30 



A COURSE ON ZOOLOGY. 



FIG. 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 j 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 ^PS- On the edges 



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

 muscles that move it are attached ; 3, i j_i_ 



3, the processes which unite the lower UCOUS membrane that 



jaw with the head; i, the lower and has been mentioned. This 



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



torC"er e Permanent ^s 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. 



