ORGANS AND PROCESSES OF DIGESTION 141 



canal and pass their liquids into it by means of small tubes, 

 called ducts. In the canal the food and the digestive 

 fluids come in direct contact a condition which the dis- 

 solving processes require. Each kind of fluid is secreted 

 by a special kind of gland and is emptied into the canal 

 at the place where it is needed. 



The Digestive Processes. Digestion is accomplished by 

 acting upon the food in different ways, as it is passed along 

 the canal, with the final result of reducing it to the form 

 of a solution. Several distinct processes are necessary 

 and they occur in such an order that those preceding are 

 preparatory to those that follow. These processes are 

 known as mastication, insalivation, deglutition, stomach 

 digestion, and intestinal digestion. As the different mate- 

 rials become liquefied they are transferred to the blood, 

 and substances not reduced to the liquid state are passed 

 on through the canal as waste. The first two of the di- 

 gestive processes occur in 



The Mouth. This is an oval-shaped cavity situated at 

 the very beginning of the canal. It is surrounded by the 

 lips in front, by the cheeks on the sides, by the hard 

 palate above and the soft palate behind, and by the tissues 

 of the lower jaw below. The mucous membrane lining 

 the mouth is soft and smooth, being covered with flat 

 epithelial cells. The external opening of the mouth is 

 guarded by the lips, and the soft palate forms a movable 

 partition between the mouth and the pharynx. In a con- 

 dition of repose the mouth space is practically filled by the 

 teeth and the tongue, but the cavity may be enlarged and 

 room provided for food by depressing the lower jaw. 



The mouth by its construction is well adapted to carry- 

 ing on the processes of mastication and insalivation. By 

 the first process the solid food is reduced, by the cutting 



