216 ELEMENTS OF ORNITHOLOGY. 
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 paetanee of these organs. 
THE CIRCULATORY SYSTEM. 
The heart 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 vencee 
cave 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-ventricular 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 awriculo-ventricular valve— pre- 
venting its regurgitation. From 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 
