588 A TEXTBOOK OF PHYSIOLOGY 



hair being known, the threshold value of the sensation of pressure 

 can be accurately determined. For ordinary clinical purposes, the 

 presence or absence of sensation to the touch of cotton-wool is usually 

 employed. To test relative sensitiveness or the power to discriminate 

 the part touched in various parts of the skin, the distance between 

 the points of a pair of compasses is measured when the points are just 

 sensed as touching two places. Temperature sensations may be 

 tested with a hollow pencil-shaped rod in which water of various 

 temperatures is circulated. A more common method is to use the 

 bottom of test-tubes containing water at various temperatures. Pain 

 may be tested with the prick of a needle-point. 



The Sensations of Touch or Pressure. Pressure-points are closely 

 related to the distribution of hairs, each hair having a pressure-point 



FIG. 330. SENSOKY NEKVB END-ORGAN IN EXTERNAL EPITHELIUM OF PIG'S SNOUT. 

 (Redrawn after Retzius, from Dahlgren and Kepner.) 



on the surface corresponding to the situation of the hair follicle. 

 Some pressure-points have no such relation to the hair follicle. In 

 hairless parts it is suggested that pressure is associated with the 

 corpuscles of Meissner and of Pacini. These pressure organs are not 

 directly stimulated, since they do not reach the exposed surface of 

 the skin. The excitation is produced by the variation in pressure 

 in the neighbourhood of the end-organ, as can be shown by the fact 

 that they are stimulated either by pushing against or pulling on a hair. 



The extent of the surface to which the stimulus is applied is an 

 important factor. For example, a greater pressure per unit is required 

 for an area of 0-25 square millimetre than one of 0-5 square millimetre. 

 The optimum surface is about 0-5 square millimetre. Above or below 

 this area the amount of pressure required is increased. It has been 

 found that the hairy parts are more excitable and more easily fatigued 

 than the smooth areas of the skin. 



In estimating weights, it is easier to compare them applied succes- 

 sively than simultaneously. Further, the rapidity with which the 



