2l6 ELEMENTS OP OENITHOLOGY. 



The bronchi lose their rings after entering the lungs, and 

 give off secondary branches at right angles (fig. 166), and these 

 again tertiary ones in a similar way, ultimately ending, in the 

 lobules of the lungs, in very minute air-sacs, which make up 

 the bulk of the substance of these organs. 



The CiECtriATOET System. 



The Iteart of a Bird consists, like our own, of four cavities — 

 two auricles and two ventricles. The blood is collected from all 

 parts of the body by the veins, which gradually unite together 

 and end in three large vessels — two called the superior venas 

 cavce and the other the inferior vena cava — which pour the venous 

 blood they contain into the right auricle, whence it passes into 

 the right ventricle, a valvular flap formed of muscle — the right 

 auriculo-ventrioular valve — preventing its regurgitation. From 

 the right ventricle a vessel goes forth called the pulmonary 

 artery ^ which bifurcates and then subdivides in either lung, 

 conveying the blood into it. Thence it is collected and brought 

 back, by converging vessels called the pulmonary veins, to the 

 left auricle, whence it passes into the left ventricle, another 

 valvular structure — the left auriculo-ventricular valve — pre- 

 venting its regurgitation. Prom the left ventricle it is sent 

 forth by a single great artery called the aorta, which ramifies, 

 sending blood all over the body, its branches ultimately ending 

 in most minute vessels called capillaries, which lead to the 

 commencement of the veins of the body. Thus all the vessels 

 proceeding from the heart are called " arteries," whilst those 

 advancing towards it are termed " veins," and a complete circu- 

 lation goes on as above indicated. There is, however, another 

 subordinate circulation which takes place in what is called the 

 ■portal system, or system of vessels ramifying in the substance 

 of the liver. The vessels which thus convey blood to the liver 

 and therein ramify are called portal veins, while those which 

 collect the blood from the liver and carry it on towards the 

 heart and into the inferior vena cava are termed hepatic veins. 

 The blood collected by the portal veins comes from the stomach, 

 intestines, and also from a caudal vein. The heart of a bird is 

 more pointed than that of Mammals, and its apex lies between 

 the lobes of the liver. 



The aorta as it proceeds from the heart arches over the right 



