IMHsTlXAI, TRACT OF BlKDs. 2{; 



).j 



"org-ans, of their compromises and eo-ordiiiatioiis, in fact of their phices with regard to 

 one another and to the whole corporeal republic in every staije of the growing eiubrvo, 

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

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

 such value. 



ThK SlPUA-DlJODKNAI, LoOl'. 



In its natural condition the duodenum lies folded ventrally under the other portions 

 of the gut, and comes in vci-y close relation witii the distal portion of Meckel's tract. 

 Cuvier drew attention to tlie fact that in birds as in mammals the duodenum comes int.. 

 intimate relation witli a posterior portion of tlie gut, a relation so intimate tiiat the 

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

 Klaatscli. This portion of the gut which comes into relation with the ca^ca I have 

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

 since that term is applied in ]\tammalian anatomy to a portion of the gut posterior t(» 

 the cteca, and thereroro belonging to what I have been terming the rectum. Within 

 the group of liirds various stages in the evolution of tliis curious inter-relation 

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

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

 difficulty or cutting of blood-vessels. The same conditions obtain in a nundicr of tin- 

 archecentric types, and in the systematic portion I have referred to these. lOveu 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 draininu- 

 the caeca. As the ca'ca 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 tliird accessory recurrent vein i-iui 

 to nearer the apex of the duodenum, having traversed the mesentery as " l)ridging " veins, 

 and arisen from the long ca-ca and the posterior portion of Meckel's tract. Tn sucli a 

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

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

 nerves. The portion of the Tract to which I he ca'ca are adhercMit is not distinctly 

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

 apocentric types. But, as I have shown in the systematic \Kn-{un\, pari puxsu with the 

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

 loop clearly marked off from the general sweej) of the Tract, finally, in tiiose birds 

 where the eieca 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 sucli a form the supra-duodenal loop generally 

 hecomes 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, bvit still retaining evidence of past complexity. I do not tliink it can be doubted 



