30 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



pre-hepatic. The post-hepatic portion of the alimentary canal is 

 more or less clearly divided into two regions, an anterior mid 

 gut, the small intestine, and a posterior hind gut, the large 

 intestine of higher forms. In the lower vertebrates this distinc- 

 tion is not so sharp, being largely indicated by the character of 

 the internal walls, or, as in the elasmobranchs, by the develop- 



FlG. 38. Diagram of ruminant stomach, after Wiedersheim. A, abomasum; 

 P, psalterium (manyplies); RT> reticulum (honeycomb); RU, rumen (paunch). 



ment of a caecal tube (rectal gland or digitiform appendix) at 

 the boundary between the two. From the amphibia upwards 

 the line of division is more sharp, an internal constriction, the 

 ileocolic valve, forming the line of demarcation. 



The mid gut is the chief seat of intestinal absorption, and 

 various means are introduced of increasing the intestinal surface. 

 In the cyclostomes there is an infolding of the inner wall which 

 follows a slightly spiral course. In the elasmobranchs this spiral 

 valve acquires great development, either growing out so that 

 the interior of the intestine resembles a spiral staircase, or more 

 like a roll of paper, the free edge projecting into the lumen of 

 the tube. This spiral valve reappears in the ganoids, but is not 



