296 HISTORY OF THE HUMAN BODY 



sence in the other. A good example of this is that of the 

 pigeon and the common fowl, in the latter of which a well- 

 developed gall-bladder occurs while absent in the former. The 

 pancreatic duct is normally without such a resevoir, but a pan- 

 creatic bladder has occasionally been observed as an abnor- 

 mality in the common cat, existing side by side with a normal 

 gall-bladder, the two exhibiting about the same size and pro- 

 po^tions. 



,/The terminal portion of the alimentary canal in Amphibia 

 / and Sauropsida, and in some fishes (e. g., selachians), enlarges 

 into a cloacal chamber which bears within its walls the out- 

 lets of the urinary and reproductive organs, and receives their 

 products as well as that of the intestines. In the Sauropsida 

 and in monotremes the terminal portion of this serves as the 

 functional cloaca and receives also the urinary and reproduc- 

 tive products, but in all mammals except these last the uro- 

 genital outlets are emancipated from the alimentary canal, 

 which thus terminates in a rectum instead of a cloaca, and its 

 external opening is a true anus and not a cloacal orifice. 



At the junction of the small intestine with the large, there 

 is a strong tendency to form one or more cceca, or blind sacs, 

 which often become digestive organs of great physiological 

 efficiency. The characteristic form in reptiles is that of a single 

 rather short and wide ccecum, symmetrically placed. In birds 

 there are usually two symmetrical ones, which attain great 

 length in scratching birds (e. g. f the common fowl), and in 

 ducks and geese, but are quite rudimentary in certain others 

 (woodpeckers, parrots, etc.). Ostriches possess a single 

 coecum of great length (7 to 8 meters) and furnished with 

 an internal spiral partition, which greatly increases its ef- 

 fective surface. 



In mammals a single coecum is developed,* which varies 

 greatly in size and functional importance. Rudimentary in 



* There are two very short coeca in the arboreal ant-eater, and in 

 the manatee a single coecum bears two supplementary diverticula. In 

 Hyrax, in addition to a moderately large coecum, there are two smaller 

 diverticula situated farther down on the colon. 



