TEETH 239 



The small intestine is in most cases the longest section of the 

 alimentary tract : the bile and pancreatic ducts open into its 

 anterior portion. 



In almost all cases the alimentary canal becomes more or less 

 coiled, and thus presents a greater surface for absorption. As a 

 general rule, it is relatively longer in herbivorous than in carni- 

 vorous animals. A considerable increase of surface also commonly 

 results from the elevation of the mucous membrane to form folds, 

 villi, and papillae (p. 269). 



Certain appendages are present in connection with the ali- 

 mentary canal. These are all developed primarily from the 

 hypoblast and are thus of epithelial origin : mesoblastic elements 

 are added to them secondarily. Whether they function as 

 glands throughout life or not, they are all formed on the same 

 type as glands. 



Beginning from the mouth the following appendicular organs 

 may be distinguished (Fig. 190) : 



(1) Mucous and salivary glands. 



(2) The thyroid. 



(3) The thymus. 



(4) The lungs or air-Madder. 



(5) The liver.. 



(6) The pancreas. 



In addition to these, gastric and intestinal glands are embedded 

 in the wall of the gut. 



MOUTH. 



In Amphioxus the entrance to the mouth (oral hood) is pro- 

 vided with cirrhi, and in Petromyzon 1 it is surrounded by a ring of 

 cartilage (Fig. 54) : all other Vertebrates are provided with jaws. 



Definite lips provided with muscles first appear in Mammals, 

 but are wanting in Monotremes. The space between them and the 

 jaws is spoken of as the vestibulum oi*is ; this may become 

 extended on either side to form cheek-pouches, which serve as food 

 reservoirs (many Monkeys and Rodents). 



The chief organs of the oral cavity are the teeth, the glands, 

 and the tongue. 



Teeth. 



The teeth are developed quite independently of the endo- 

 skeleton, and both epiblast and mesoblast take part in their forma- 

 tion (comp. p. 30). The first traces of the teeth are seen primarily 



1 The mouth of the Lajnprey serves as a suctorial organ for attaching the 

 animal to foreign objects. The larva? of Lepidosteus and Anura are temporarily 

 provided with suctorial organs 



