ORGANIC EVOLUTION 185 



and esophagus, and these are succeeded by stomach, 

 small intestine, large intestine and cloaca. All these, 

 together with the appendages, are indicated in the accom- 

 panying diagram (fig. 114). Such differentiation of parts 

 bespeaks many separate localized functions along the course 

 of the enteron ; and such indeed there are, but we are here 

 concerned with form changes and can note only the more 

 important functions as bound up with the principal organs. 

 The stomach has become a sharply delimited organ for 

 the reception of food, capacious, distensible, suited to the 

 exigencies of irregular food supply. Its thick muscular 

 walls are filled with small gastric glands, whose secretion 

 initiates digestion. The churning movements of the walls 



FIG. 1 14. Diagram of the enteron of the salamander, with 

 its principal appendages, o, mouth: /, pharynx; e, eso- 

 phagus; d, stomach; ', small intestine; j large intes- 

 tine; c.cloaca; a, anus; b, urinary bladder ;g, gallbladder 

 onm, liver; n, pancreas; I, lung. 



aid in the comminution of the food and in the mixing of it 

 with the gastric secretion. At the outlet of the stomach is 

 a guarded passageway called the pylorus, through which the 

 food passes, when reduced to a more or less fluid condition. 

 The small intestine is a narrow passageway (greatly 

 abbreviated in the diagram) , well adapted to the slow pas- 

 sage of the food, to the completion of its digestion, and to the 

 extraction from it of assimilable materials. It is long and 

 tortuous. Its walls are covered internally with folds and 

 processes (villi) which greatly increase the surface in con- 

 tact with the passing stream. These secure the better mix- 

 ing of food with the digestive secretions of the liver and the 

 pancreas, and the completer absorption of it after digestion. 



