INTESTINAL TEACT OF BIRDS. 267 



suppose that want of space played a part in its ontogenetic appearance. Plainly, when 

 thepyloric valve relaxes and the contents of the stomach are poured into the duodenum, 

 the shock will be transferred to the supra-duodenal loop which lies closely applied to 

 the duodenum and sometimes in special nerve-connection with it. And thus discharge 

 of the contents of the posterior region of the gut into the rectum may he set about 

 without the necessity of peristaltic waves traversing the whole length of Meckel's tract. 



THE COLIC C^ECA. 



Gadow (12), Furbringer (9), Beddard (2), and Oppel (30), in their respective treatises 

 have devoted so much attention to the voluminous literature concerning the colic caeca 

 of birds that I need not refer to older writers. The archecentric condition of these 

 organs in birds, a condition which is probably an heritage from lieptilian ancestors, is the 

 existence of a pair of caeca growing from the point where the distal end of Meckel's 

 tract passes into the rectum. Such primitive caeca proximally are applied more or less 

 closely to the posterior portion of Meckel's tract. They are of moderate length ; their 

 walls are not specially thickened, their lumens are widely open to the gut, and their 

 contents consist of food-material in a state more akin to that in the rectum than to the 

 state in Meckel's tract. When, as happens frequently, there is a difference in colour 

 apparent through the intestinal wall and marking the different stages of metabolism, the 

 colour of the caeca approximates to that of the rectum. These primitive caeca probably 

 had a digestive function of some sort, for the presence in their walls of glands, of 

 absorbing veins, and occasionally of villi show that they were not mere reservoirs of 

 faecal matter. From the primitive condition various apocentric modifications have 

 arisen. The caeca may increase very greatly in size, and may develop spirally arranged 

 septa protruding more or less into the cavity and deeply marking the exterior, as in 

 Chauna and some of the Ratites and Gruiformes, or the whole external surface may be 

 prolonged into a number of papilliform hollow outgrowths, as in Calodromas described 

 by Beddard (2). These enlarged caeca appear to retain their digestive functions. 

 Secondly, the caeca may become very much reduced, but in such apocentric reduction I 

 am convinced that there are two quite different conditions to be noted. In the one case, 

 the reduction may he nothing more than the degeneration of an organ that has become 

 f unctionless ; and almost any stage from the archecentric size to complete absence may 

 exist. In the Columbidae, for instance, the caeca if present are always small and 

 frequently are thin- walled, irregular (I have noted many cases of individual absence of 

 one caecum), and sometimes pigmented. In the fruit-eating Pigeons, and indeed in 

 many other Pigeons apart from the nature of their food, the vestiges have disappeared 

 completely. Precisely a similar series of events occurs among most Falconiform birds. 

 The caeca are small in all, and when present appear to be functionless, thin-walled 

 vestiges, frequently unsymmetrical (here, again, absence on one side has been noted in 

 different birds by different observers). Sometimes they appear to be absent, but inflation 

 of the wall of the gut reveals the presence of slight, thin-walled rudiments in their 

 place. In the Vultures they are sometimes totally absent, and a similar absence is a 



