INTESTINAL TRACT OF BIKDS. 2(51 



certain cases nerves may " bridge," or rather bore through, the mesentery and pass across 

 a narrow portion of coelomic space. The most obvious case of this occurrence is where ? 

 as in Otis, nerves leave a ganglion in the duodenal loop and pass to the caeca or the 

 supra-duodenal loop. I am practically certain that this happens in many of the small 

 Passerines, where the supra-duodenal loop is a structure of considerable importance, but 

 I am not yet prepared to demonstrate this. 



MORPHOLOGY OF THE INTESTINAL TRACT. 



The chief writers on this subject Avithin comparatively recent times have been Toldt 

 (35), who deals chiefly with the tract in Man; Klaatsch (18), who in two extremely 

 important memoirs dealt with the relation of the tract and its ancillary viscera to the 

 mesentery; andMathes(24), who followed closely the work of Klaatsch, but dealt chiefly 

 with the development of the mesenteries in the Amphibia. None of these writers has 

 paid special attention to the conditions that exist among .Birds, but their work has been 

 of great assistance to me in interpreting ;md coordinating my own investigations. It is 

 plain that in all the higher vertebrates the intt-stinal tract is thrown into three main 

 portions which are homologous throughout the series. Of these, the first is the 

 Duodenum, which in birds is always a closed loop lying ventral to the rest of the tract. 

 Jt arises extremely early in ontogeny, and while in the majority of cases it remains 

 simple, it may develop many minor complexities, sometimes simply becoming wider, 

 sometimes being thrown into numerous minor folds, and sometimes being twisted into 

 more or less regular spirals, the spiral duodenum being in some cases (Storks) wound 

 with the spirally twisted proximal loop of Meckel's tract. Concerning the relations of 

 the duodenum to the supra-duodenal loop I shall presently have more to say. The 

 distal extremity of the duodenum, however the course of that may have been complex, 

 always returns to the dorsal edge of the mesentery very close to the starting point of the 

 anterior limb, and there passes into the second portion of the gut. This portion, which I 

 name Meckel's Tract, extends from the duodenum to the insertion of the caeca. The 

 first important point about this large region of the gut is that it represents an outgrowth 

 of only a very small section of the primitive gut. Its proximal extremity approaches its 

 distal extremity so closely in the line of the dorsal attachment of the mesentery, that in 

 the majority of cases it would be possible to remove the whole of Meckel's tract and 

 surure the cut edge of the duodenum to the cut proximal edge of the rectum, and 

 a. most without dislocation reconstruct a primitive straight intestinal canal. In actual 

 development Meckel's tract, in all the vertebrates in which it is developed, arises as a 

 simple narrow loop in the line of the principal mesenteric artery. Toldt's figures, and 

 others given by Kollman (19), show this beautifully in the case of human embryos, and 

 general comparative anatomy from the Frog to Man makes the morphological nature of 

 Meckel's tract extremely plain. There can be no doubt that this is the most recent 

 phylogenetic development of the Vertebrate gut, and that it corresponds to not more 

 than two, or possibly three, of the primitive somites of the body. "When the development 

 and comparative anatomy of the intestinal nervous chain in Birds has been worked out, 



SECOND SERIES. ZOOLOGY, VOL. VIII. 39 



