THE ANATOMY OF BIRDS. — 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, througli the caval veins into the right auricle ; 
from right auricle through 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 
the 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 
capillaries, and so to the veins and heart again. Thus the pulmonary arteries convey black 
blood, the pulmonary veins red blood j 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 cii'culatory arrange- 
ment, 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 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- walled than the 
left J 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 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 ven(E cavce, bringing blood from the body at large, are 
thi'ee — 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 off 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 mth certain families and orders of birds. 
They are the first branches of the innominates. 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 will be perceived by the diagrams taken from Garrod's admirable paper (P. Z. S., 1873, 
p. 457). In nearly the words of this 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 vertebrae, in the hypapophysial canal. Birds with this 
arrangement Garrod calls aves hicarotidince normales (fig. 90). 2. In most birds, the carotid 
branch of the right innominate being not developed, 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 aU the way up. Such birds are said to have a left carotid, 
