334 ZOOLOGY 



the phalanges difier? Which is the thumb? How can you be sure? Compare 

 this figure with figures (in reference texts) of the appendages, both front and rear, 

 of the frog; of some bird; or some carnivore; of the horse; of man. Where are the 

 greatest variations, i.e., which bones depart most from this typical condition? 



layers of unstriped muscle fibres, circular and longitudinal, 

 by which the food is forced onward. The muscles are espe- 

 cially developed in certain regions, as in the stomach. Outside 

 of all these, in the portion passing through the body cavity, is 

 the serous . membrane derived from the mesoderm, a portion 

 of the lining of the body cavity. The mucous surface which 

 is, nattu-ally enough, the important portion in digestion and 

 absorption may be increased by the lengthening of the tube as 

 a whole or by means of outgrowths (the glands) or by ingrowths 

 (folds of various kinds). The highly nourished condition 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. 



352. 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 as the part of the digestive tract in con- 

 nection with which the lungs or gills are developed. In fishes 

 and in the embryos of higher forms there are slits in the side 

 walls connecting the pharynx with the outside. Gills are 

 developed in the walls of these slits. In forms above fishes the 



