I 



ORAL CAVITY .JJk • 223 



THE ORAL CAVITY 



The cavity of the mouth is limited anteriorly by the Une of the 

 stomodeal involution and extends back to the pharynx. It is lined 

 in part by ectoderm, in part by entoderm, the line between the two, 

 as stated above, not being recognizable in the adult. In the amphi- 

 bia the lining is ciliated, the ciUa extending back to the stomach. In 

 the cyclostomes the oral cavity is funnel-shaped, with a circular or 

 quadrangular opening supported by a cartilaginous ring and has the 

 name of oral hood. It is permanently open, there being no jaws 

 capable of closure (see skeleton, p. 84), thus furnishing a marked con- 

 trast to all other vertebrates in which there are jaws and which are 

 consequently known as gnathostomes. (Development gives little 

 support to the view that the cyclostome tongue is the homologue of 

 the lower jaw of the gnathostomes). 



In development the vertebrate mouth arises on the ventral side ot 

 the head, some distance from the anterior end of the body. This 

 position is retained throughout life in most elasmobranchs and in the 

 sturgeons; but elsewhere, by the development of the bony upper jaw 

 in front of the pterygoquadrate (p. 77) and the concomitant exten- 

 sion of Meckel's cartilage, the mouth opening is gradually transferred 

 to the anterior end and becomes terminal. 



In most lower gnathostomes (the holocephaU and other isolated 

 forms are exceptions) the mouth opening is bounded by folds of epithe- 

 Hum which meet when the mouth is closed. Usually these folds are 

 soft and are supported below by connective tissue, but in birds, tur- 

 tles and monotremes they are cornified. It is only in the mammals 

 that true lips occur. These are fleshy folds around the mouth and 

 their development in this group is correlated with the presence of the 

 dermal facial muscles (p. 140), by which they are moved. With the 

 development of Hps there is formed a space between lips and teeth, 

 the vestibule of the mouth, which sometimes (e.^., some rodents) 

 forms cheek pouches, lined with hair, of considerable size. 



Teeth 



The primitive function of the teeth was apparently to hold the 

 prey taken into the mouth and this is their sole use in many forms. 

 In other species they have become efficient organs for the comminu- 

 tion of food, either by cutting or by crushing it. 



There are two types of teeth, much alike in function, but differing 



