900 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



which it exerts upon the skin — and to a certain extent its temperature. 

 If the degree of pressure be greatly increased, the sensation of pressure 

 gives place to one of pain, and so, also, for extremely hot or extremely 

 cold bodies. On the other hand, impressions which are not localized on 

 the terminal organs, as, for example, the passage of the electric current 

 through the skin, are not capable of being regarded as tactile sensations. 

 In other words, we are unable to distinguish one such mode of stimula- 

 tion, except within narrow limits, from another. Thus, for example, the 

 sensation of mechanically pricking the skin is probabty identical with 

 that produced by the passage of the current. Again, the contact of a fluid 

 with the skin will cause a tactile sensation, since it is in contact with the 

 nerve terminations, and if that fluid be an acid and pass below the terminal 

 organs and implicate the nerve-fibres a sensation of pain will be caused ; 

 so that stimulus which when applied only to the nerve terminations may 

 cause a tactile impression, when applied to the nerve-trunk results in a 

 painful sensation. 



The intensity of the sensation produced by pressure depends almost 

 as much upon the rapidity of the application of the pressure as upon the 

 degree of the pressure. If the increase be gradual, more pressure may 

 be applied with scarcely a perceptible sensation ; while, on the other 

 hand, the rapid application of a less pressure may cause a much more 

 intense sensation. All parts of the skin are not equally susceptible to 

 pressure for the simple reason that all parts are not equally supplied 

 with tactile corpuscles. In man the most sensitive localities are on the 

 palmar surface of the fingers, on the forehead, and on the flexor surfaces 

 of the limbs as contrasted with the extensor surfaces. 



If two points of the skin are subjected to pressure the sensation 

 becomes fused when the two points are sufficientl}- close. When an im- 

 pression is made upon any part of the bocty, not only the character but 

 the locality are appreciated. This power, however, of localizing pressure 

 sensations varies in different localities. 



The following table from Weber gives the minimum distances at 

 which simultaneous stimulation of two points may be recognized as two 

 distinct sensations : — 



Tip of tongue, .... 



1.1 millimeter 



Palm of last phalanx of finger, . 



22 



Palm of second phalanx of finger, 



4.4 



Tip of nose, . . . . 



6.6 



White part of lips, 



8.8 



Back of second phalanx of finger, 



11.1 



Skin over malar bone 



15.4 



Back of hand, 



29.8 



Forearm, . . ... 



39.6 





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