THE ANATOMY OF BIIWS. — ANGEIOLOGY. 197 



which then closes. Arteries proceed from the strong muscular ventricles ; veins are received by 

 the weaker auricles. The course of the blood is: From the body excepting the lungs it comes, 

 dark and heavy with products of decomposition, through the caval veins into the right auricle ; 

 from right auricle througli the auriculo- ventricular opening into right ventricle ; from right ven- 

 tricle through the pulmonary arteries to the lungs ; in the capillaries of which it is relieved of 

 its burden. There decarbonized and oxygenized, the bright red aerated blood returns through 

 xhe pulmonary veins to the left auricle ; through the corresponding auriculo- ventricular open- 

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

 ('ai)illaries, and so to the veins and heart again. Thus the pulmonary arteries convey black 

 hl( lod, 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 allantois, an embryonic organ which then sustains 

 a respiratory function. Besides the pulmonary there is another special circulatory arrange- 

 ment, the liepatic portal system <.)f 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 portal 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 completed diaphragm, the pericardial sac which holds it is received 

 in a recess between lobes of the liver. The right ventricle is much thinner-waUed than the 

 left; the auricles have less of the elongation 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 applies itself closely to the walls of the cavity. The pulmonary artery aiid 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 vente caves, bringing blood from the body at large, are 

 three — two pre-caval, from head and upper extremities, one post-caval, 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, ri, 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 statement is, perhaps, 

 that the aortic root, h, first gives oflF the left innominate, li, 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 of one pre-caval vein, 

 right instead of left aortic arch, and mode of origin of the primary aortic branches. 



The zoological interest of the avian blood-vessels centres in the carotid arteries, which, 

 with the vertebral arteries, supply the neck and head. The carotids may be single or double ; 

 and other details of their disposition correspond well with certain families and orders of birds. 

 They are the first branches of the iunominates. In most birds, there is but one carotid, the 

 left ; in a few, one, formed by early union of two ; in many, two, long distinct. The arrange- 

 ment wiU be perceived by the diagrams taken from Gari'od's admirable paper (P. Z. S., 1873, 

 p. 457). In nearly the words of thig author: 1. In what may be termed the typical arrange- 

 ment (though it is not the usual one), two carotids, of equal size or nearly so, run up the front 

 of the neck, converging till they meet in the middle line, and so continue up to the head, on the 

 front of the bodies of the cervical vertebra, in the hypapophysial canal. Birds wath this 

 arrangement Garrod calls aves hicarotidincB normales (fig. 90). 2. In most birds, the carotid 

 branch of the right innominate being not develojied, only the left, of larger size, traverses the 

 hypapophysial canal ; but it bifurcates before reaching the head, thus producing two carotids, 

 distributed as if there had been two all the way up. Such birds are said to have a left caroli '. 



