156 FOUNDATIONS OF BIOLOGY 



lines the lungs is, from the standpoint of development, a 

 specialized part of the epithelium of the alimentary canal. 

 (Fig. 94.) 



The STOMACH is the first stopping place of food which has 

 been swallowed and where the work of the digestive tract 

 (alimentary canal) essentially begins by the dissolving action 

 of chemical substances (enzymes) secreted by its walls. The 

 stomach leads by a constriction (PYLORIC VALVE) into a long 

 and usually convoluted INTESTINE. The anterior portion of 

 this is known as the SMALL INTESTINE, and it is here that the 

 major part of digestion is accomplished directly or indirectly 

 by means of chemical secretions supplied by its walls and by 

 the PANCREAS and LIVER. In the small intestine ABSORPTION 

 also begins; that is, the passage of the soluble food materials 

 through the wall of the digestive tract into the body proper. 

 The soluble proteins and carbohydrates are taken up 

 directly by the blood vascular system and conveyed to the 

 liver, while the fats enter the lymph vessels which later 

 deliver it to the blood. A constriction marks the origin of 

 the LARGE INTESTINE which continues the absorption of 

 water and carries the undigested material, or FAECES, to the 

 exterior through the anus. This either opens into a terminal 

 sac, the CLOACA, in which also are situated the orifices of 

 the urogenital ducts, or directly on the ventral surface, as 

 in Man. (Fig. 93.) 



The wall of the alimentary canal consists of three chief 

 cellular layers: a lining epithelium, a connective tissue layer, 

 and a muscular layer. The epithelium, however, together 

 with its derivatives is the digestive tract proper in the sense 

 that it is of prime functional importance; the other layers 

 performing accessory functions such as support, conduction 

 of blood vessels, and movements of the canal. (Figs. 20, 

 103.) 



