530 Comparative Morphology of Chordates 



referring to diameter, not length. The long small intestine is disposed 

 in turns and coils which have several different patterns, each charac- 

 teristic of a particular group of birds. The large intestine, better called 

 rectum, is relatively short — usually a small fraction of the length of 

 the body. The ostrich is a notable exception, having a very long and 

 capacious rectum. Since the chief function of the rectum is storage of 

 the waste residue of digestion, the extreme abbreviation of this region 

 of intestine in the flying birds is significant in its economy of useless 

 weight. 



In most amniote vertebrates the region of junction of small and 

 large intestines is marked by outgrowth of the intestinal wall to form 

 one or two blind pouches, ceca, which serve to augment the secretory 

 and absorbing surface of the intestine. In the heron and a few other 

 birds, the cecum is single and quite short. In parrots, woodpeckers, and 

 toucans, the ceca are absent or very rudimentary. In most birds they 

 are paired and in many cases attain extraordinary length and give 

 evidence of great functional importance. In the common pigeon they 

 are less than an inch long; in the domestic fowl, about one-third the 

 length of the body; in a grouse, according to Richard Owen, each 

 cecum is "a yard long" and, on the same authority, the ostrich has two 

 ceca, each "upwards of two feet" long and having its internal epithe- 

 lium uplifted to form a projecting membrane extending spirally length 

 wise of the cecum — a "spiral valve" structurally similar to that in the 

 intestine of a shark. In the grouse, too, the internal surface of the long 

 ceca is much increased by the presence of several projecting longi- 

 tudinal folds of the lining, but they are not spirally arranged. Unques- 

 tionably the ceca in birds are important digestive organs. Partly 

 digested food from the small intestine passes into the ceca to undergo 

 further or complete digestion and absorption. Thus the ceca compen- 

 sate for the relative shortness of the main intestinal tract. 



The liver and pancreas are well developed — the pancreas espe- 

 cially so — and, as usual, connect by ducts with the extreme anterior end 

 of the small intestine. An unusual feature is the multiplicity of ducts — 

 commonly two hepatic or bile-ducts and two or three pancreatic ducts. 

 Many birds, including pigeons, have no gallbladder. 



The rectum opens into a capacious cloaca, in whose cavity may 

 be recognized three regions delimited by transverse folds or constric- 

 tions of the wall (Fig. 413). Most anterior is the coprodeum, into 

 which opens the rectum only. The middle region, urodeum, contains 

 the apertures of the urinogenital ducts. These two chambers are de- 

 rived from the endodermal wall of the embryonic digestive tube. The 

 posterior chamber, the proctodeum, is lined by infolded ectoderm. In 

 young birds a thick-walled glandular pouch, the bursa Fabricii, opens 



