They are

sometimes completely absent, as in the majority of that large assemblage of birds the picarians, but when present they show every degree of relative size. In the Passeres, and in some other birds, the two cæca are the merest nipples, which cannot be believed to serve any function. in the same way reduced in size in the hawks and storks. On the other hand, in the gallinaceous birds, in the Limicolæ, and in some others the cæca are large tubular diverticula of the gut, the length being sometimes to be measured by inches. Among the owls the сӕса are large, and have the additional peculiarity of being swollen at the blind extremity.

FIG. 15.-Parus major.

r, as in fig. 12. (After CHALMERS MITCHELL.)

The cæca are most complicated in the ostrich, the screamers, and the tinamou Calodromas, under the descriptions of which birds will be found an account of this organ. The herons are remarkable for the fact that one of the two cæca has disappeared, the remaining one being but small.

The liver is invariably composed of two lobes, of which the left often shows a more or less distinctly marked secondary division. The size of the lobes varies greatly, as does their relative size. Thus in some birds the liver lobes are quite hidden by the sternum; in others again they descend some way down below the shelter of that bone and are apparent when the muscular walls of the abdomen are cut through or removed. The two lobes are occasionally equal or subequal in size; more generally there is a discrepancy, the right or left, as the case may be, being the larger, sometimes very much the larger. The two lobes of the liver are commonly firmly attached to each other by a bridge of hepatic tissue. In Chauna they are nearly separate, being only united by a very narrow isthmus of liver substance. The liver sometimes (e.g. in Rhynchotus rufescens) has two or three small vessels, belonging to the portal system, entering its substance at the free edge, a state of affairs which has a

very lizard-like appearance. The relative sizes of the liver lobes appear to be of no importance systematically.

The gall bladder is an organ which is not invariably present in birds. It is even sometimes present and sometimes absent in the same family (e.g. parrots). As a rule this vessel is of a rounded or oval contour and is embedded on the surface of the right lobe of the liver. The Picidæ, Capitonidæ, and Rhamphastidæ are remarkable for the extraordinarily elongated gall bladder, which reaches a long way down the abdominal cavity; this is described more fully below. The penguin has an almost equally elongated

gall bladder. The position of the apertures of the cystic and hepatic ducts upon the small intestine varies. The ostrich is remarkable for the fact that the single duct opens practically into the stomach.

The pancreas lies in the fold of mesentery that unites the two arms of the duodenal loop. It is commonly more or less distinctly composed of two parts, and in relation to this. there are two pancreatic ducts which pour its contents into the duodenum. Apparently, however, no value can be

D

attached to either the form of the gland or the number and position of the orifices of its ducts. In Syrrhaptes paradoxus, for instance, both of the arrangements figured in the accompanying cuts have been found by BRANDT, who investigated the structure of the bird. In one of them both ducts open close to each other and to the cystic duct on the ascending part of the duodenal loop; in the other the cystic and hepatic ducts were on opposite sides of the duodenal loop, and in common with each opened a single pancreatic

duct. This latter arrangement was found by GADOW in Pterocles. In two species of Rhea the relative positions of the pancreatic and bile ducts were as is shown in the figures. In the owl Photodilus badius I found that the cystic duct opened near to the summit of the ascending arm of the duodenal loop; below this opened the hepatic duct, and some way below this again, and near together, the two pancreatic ducts. A good many details upon this subject will be found in GADOW's paper on the digestive organs of birds.

The cloaca of birds is the terminal chamber of the alimentary canal, which also receives the urinary and genital ducts, and is provided with an appendix of unknown function,

1

the so-called bursa Fabricii. GADOW, in a recent work upon this region of the alimentary canal, recognises three chambers in the cloaca. Above, and separated by a constriction from it, is the coprodæum, into which the rectum opens; this is divided by a constriction from the middle chamber, or urodæum, which receives the genital and urinary ducts;

FIG. 21.-CLOACA OF Chauna derbianu LAID OPEN FROM IN FRONT.

a, rectum; b, orifices of ureters; b', genital papillæ; c, fold separating coprodæum from urodæum; d, fold separating urodæum from proctodæum; e, opening of f, bursa Fabricii. (After FORBES.)

then follows the proctodæum, of which the bursa Fabricii is a diverticulum.

2

The bursa Fabricii has been chiefly investigated by FORBES and WENCKEBACH.3 It is a dorsal diverticulum of the proctodæum, and therefore has nothing to do with the

'Remarks on the Cloaca and on the Copulatory Organs of the Amniota,' Phil. Trans. vol. clxxviii. p. 5.

2 On the Bursa Fabricii in Birds,' P. Z. S. 1877, p. 304.

* De Ontwikkeling en de Bouw der Bursa Fabricii,' Inaug. Diss. Leyden

1888. See also E. RETTERET, Contribution à l'Etude du Cloaque,' &c., J. de l'Anat. xxi. 1885, p. 369.