The Inner (Alimentary) Tube and Its Respiratory Derivatives 29 



direct passage through the intestine and compelling food to follow the 

 turns of the spiral membrane. The functional length of the intestine is 

 thereby greatly increased. 



Further increase of intestinal surface is achieved by development of 

 externally projecting hollow appendages of the tube. Most important 

 of these and most constant in occurrence are the liver and pancreas, 

 which are highly complex glandular structures. Of simpler nature are 

 the pouchlike ceca commonly produced by the intestine. In many of 



•^ Greater Curvature 



Pyloric Stomach 



Fig. 21. The right half of the human stomach, 

 viewed from within. (Redrawn from Braus, after Elze. 

 Courtesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



the bony fishes (Teleostei), there may be from 1 to upward of 100 ceca 

 protruding from the intestine just behind the pylorus (Fig. 24). When 

 very numerous, these pyloric ceca form a conspicuous cluster of long, 

 slender tubes. The rectal gland of sharklike fishes is a narrow cecum 

 arising from the dorsal wall of the rectal region of the intestine. Its 

 function is uncertain. In reptiles, birds, and mammals, there is usually 

 a cecum, or a pair of them, at the anterior end of the "large" intestine. 



Liver and Pancreas 



These two glandular organs, mainly digestive in function but pos- 

 sessing important secondary functions, are highly characteristic organs 

 of vertebrates (Figs. 10, 25, 26). The liver, most massive of all the vis- 

 ceral organs, develops as an outgrowth from the ventral wall of the 



