INTESTINES 



243 



The posthepatic intestine is the seat of most of the digestive proc- 

 esses and of absorption of the products of digestion. Here the food, 

 coming from the stomach, is mixed with the bile from the liver and 

 with the pancreatic juice and with the secretions of numerous small 

 glands in the intestinal wall. The increase of surface needed for 

 adequate digestion and absorption is provided in several ways. 



Fig. 257. — Spiral valve of Raia, after Mayer. 



There may be an elongation of the tube which results in its becoming 

 coiled in the body cavity; the mucous lining may develop folds, both 

 longitudinal and circular; or the folds may break up into numerous 

 minute, finger-hke processes (villi) which give the surface a velvety 

 appearance. The food undergoing digestion is moved back and forth 

 (peristaltic motion) by the antagonistic action of the muscles of the 

 intestinal wall (p. 130) ; bringing all of it in con- 

 tact with the absorptive surface. 



The length of the intestine is roughly re- 

 lated to the food, being longer in the plant- 

 eating than in the carnivorous species. This 

 is strikingly shown in the frogs, where the 

 tadpole (larva) has a very long intestine, corre- 

 lated with the vegetable food, while the adult 

 flesh-eating frog has a canal hardly longer than 

 that of the tadpole of half the size. 



In the intestine there are two divisions, an 

 anterior small intestine and a posterior large in- 

 testine, terms adapted from the digestive tract of man, though not 

 always appropriate in the lower groups. The line between the two 

 may be marked externally by the development of one or two blind 

 pouches or caeca at their junction or by a circular fold or a pair of 

 internal folds of the lining, consituting an ileo-colic (ileo-caecal) 

 valve, both valve and caeca coexisting in many cases. Both large 



Fig. 258. — Diagram 

 of spiral valve of Carcha- 

 rias. 



