302 HUMAN PHYSIOLOGY 



2. In those regions of the body not provided with hairs r 

 the division of the skin into tactile points corresponds with 

 the distribution of the Meissner corpuscles. 



The number of tactile points varies in different parts of the 

 skin. In the palm of the hand there are from 40 to 50 in 

 I sq. cm. 



The liminal intensity of the stimulus for the tactile sensa- 

 tion depends upon: 



I. The place of stimulation. The lips, finger-tips, and 

 forehead are the most sensitive. For the finger-tips the 

 liminal intensity is 0.03 g acting upon I sq. mm. surface. 

 The least sensitive portions are those covered with thick 

 epidermis, for example the soles of the feet. 2. The extent 

 of surface stimulated. 



With increase of surface, the liminal intensity first decreases 

 rapidly, then increases slowly. 



3. The rapidity of the stimulus. 



The liminal intensity is, to a certain extent, the smaller the 

 more rapidly the pressure is exerted. 



The ability to distinguish two unequal pressure stimuli of 

 moderate intensity corresponds with the law of Weber (see 

 page 256). The difference-threshold is about -^ of the 

 already acting weight, i.e. we can recognize the difference 

 between two weights if they have the ratio of 29 : 30. 



The law of Weber does not hold good for very small or very 

 large weights. 



The tactile sensations appear and disappear very rapidly. 

 Even 460 shocks per second are still perceived as individual 

 shocks. 



(b) and (c) The sensations of heat and cold. These 

 sensations are produced either by increasing or decreasing 

 the heat conveyed to the skin while the loss of heat remains 

 constant, or by increasing or decreasing the loss of heat by 

 the skin while the heat conveyed to it remains the same. 

 The last is the case, e.g., in applying warm or cold objects 

 to the skin. Sensations of temperature are produced chiefly 



