BUCCAL CAVITY OF VERTEBEATA. 



549 



number of organs, wliicli were seen in the primitive arrangement, 

 are assigned to the buccal cavity, while other organs appear only 

 as later developments. The teeth, the tongue, and various glandu- 

 lar organs belong to the former series. The soft palate, or velum 

 palati, which is only found in Mammals, is a new organ. This 

 muscular apparatus forms the posterior boundary of the buccal 

 cavity, and separates it from the pharynx. The uvula is a median 

 prolongation of the velum palati ; this structure is apparently found 

 in the Primates only. 



In Reptiles and Birds the anterior and lateral boundaries of the 

 buccal cavity are formed by the margins of the jaws, which are 

 invested by the integument, and by the hard structures which are 

 found on the jaws. In the Saurii and Ophidii the integument de- 

 velops pad-like lips along the edge of the jaw. In all Mammals, 

 except the Monotremata, the integument is separated from the edge 

 of the jaws, and invests a complicated muscular layer which has its 

 origin in them ; this layer forms the ground- work of the lips, and 

 gives them their mobility. In this way a space is developed which 

 lies in front of the buccal cavity — the vestibulum oris. The lateral 

 portions of this space form the cheek -pouches, and, when capable of 

 great extension, develop into the pouch-like diverticula of many 

 Mammals (buccal pouches of the Rodents and Apes). 



Organs of the Buccal Cavity. 

 § 414. 



Of these organs those hard structures which serve for the pre- 

 hension and comminution of food are of 

 various kinds. Some of them are formed 

 by the cornification of epithelial cells. The 

 sucker-shaped mouth of the Cyclostomata 

 (Fig. 308) is beset with horny teeth of this 

 kind, which are also present on a tongue- 

 like organ in these animals. In the Am- 

 phibia the edges of the jaws are provided 

 with a similar covering; these structures, 

 which are partly limited to the larval stages, 

 are formed of a number of closely-set den- 

 ticles (Anura) ; in Siren some are retained 

 throughout life. 



The larger horny coverings on the mar- 

 gins of the jaws of the Chelonii, Aves, and 

 Monotremata differ somewhat from these 

 horny denticles ; they are compensatory 

 arrangements, the presence of which is due 



to the absence of true teeth. Although 



Fig. 308. Mouth of Pc- 



tromyzon Biariuus, with 



its "homy teeth" (after 



Heckel aud Kuer) . 



these structures are used for the comminution of food, they have 

 nothing to do with true teeth ; they are purely epidermal structures, 

 as is also the whal^-bone of Whales. 



