INTESTINAL TKACT OF BIRDS. 2G1 



certain cases nerves may " bridge/' or rather bore tliroui-li, the m.'sentery and pass across 

 a narrow portion of coelomic sjiace. The most obvious case of tills occurrence is where^ 

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

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

 Passerines, Avhere tlie supra-duodenal loop is a structure of considerable imjjortance, but 

 I am not yet prepared to demonstiati- this. 



MOKPHOLOGY OF THE L\T1£ST1.\.\L TkACT. 



The chief writers on this subject witliin conii)aratively recent times have l)een Tohit 

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

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

 mesentery; andMathes (24). who folloa^Ml closely tln^ work of Klaitsch, but dealt chiefly 

 with the development of tiie mesenteries in tiie Amphil)ia. None of these writers has 

 ])aid special attention to the conditions tliat exist among liii-ds, but tlieir work has been 

 of great assistance to me in interpreting and coordinating my own investio-ations. Jt is 

 plain that in all the higher vertebrates the hitistinal tract is thrown into three main 

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

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

 It arises extremely early in ontogeny, and while in the majority of eases it remains 

 simple, it may develop many minor complexities, sometimes simply becomiuo- wider, 

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

 more or less regular spirals, the sj)iral 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 preseatly have more to sav. The 

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

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

 anterior liml). and there passes into the second j)ortion of the gut. 'I'his portion, Avhich 1 

 name Meckel's Tract, extends from the duodenum to the insertioii of the cicea. The 

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

 of only a very small section of the primitiA'c gut. Its proximal extremity approaches its 

 distal extreuiitv so closely in the line of the dorsal attachment of the mesenterv, that in 

 the majority of eases it would be possilde to remove the Mhole of Meckel's tract and 

 suture the cut edge of the duodenum to the cut proximal edge of tlie rectum, and 

 a. most without dislocation reconstruct a primitive straight intestinul 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 tlie principal mesenteric artery. Toldt's figures, and 

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

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

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

 phylogenetic development of the A'ertebrate gut, and that it ecu-responds to not mon- 

 tlian two, or possibly three, of tlie primitive somites of the budy. When tlie development 

 and comparative anatomy of the intestinal nervous chain in 13irds has been worked out. 



SKCONIi .SERIES. — ZOOLOGV, VOL. V1J[. 39 



