30 



A COURSE ON ZOOLOGY. 



FlG. 15. 



digestive apparatus is the collection of organs which 

 serve as agents in digestion. 



The digestive apparatus is composed of a long tube 

 open at both extremities, and a "number of auxiliary 

 organs, among which are the salivary glands, the liver, 

 the spleen, and the pancreas, most of these organs being 

 contained in the abdominal cavity. 



The walls of the digestive tube are formed by the 

 union of several layers of various membranes ; interiorly 

 a mucous membrane is a continuation of the skin : under 

 this a fibrous membrane determines the form of the 



tube ; next to this a mus- 

 cular coat gives the power 

 of contraction, and lastly, 

 a serous membrane iso- 

 lates the tube from other 

 organs. 



The mouth is a sort of 

 vestibule to the digestive 

 canal; its exterior open- 

 ing is surrounded by a cir- 

 cular muscle that forms 



1. The body of the lower jaw; 2, ramus, tne lips. On the edges 

 or branch of the jaw, to which the of the lips begins the 



that 

 This 

 differs from 



muscles that move it are attached ; 3, 

 3, the processes which unite the lower 

 jaw with the head ; i, the lower and 



mucous membrane 

 has been mentioned; 



lateral incisor teeth of one side; 6, the ^ 



hicuspid teeth; c, the cuspids or eye T 



teeth; TO, the three molar teeth; A, the skin by its thinness, 



shows the relation of the permanent 



to the temporary teeth. 



its moisture, and its rosy 

 color. The lips retain the 

 food during mastication, prevent the saliva from running 

 out the mouth, and aid in the production of sound and 

 speech. 



