hTO Comparative Morphology of Chordates 



193). The sensory "corpuscles" known as "Krause's," " Meissner's," 

 and "Pacini's" are especially characteristic of mammals (Figs. 164- 

 166). They are commonly called "tactile corpuscles," although it is 

 likely that some of them are stimulated by agencies other than pres- 

 sure. They are widely distributed in the mammalian skin, but are few 

 or absent in regions covered by dense fur, and especially numerous on 

 such hairless areas as the ventral surfaces of the paws, the lips and 

 snout, the nipples of the mammary organs, and the external genitalia. 

 They occur in the epidermis, but much more abundantly in the dermis. 

 In such acutely sensitive regions as the tips of human digits, they are 

 found each within a cylindric protrusion of the dermis into the epi- 

 dermis — a dermal papilla (Fig. 502). They are thus brought into 

 closer relation to the external surface of the skin. Dermal papillae, 

 many of them sensory, occur in rows conforming to the cutaneous 

 ridges which constitute the human "fingerprint" patterns. 



There is considerable experimental evidence that a particular 

 sensory structure of the skin is capable of giving rise to sensations of 

 only one kind — i.e., pressure ("touch"), heat, cold, or pain. Sensations 

 of pressure and of heat, referred to the same square centimeter of 

 skin, result from stimulation of two totally distinct sets of receptors. 



The highly developed sensitiveness of mammalian skin is one of 

 the most important differences between the sensory equipments of 

 reptiles and mammals, and of great significance in that it gives the 

 mammal a far more intimate acquaintance with its immediate environ- 

 ment. Integumentary organs of chemical sense, however, are totally 

 lacking, as in reptiles and birds, and there are not even embryonic 

 vestiges of the lateral-line organs of Anamnia. 



ORGANS OF TASTE 



The organs of taste, or "taste-buds," are much alike in all 

 vertebrates (see p. 196). In fishes organs of this sort exist not only in 

 the mouth but also more or less extensively over the external surface 

 of the body. They are evidently organs of chemical sense, and fishes 

 may be said to possess a cutaneous sense of taste. In amphibians the 

 taste-buds rarely occur externally, and never so in adult frogs and 

 toads. In mammals, as in reptiles and birds, they are never external. 



The taste-buds of mammals occur in the lining of the mouth 

 and pharynx, but are less widely distributed than in Anamnia. tending 

 to be restricted to the more posterior surfaces. They are found on the 

 tongue, the soft palate, and in relatively small numbers on the larynx 

 and epiglottis. The dorsal surface of the tongue is thickly beset with 

 papillae of a variety of forms — slender, more or less branched filiform ; 

 mushroom-shaped fungiform; and, at the posterior region of the 



