608 BIRDS. 



In the fold of the long duodenum lies the pancreas with 

 three ducts, whose number points to the triple origin of the 

 pancreatic rudiment in the embryo. Into the same region 

 open two bile ducts from the two-lobed liver, which is 

 without a gall-bladder in the common pigeon, though this is 

 present in some birds, and even in some species of pigeon. 



The small intestine is long ; the large intestine is very 

 short ; in fact, it is not more than a rectum two inches in 

 length. At the junction of the small and the large intestine 

 there are two short caeca. In some birds, e.g. the fowl, 

 these are of considerable length ; in the ostrich they are 

 very long ; in the hornbills, etc. , they are absent. 



The cloaca has three divisions (see Fig. 267), — an upper 

 part into which the rectum opens, a median part into which 

 the ureters and the genital ducts open, and a posterior region 

 (proctodeum), opening into which from the dorsal surface 

 is a vascular and glandular sac of obscure function, the 

 bursa Fabricii, which usually disappears during adolescence. 



Vascular system. — The relatively large four-chambered 

 heart, the complete separation of arterial and venous blood, 

 the single aortic arch bending over to the right side, and 

 the hot blood (about 38° C, 100° F.), are important 

 characteristics. The heart beats are more rapid in birds 

 than in other Vertebrates, being about 120 per minute when 

 the bird is at rest, and far more when it is flying. 



The impure blood returned by the venae cava; to the 

 right auricle passes into the right ventricle through the 

 auriculo- ventricular valve (which has two muscular flaps 

 without chorda; tendinea; or papillary muscles). From the 

 right ventricle it is driven to the lungs. From the lungs 

 the purified blood returns to the left auricle, and passes 

 through two membranous valves (with chordae tendinea; and 

 papillary muscles) into the left ventricle. Thence it is 

 driven through the arterial trunk into the carotids, the 

 subclavians, and the dorsal aorta. The bases of the aortic 

 and pulmonary trunks are guarded by three semilunar 

 valves. From the capillaries the impure blood is collected 

 anteriorly in two superior venae cava; (precavals), and 

 posteriorly in an inferior vena cava (postcaval), composed 

 of veins from hind-legs and kidneys, and receiving as it 

 approaches the heart the hepatic veins from the liver. 



