CHORDATA. 331 



a whole or by means of outgrowths (the glands) or by in- 

 growths (folds of various kinds). The highly nourished con- 

 dition of the entodermal sheet of cells presumably leads to 

 their rapid growth and foldings. The folds are often so 

 arranged across the axis of the tube as to retard the progress 

 of the food through the tract, thus making digestion and 

 absorption more complete, by increasing the time during which 

 the food is exposed to the action of the digestive juices, and 

 to the absorbing surface. 



344. The Divisions of the Tract. The mouth, which may 

 be either terminal or ventral, opens into the buccal cavity, 

 which is bounded dorsally by the floor of the brain case, on the 

 sides and in front by the jaws, and ventrally by a muscular 

 floor from which the tongue arises as a fold. The jaws are 

 made up of bony elements from two sources; a core of bones 

 from the internal skeleton (the first visceral arch) and a 

 covering of dermal bones which fuse with it. The latter are 

 the bones which (typically) bear the teeth. Teeth however 

 occur in the lower vertebrates in the roof of the mouth or on 

 the tongue. Their place may be taken by horny epidermal 

 structures, as in the beak of birds. When present the salivary 

 glands open into the mouth cavity. Posteriorly the buccal 

 cavity communicates with the pharynx, which may be defined 



FIG. 1 60. Stomach of Dog (A) and of Rat (B). c, cardiac portion; p, pyloric por- 

 tion; o, oesophagus; t, intestine. 



