DIGESTIVE SYSTEM 141 



convolutions reveals their taxonomic value, and enables me to say 

 that the Digestive System, taken in its entirety — that is to say, 

 the crop, glandular and muscular stomach, liver, gall-bladder with 

 its ducts, cteca, and the relative length and convolutions of the in- 

 testinal canal — aftbrds more diagnostic features than any other 

 organic system — the osseous excepted. Moreover, it has the great 

 advantage that through reference to the food we can in many cases 

 account for the aberrant features of the digestive organs displayed 

 by birds otherwise closely allied. So much cannot be said for char- 

 acters furnished by Pterylosis, and attempts to explain taxonomic- 

 ally the more important difierences observable in the Muscular 

 System have hitherto been futile because of the complex problems 

 involved. At any rate, we ought not to treat recent birds as if " 

 they' were fossil and had left us nothing but their bones, unless, 

 indeed, the specimens be skinned and all their other important char- 

 acters thrown away. 



It is hoped therefore that a brief general account, condensed 

 from a paper in the Zoological Proceedings for 1889 (pp. 303-316), 

 of the chief types of intestinal structure in birds may here have 

 interest, especially as, with the exception of Cuvier, British Ana- 

 tomists only^ have treated the subject, and since the days of 

 Macgillivray, who alone attempted it systematically, this branch 

 of Ornithotomy has been neglected, perhaps from the apparent but 

 not real difficulty of studying these easily-putrefying organs. 



In a typical loop of the intestines of a bird we distinguish 

 between a descending and an ascending branch ; both meet at the 

 distal end or apex of the loop, and this forms its turning-point. 

 The starting-point is the pylorus, the goal the cloaca. Each looj) 

 is either closed or open. It is closed when both the descending 

 and the ascending branches are throughout the length of the loop 

 closely bound together by an extension of the mesentery and its 

 vessels. Of these vessels, as a rule, each principal loop receives 

 one bigger branch from the middle mesenteric artery. A loop is 

 open when its two branches are not closely connected by mesentery 

 and vessels ; the mesentery is wider, and the two branches of the 

 looji may receive another loop or intestinal fold between them, the 

 latter then resting upon the mesentery of the former open loop. 



The duodenum is always a typically-closed loop. Its first or 



^ E. Home, The course of the intestine with the varieties in the form of the 

 caeca in carnivorous, piscivorous, and granivorous birds, Phil. Trans, 1814. G. 

 Cuvier, Lemons d'anatomie comimree, ed. 2, 1835. K. Owen, Todd's Cyclopmdia 

 of Anatomy and Physiologij, article " Aves," 1836. W. ]\Iacgillivray, "Obser- 

 vations on the Digestive Organs of Birds," 3£ag. Zool. and Bot. 1837. Occasional 

 notes on the intestinal canal are extremely numerous, among others by Burton, 

 Crisp, Duvernay, Forbes, Garrod, Jobert, Leuckart, L'Herminier, Martin, Nitzsch, 

 Pavesi, Perrin, and Yarrell , 



