206 DR. P. CHALMERS MITCHELL ON THE 



that wherever in short-gutted birds without large caeca the supra-duodenal loop is present 

 in its elaborate form, these birds at one time had a longer gut, and almost certainly 

 possessed longer caeca. Where a gut is long, and where its irregularly folded loops lie 

 closely together in a narrow space, the establishment of an intimate connection between 

 any of the crowded loops would call for little remark. Where, as in the Passeres and 

 many other short-gutted forms, there appears to be abundant space, and yet the 

 duodenum and the supra-duodenal loop are accurately moulded, the one on the other, 

 and in intimate vascular, nervous, and mesenterial connection, we must seek for the 

 origin of the complexity in the past history. 



There can be no doubt, I think, that the presence of a specialized supra-duodenal 

 loop marks a high degree of apoceutricity in the intestinal tract possessing it, but it is 

 equally plain that this apocentricity is multiradial and no guide to affinity. The 

 presence of caeca of at least moderate length is a fundamental or archecentric character 

 of birds ; and, if the supra-duodenal loop has arisen in the mode I have indicated, it is 

 clear that it may have arisen repeatedly, and in my systematic description I showed that 

 actually it does appear, repeatedly and apparently independently, in the different groups. 

 The probably multiradial nature of the structure is also supported by the occurrence 

 of similar formations among Mammals, these formations not even being on exactly 

 homologous parts of the gut. The mammalian structures with which the Supra- 

 duodenal Loop may be compared are the loop which in Man has the transverse colon as 

 its apical portion, and the sigmoid flexure, which in embryonic Mau reaches much 

 further towards the duodenum. Naturally, I do not propose to enter at present into the 

 various modifications presented by these structures ; it is enough to say that the 

 connections in Mammals between the duodenum and posterior regions of the gut are 

 frequently present, but less frequently than in the case of Birds. 



Dealing with its occurrence in Man, Toldt sees in these connections between the 

 proximal and distal portions of the gut the mere result of apposition. Klaatsch 

 criticises this view, and rightly points out that, although many of the other loops are in 

 equally close spatial relations, fusion does not necessarily occur among them. In Birds, 

 however, as I have shown, secondary fusions and " bridging " veins are not absolutely 

 confined to the duodenum and supra-duodenal loop. Thus in Psittaci they are frequent 

 among the numerous long and narrow loops into which Meckel's tract is thrown. In 

 Anatidae secondary fusions and " bridging " veins are of frequent occurrence in the case 

 of different portions of the same loop, notably in the axial loop, and in Spatula in the 

 long loop distal to Meckel's diverticulum (fig. 24). Finally, in the Cicouiae bridging 

 veins and secondary fusion of the mesentery occur between the duodenum and the 

 proximal minor loop of Meckel's tract. These occurrences, however, are less important 

 and much less frequent than the supra-duodenal connections. Klaatsch accounts 

 for these in the case of Reptiles and Birds by the mode in which the coelomic divisions 

 are broken up by the intruding blood-vessels and viscera. I think, however, that, in 

 birds, the formation may serve a useful purpose ; the supra-duodenal loop is maintained 

 and made even more elaborate after the degeneration of the caeca, although it arose in 

 relation to the cseca ; moreover, it is specially perfect in cases where there is no reason to 



