CHAPTER XL 

 THE DIGESTIVE SYSTEM. 



Arising from tlic simple eml)ry()nic invaginations which form 

 the fore and hind gut, there develops in vertebrates a relatively 

 elaborate system for service in the digestion, absorption, and 

 defecation of food. This system is composed not only of the long 

 canal that extends from mouth to anus, or cloaca, but includes also 

 the tongue, teeth, a few large and a great many small glands derived 

 from embryonic outjjocketings of the endodermal (epithelial) lining. 



Food is taken in at the mouth, where it is usuall,y subjected to 

 the mechanical ])rocess of mastication by the tongue and the teeth. 

 The secretions of the glands of this region pass into the oral cavity 

 to aid in lubricating the food for passage. Food thus crushed and 

 lubricated passes into the esophagus, which conducts it to the 

 stomach, where digestive juices are poured upon it from the glands 

 of the stomach walls. Very little, if any, absorption occurs in the 

 stomach where the food is stored temporarily and is actively worked 

 upon by digestive ferments (enzymes). It is passed from the lower 

 end of the stomach by peristaltic waves into the small intestine, 

 whose surface is greatly increased by folds. Secretions from small 

 glands in the intestinal wall, together with the secretions from the 

 pancreas and the liver, which are introduced here by ducts from 

 these two organs, are all mixed with the food mass in the lumen. 

 Under the combined chemical acti^'ities of the secretions added to 

 it, the food is broken into products which are capable of being 

 absorbed through the epithelial wall of the intestine. Passing 

 through the small intestine the unabsorbed portion of the food 

 enters the large intestine where absorption of water takes place. 

 The glands lining the walls of the large intestine are probably 

 mainly acti\'e in lubricating unusable materials and waste by- 

 products of digestion for passage and elimination. 



THE MUCOUS MEMBRANE. 



The intestinal tube extends from the lips to the posterior aperture. 

 There is a line of demarcation at each of these places where the skin 

 ends and the membrane lining the cavity of the intestinal tract 



