292 GENERAL ORNITHOLOGY part ii 



into the right auricle ; from right auricle through the auriculo-ven- 

 tricular opening into right ventricle ; from right ventricle through 

 the pulmoncmj arteries to the lungs ; in the capillaries of which it is 

 relieved of its burden. There decarbonised and oxygenised, the 

 bright red aerated blood returns through the pulmonary veins to the 

 left auricle ; through the corresi3onding auriculo-ventricular opening 

 to the left ventricle, which pumps it out through the aorta and other 

 arteries to the capillaries, and so to the veins and heart again. 

 Thus the pulmonary arteries convey black blood, the pulmonary 

 veins red blood ; the reverse of the usual course. Before lungs come 

 into play, in the egg, the blood is purified in the alhmfois, an em- 

 bryonic organ which then sustains a respiratory function. Besides 

 the pulmonary there is another special circulatory arrangement, the 

 hepatic portal system of veins, by which blood coming from the 

 chylopoetic viscera (stomach, intestines, etc., which make chyle in 

 the process of digestion) strains through the liver before reaching 

 the heart. There is no renal j^ortal system in birds. 



The heart of birds is not peculiar in its conical shape, but is 

 more median in position than in mammals. There being no com- 

 pleted diaphragm, the pericardial sac which holds it is received in 

 a recess between lobes of the liver. The right ventricle is much 

 thinner-walled than the left ; the auricles have less of the elonga- 

 tion which has caused their name ("little ears" of the heart) in 

 mammals. The right auriculo-ventricular valve, which prevents 

 regurgitation of blood, instead of being thin and membranous, is a 

 thick fleshy flap which during the ventricular systole a])plies itself 

 closely to the walls of the cavity. The pulmonary artery and the 

 aorta are each provided at their origination with the ordinary three 

 crescentic or semilunar valves, as in mammals. The pulmonary 

 artery arises single, forking for each lung. The pulmonary veins 

 are two. The systemic veins, or vence cavce, bringing blood from the 

 body at large, are three — two precaval, from head and ujjper ex- 

 tremities, one piostcaval, from trunk and lower extremities. The 

 aorta, almost immediately at the root of that great trunk, Figs. 90- 

 95, h, divides into three primary branches ; right, n, and left, li, 

 innominate arteries, conveying blood to the neck, head, and upper 

 extremities ; and main aortic, a, which curves over to the right (left 

 in mammals) and supplies the rest of the body. More precise state- 

 ment is, perhaps, that the aortic root, h, first gives oft" the left in- 

 nominate, //, then at once divides into right innominate, ri, and 

 main aortic trunk, a, (right). It represents the fourth primitive 

 aortic arch of the embryo. On the whole, the avian heart is a great 

 improvement on that of most reptiles, though nearly resembling 

 that of Crocodilia ; it is substantially as in any mammal, though 

 differing in its fleshy right auriculo-ventricular valve, two instead 



