ALIMENTATION 



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Fig. 54. — Maxillary teeth of 

 Rana pipiens, shown in normal 

 position, attached above to bone, 

 below which transparent gum tissue 

 is shown. 



differs in its embryological origin from the lining of the pharynx, 

 though in the frog or in the adult of any air-breathing vertebrate 

 it is impossible to establish any 

 sharp histological difference at the 

 line of fusion of ectoderm and 

 endoderm. In fishes the pharyn- 

 geal region is definitely distin- 

 guished from the oral cavity because 

 it is provided with gill clefts and 

 gills. In the frog gill clefts are 

 present in the embryo and tadpole, 

 but all of them except the first dis- 

 appear during metamorphosis. 

 Since the persisting first gill cleft is 

 converted into the cavity of the middle ear, connecting with the 

 pharynx by the Eustachian tube, the location of the opening of 



the Eustachian tube gives a rough idea 

 of the anterior limit of the pharynx. 

 Otherwise in the frog and in all higher 

 vertebrates there is no simple way of 

 establishing the line of demarcation 

 between the oral cavity and pharynx. 

 The oral cavity may be provided with 

 teeth, a tongue and the openings of 

 oral glands. 



Teeth. — True teeth, such as human 

 teeth, are hard bony structures found 

 in the oral cavity of most vertebrates, 

 notable exceptions being birds and some 

 reptiles (turtles). The teeth of the frog 

 consist of a single row attached to the 

 inner edge of the premaxillary and 

 maxillary bones of the upper jaw and a 

 small group in the ventral surface of 

 each vomerine bone. The tooth con- 

 sists of a conical bony core v of dentine, 

 whose root is attached to the bone of the 

 jaw by a cement substance and whose 

 exposed crown is covered with enamel thickened at the apex 

 (Fig. 54). The enamel in turn is covered with the cuticula dentis, 



Fig. 55. — Diagrammatic 

 longitudinal section of human 

 tooth. B, bone of jaw; C, 

 cement; D, dentine; E, en- 

 amel; G, gum; P, pulp cavity. 



