552 COMPAEATIVE ANATOMY. 



teeth are merely formed in rudiment^ and are atrophied while stil 

 within the alveolar cavities. 



The replacement of the worn-out, and subsequently-shed teeth, 

 is effected in Fishes by the continual development of new teeth close 

 to the old ones. Teeth are, therefore, developed by a process which 

 is continued throughout the whole life of the animal, and is always 

 being renewed. Even in the Amphibia and Reptilia we also meet 

 with successional teeth, so that by the continual development of 

 fresh teeth the dental apparatus is kept complete. In most 

 Mammals this process is limited to a single change^ the first (milk) 

 dentition being replaced by a second one, in which there is a larger 

 number of teeth (Diphyodonts). This change does not happen in 

 the Cetacea (Monophyodonts). In the Marsupialia the diphyodont 

 condition is in a rudimentary stage, for it is confined to one tooth 

 only on either side of the jaw. This is the case also in several other 

 Mammals (Elephas, Halicore), while the Rodentia would seem to 

 belong to this series. The two series are therefore connected, and 

 the change of teeth in the Mammalia may be regarded as a process 

 which has been developed from a polyphyodont condition. 



Tomes, Ch. S., Manual of Dental Anatomy, Human and Comparative. London 

 1876. 



§ 415. 



The tongue is a second organ of the buccal cavity. In Fishes it 

 is generally a process formed by the investment of mucous mem- 

 brane of the body of the hyoid. It is flat, and movably connected 

 with the general branchial s-keleton only. Like the other skeletal 

 portions of the wall of this cavity it frequently carries a number of 

 teeth. This organ is not provided with a special musculature below 

 the Amphibia, where it forms a thick, and in many, a protrusible 

 structure. It is not developed in Pipa and Dactylethra. As a rule, 

 the anterior end alone is connected "with the floor of the buccal 

 cavity, and the posterior, and more movable portion, is drawn out 

 into two processes. In the Reptilia there is likewise a muscular 

 tongue, which, in the Ophidii and Saurii, can be drawn out of a 

 special sheath. The epithelium of the tongue, which is ordinarily a 

 delicate organ, frequently develops scales and knobs on its upper 

 surface, while the anterior end is drawn out into two fine points 

 (Fissilingues) (Fig. 310,2). In the Chelonii, and especially in the 

 Crocodilini, the tongue is broad and flat. In Birds, the anterior 

 end of the tongue is ordinarily covered by a cornified layer of 

 epithelium, and is sometimes beset by lateral barbs (Woodpeckers), 

 or fine setee (Toucan) ; it is in the Psittacidte only that the tongue 

 forms a larger fleshy organ. In the Mammalia we find that the 

 tongue is very large, owing to the greater development of its 

 musculature, while at the same time its investing mucous mem- 

 brane is provided with a number of differentiated papillae. The 

 function of the organ is chiefly that of aiding in the ingestion of 



