INTESTINAL TRACT OV lURDS. 267 



suppose that want of space played a i)art in its ontogenetic appearance. JMainly. wlien 

 thepyloi-ic valve relaxes and the contents of the stoniacli are poured into the duodenum, 

 the shock will he transferred 1o the supra-duodenal loop wliicli lies closely applied to 

 the duodenum and sometimes in sjieeial nerve-connection with it. Aiul thus discharffc 

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

 without the necessity of peristaltic waves traversinij the wliole len<,'tii of Meckel's tract. 



Thk Colic C.kca. 

 (iadow (i2), Fiirbringer (9), Beddard (2), and Oppel (30), in their respective treatises 

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

 of birds tliat I need not refer to older writers. The arcIuH-entric condition of these 

 organs in Itirds, a condition which is probably an heritage from Itrptilian ancestors, is the 

 existence of a pair of cu'ca growing from the point when; the distal end of Meckel's 

 tract passes into the rectum. Such primitive ca-cn proximally are applied more or less 

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

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

 contents consist of food-material in a state more akiu to that in the rectum tlian 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 diiferent stages of metabolism, tlie 

 colour of the caeca approximates to that of the rectum. These primitive caecii probablv 

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

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

 hecal matter. From the primitive condition various apocentric moditi('ations have 

 arisen. The c;eca may increase very greatly in size, and may develop spirally arranged 

 septa protru(iin<; nu)re or less into tlie cavity and deeply marking the exterior, as in 

 L'haiina and some of the Ratites and Gruitbrmes, or the whole external surface may bi' 

 prolonged into a luunber of papilliform hollow outgrowths, as in Calodromda described 

 by Beddard (2). These enlarged c;eca appear to retain their digestive functions. 

 Secondly, the ctpca 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 be nothing more than the degeneration of an organ that has l)ecame 

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

 exist. In the Columbidoe, for instance, the ca'ca if present are always small aiul 

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

 one cjecuittl, and sometimes pigmented. In the fruit-eatinii' Pigeons, and indeed in 

 many other Pigeons apart from the nature of their food, thr \estigrs have disappeared 

 completely. Pr(^eisely a similar series of events occurs among most Kalconiform birds. 

 The caeca are small in all, and when present appear to he 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, hut inflation 

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

 place. In the Vultures they are ^onu'times totally absent, and a similar absence is \x 



