128 The Vertebrate Organ Systems 



THE FUNCTION OF THE DIGESTIVE SYSTEM 



The function of the digestive system is to prepare food for absorp- 

 tion and use by the cells of the body. Digestion is defined as the proc- 

 ess by which foods are changed from the complex to the simple 

 through the action of secretions containing enzymes. 



Any material which contributes to the growth, energy, repair of 

 tissue, or maintenance of vital processes may be considered a food. 

 This includes a variety of materials : carbohydrates, proteins, fats, 

 vitamins, inorganic salts, and water. 



PARIETAL 

 PERITONEUM 



Fig. 39. — Diagram of cross section of the body of a vertebrate to show the relation- 

 ships of the intestine, coelom, peritoneum, and mesentery. 



THE DIGESTIVE TRACT 



Essentially the vertebrate digestive tract is a tube modified in 

 different portions to perform particular functions. It has two open- 

 ings : the anterior one is the mouth, the posterior, the amis. In 

 close association with the tract are special glands which secrete enzymes 

 for the digestion of the food. 



The greater part of the digestive tract is located in the body cavity, 

 the coelom. This cavity is lined with a thin membranous tissue, the 

 parietal peritoneum, which extends from the body wall as sheets known 

 as mesenteries. These membranous sheets support the digestive tract 

 and continue as the visceral peritoneum which closely surrounds the 

 internal organs. Many other essential organs of the body are lo- 

 cated in the coelom, and are collectively called the viscera. In mam- 

 mals, the coelom is divided into two regions by the muscular diaphragm 

 which is located just below the ribs. Anterior to the diaphragm, the 



