MULTICELLULAR ANIMALS 



159 



absorption, namely, time; more digested food will pass through the 

 membranous wall of the digestive system the longer the time that 

 the food remains in contact with the absorbing walls. In Man the 

 digestive tube is quite long as compared with the length of the body 

 w^all, and the time required for food to traverse this tube more than 

 twenty feet long permits almost complete absorption of the products, 

 of digestion. 



The Human Digestive Sys- 

 tem. The human digestive sys- 

 tem (Fig. 108) consists of the 

 ORAL CAVITY, iuto which three 

 pairs of salivary glands empty, 

 the (ESOPHAGUS, a narrow tube 



leading from the base of the oral 

 CAVITY, the STOMACH, iuto which 

 the oesophagus opens, the small 

 INTESTINE, with the glands that 

 empty into it, the liver and the 

 PANCREAS, the large intestine or 

 COLON, the rectum, and the anus. 

 The small intestine consists of 



three regions, the duodenum, which receives the food from the stom- 

 ach through a valve called the pyloris and into which the bile duct 

 from the liver and the pancreatic duct from the pancreas open, the 

 JEJUNUM, and the ileum, which opens into the large intestine 

 through the ileo-cgelic valve. The ileum does not open directly into 

 the end of the colon, but at some distance from the extreme upper 

 end. This leaves a sac at the upper end of the colon, beyond the ileo- 

 cceHc valve, known as the c.^cum. The end of the csecum opens into 

 a small blind tube, the vermiform appendix. 



The walls of the digestive tube contain an intricate network of 

 blood and lymph vessels and are composed chiefly of two layers of 

 muscles, an outer longitudinal layer and an inner circular layer (Fig. 



Fig. 106. — Diagram of the opened 

 small intestine of Man. Note the folds 

 and compare with the linings of the 

 intestine of the dogfish and the earth- 

 worm. 



