SI2 



BIRDS. 



glandular part of the stomach, known as the proventriculus. 

 Here the gastric juice is secreted. 



Beneath the proventriculus is the gizzard, in which the 

 food is ground. The walls are very muscular, the fibres 

 radiating from two tendinous discs ; the internal surface is 

 lined by a hard cuticle, and within the cavity are small 

 stones which the bird has swallowed. The pyloric opening 

 from the gizzard into the duodenum, is very near the open- 

 ing from the proventriculus into the gizzard. 



In the fold of the long duodenum lies the pancreas with 

 three ducts, and into the same region open two bile^ducts 

 from the liver, which is without a gall-bladder. In most 

 birds the gall-bladder is present. The subtle psychical 

 theory by which the ancients explained its presence in birds 

 of prey and its absence in doves, is untenable. 



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. 



The cloaca has three divisions, an upper part into which 

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

 the genital ducts open, and a posterior region opening into 

 which from the dorsal surface is a vascular and glandular 

 sac called the bursa Fabricii, which usually disappears during 

 adolescence. The function of this bursa is unknown. 



The Vascular System. — The four-chambered heart, the 

 single aortic arch bending over to the right side, the hot blood, 

 are the most important characteristics. 



The impure blood having returned by venae cavse to 

 the right auricle, passes through the auriculo-ventricular 

 valve which has two muscular flaps into the right ven- 

 tricle, and is thence driven to the lungs. From the lungs, 

 the purified blood returns to the left auricle, and passes 

 through two membranous valves into the left ventricle. 

 Thence it is driven up 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 cavae, and posteriorly in an 

 inferior vena cava, composed of veins from hind-legs and 



