170 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



sparrow, in which the yolk is almost or altogether 

 used up before the bird leaves the egg, rapidly acquire 

 the enteric proportions of the adult, those which leave 

 the egg, like the chick, before the yolk material has 

 been all used, are longer in acquiring the proportions 

 of intestine which obtain in adult forms (Gadow). 



In the Mammalia the small intestine is artificially 

 divided into a duodenum, a jejunum, and an 

 ileum ; the large intestine is of greater length than 

 in other Vertebrates, and the terminal portion only is 

 straight or rectal (Fig. 77). 



The rectum in. many Fishes does not open into a 

 cloacal pit, but lies in front of the u rino-genital oritice ; 

 in all other Vertebrata, except the Eutheria, the rectal 

 and renal orifices open into a cloaca, while in the 

 higher Mammals the anus lies behind the urinogenital 

 opening. In the Chelonia, where the walls of the 

 abdomen exert little or no influence on the move- 

 ments of the intestine, part of the terminal portion of 

 the gut may be enlarged, and its walls provided with 

 abundant muscular tissue (elephant tortoise). The 

 walls of the rectum are sometimes provided with 

 glandular appendages, such as the foursa fabricii of 

 Birds, and the bursar anales of Chelonia ; the func- 

 tion of these organs is unknown. 



The large is sometimes separated from the small 

 intestine by a blind ingrowth or diverticulum, the 

 ceecum ; this is first seen in the Reptilia, where it 

 varies somewhat in size in different forms, but it is 

 never of any considerable extent. In most Birds the 

 caecum is double, and the length of these outgrowths 

 are in direct relation with that of the intestine, and 

 therefore with the habits of the bird ; they are, in 

 other words, longer in herbivorous than in other forms ; 

 it has, however, been pointed out by Gadow, that 

 when the intestine is narrow the tract is long and 

 the caeca short, while other short caeca may also be 



