaching the spinal 

 cord from a finger-tip enter the gray matter and tend to 

 spread or radiate in it; from the gray region through which 

 they spread, impulses are sent on to perceptive tactile centres, 

 in the brain ; if two skin-points are so close that their regions 

 of irradiation in the cord overlap, then the two points touched 

 cannot be discriminated in consciousness, since the brain region 

 excited is in part common to both. 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 travelled, 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 and warmth ; 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 upper part of gullet, and 

 the entry of the nares. /Direct heating or cooling of a sensory 

 nerve may stimulate it and cause painjbut not a trite 'tem- 

 perature sensation; and the amount oTheat 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 



