MUSCULAR SYSTEM. 667 
which from the dorsal surface is « sac of obscure function, the bursa 
Fabricii, which usually disappears during adolescence. It is at first 
a blood-forming organ, but often becomes a mass of fibrous connective 
tissue. t 
Vascular system.—The relatively large four-chambered 
heart, the complete separation of arterial and venous blood, 
the single aortic arch bending over to the ght side, and 
the hot blood (about 38° C., 100° F.), are important 
characteristics. The heart-beats are more rapid in birds 
than in other Vertebrates, being about 120 per minute 
when the bird is at rest, and far more when it is 
flying. 
The impure blood returned by the 
ven cave to the right auricle passes 
into the right ventricle through the cd. 
auriculo-ventricular valve (which has y, 
two muscular flaps without chorde 
tendinez or papillary muscles). From x a 
pd.. 
the right ventricle it is driven to the |. \ 
lungs. From the lungs the purified is 
blood returns to the left auricle, and ; 
passes through two membranous valves Fic. 369.—Diagram- 
(with chorde tendinez and papillary age uae be 
muscles) into the left ventricle. After Gadow. — 
“Thence it is driven through the .7 Upper region of clo- 
arterial trunk into the carotids, the aca into which rectum 
subclavians, and the dorsal aorta. ae 
The bases of the aortic and pul- te, ae ae 
monary trunks are guarded by three side; _d., posterior re- 
gion into which bursa 
semilunar valves. From the capil-  Patvicii (7) opens. 
laries the impure blood is .collected 
anteriorly in two superior venz cave (precavals), and 
posteriorly in an inferior vena cava (postcaval), composed 
of veins from hind-legs and kidneys, and receiving as it 
approaches the heart the hepatic veins from the liver. 
The right auricle of the heart is larger than the left ; the right ventricle 
has thin walls, and partly surrounds the more muscular left ventricle. 
The muscular right auriculo-ventricular valve does not quite encircle 
the opening from the auricle, an imperfect differentiation which recurs 
in the Monotreme Mammals. 
The arterial system consists of the following vessels (Fig. 370) :— 
