INTESTINAL TRACT OF BIRDS. 265 



organs, of their compromises and co-ordinations, in fact of their places with regard to 

 one another and to the whole corporeal republic in every stage of the growing embryo, 

 nothing more can be said but that such complex uniradial apocentricities, if not in them- 

 selves possessed of " selection-value," may stand in. correlation with structures that have 

 such value. 



THE SUPRA-DUODENAL Loop. 



In its natural condition the duodenum lies folded vent rally uuder the other portions 

 of the gut, and comes in very close relation with the distal portion of Meckel's tract. 

 Cuvier drew attention to the fact that in birds as in mammals the duodenum comes into 

 intimate relation with a posterior portion of the gut, a relation so intimate that the 

 mesenterial folds suspending the two portions may fuse after the fashion explained bv 

 Klaatsch. This portion of the gut which comes into relation with the caeca I have 

 called the supra-duodenal loop, preferring not to call it " colon " as was done by Cuvier, 

 since that term is applied in Mammalian anatomy to a portion of the gut posterior to 

 the caeca, and therefore belonging to what I have been terming the rectum. Within 

 the group of birds various stages in the evolution of this curious inter-relation 

 between the proximal and distal portions of the gut are displayed. Thus in Palamedca 

 (fig. 1) there is no supra-duodenal loop, and the gut may be unfolded witiiout any 

 difficulty or cutting of blood-vessels. The same conditions obtain in a number of the 

 archecentric types, and in the systematic portion I have referred to these. Even in 

 Palamedea, however, the earliest stage in the formation of the connection is apparent. 

 A short recurrent factor of the duodenal vein runs in the mesentery at the dorsal portion 

 of Meckel's tract across from the terminal portion of that tract and assists in draining 

 the cseca. As the caeca increase in size this vessel becomes larger and of more 

 importance, and, in many cases which I have referred to in the systematic portion, the 

 recurrent vein along with a second and sometimes a third accessory recurrent vein run 

 to nearer the apex of the duodenum, having traversed the mesentery as " bridging " veins, 

 and arisen from the long caeca and the posterior portion of Meckel's tract. In such a 

 mode, an intimate relation is established between the duodenum and the distal portion of 

 Meckel's tract, and, as in Otis, this relation may involve not only the veins but the 

 nerves. The portion of the Tract to which the caeca are adherent is not distinctly 

 marked off from the more proximal portion of the Tract in very many of the less 

 apocentric types. But, as I have shown in the systematic portion, pari passu with the 

 establishment of the " bridging" veins, the distal portion of the Tract becomes a distinct 

 loop clearly marked off from the general sweep of the Tract. Finally, in those birds 

 where the caeca have degenerated either completely or have shortened to glandular 

 nipples, the supra-duodenal loop is retained with its separateness from the rest of the 

 Tract and with its " bridging " veins. In such a form the supra-duodenal loop generally 

 becomes very accurately moulded to the contour of the duodenum, and its presence and 

 completeness are important reasons for seeing in the apparent simplicity of the gut in 

 many of the higher forms, such as the Passeres, a pseudocentricity a condition apparently 

 simple, but still retaining evidence of past complexity. I do not think it can be doubted 



