202 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



over the forefinger, and a small object, like a bullet or 

 a pea, be rolled beneath the crossed tips, two objects are 

 distinctly felt (Fig. 129). It is because the impressions 

 are on unaccustomed or non-corresponding points of the 

 fingers. 



COMPARATIVE PHYSIOLOGY OF TOUCH. 



In our rapid survey in passing down the scale, it is 

 necessary to keep in mind the distinction between gen- 

 eral sensibility and special sense. They are indeed 

 almost in inverse proportions. 



Mammals. — Man differs from mammals chiefly in the 

 use of the hand as an organ of touch. Indeed the fore 

 limbs are liberated from the function of support and 

 progression expressly for this purpose. In other mam- 

 mals, with the exception of apes, the special tact organ 

 is localized elsewhere, generally in the prehensile organ 

 wherever that may be, most usually about the mouth, 

 as the lips, the snout, the tongue. In such cases we 

 may often find a strong development of papillae, as on 

 the nose of the dog or on the sole of the foot of all un- 

 hoofed animals. 



In birds the beak is an exquisitely sensitive organ of 

 touch. Sensory nerves are abundantly distributed at its 

 base and beneath the horny sheath, exactly as in the 

 case of our finger nails, which are also delicate organs 

 of touch. The strong papillae on the feet of birds show 

 that they are good organs of touch. The general sensi- 

 bility of birds is probably inferior to that of mammals 

 on account of the thick covering of feathers. 



Reptiles are all of them more or less covered with 

 dry horny or bony scales which must diminish their gen- 

 eral sensibility, nor have they any well-marked organ 

 specially organized for touch. They are probably 

 poorly endowed both in general and special sensibility. 



