On the dorsal wall of the cloaca between it and the spine, is a small sac, called the _bursa of Fabricius_, which has a duct communicating with the cloaca. The mucous membrane of this sac is thrown into folds and is studded with glands. The bursa of Fabricius is larger in the young than in the adult bird. It apparently atrophies as the bird becomes older. When the bird is four months old this bursa is best developed, and at this age it may be as large as 2 or 3 centimeters in diameter. _Function._—The function of the cloaca is to give passage way to the feces, the urine, and the egg and to act as an organ of copulation. =Course of the Food.=—The food first enters the mouth, after being picked up by aid of the beak. From here it passes through the pharynx and first portion of the esophagus to the crop, without mastication, as the bird is not provided with teeth. The food is passed from the crop by aid of its muscular walls as needed; thence through the second portion of the esophagus to the proventriculus, an expansion in the digestive tube just before it terminates in the gizzard. The glands of the proventriculus produce a secretion in which the food is soaked before passing into the gizzard. The gizzard is provided with strong muscular walls which, by aid of grit, thoroughly reduce the food to fineness. From the gizzard the food passes through the first portion of the small intestine, where it is subjected to the action of the bile from the liver, the pancreatic juice from the pancreas, and of the succus entericus from the glands of the intestinal wall. The food is then passed into the cæca. The indigestible portion of the food passes from the cæca through the large intestine, or rectum, to the cloaca and thence to the external world. The digestive functions of the bird are very potent and rapid. This compensates for the waste caused by their extensive, frequent, and energetic motions, and is in accordance with the rapidity of their circulation and their high state of irritability. THE ACCESSORY ORGANS OF DIGESTION The accessory organs of digestion are the liver, pancreas, and some anatomists include also the spleen. The first two manufacture fluids containing ferments which aid in splitting or digesting the food. =The Liver= (Figs. 31, 33 and 34). _Location._—The liver, hepar, lies ventrally and posteriorly to the heart. It is related anteriorly with the diaphragm, inferiorly with the sternum, posteriorly with the gizzard and intestine, and superiorly with the ovary, oviduct and proventriculus and laterally with the abdominal wall. _Shape._—The liver is a voluminous deep livid brown gland, soft and friable in texture. It is divided into two principal lobes, a right and a left.