MUSCULAR SYSTEM. 



667 



vud, 



which from the dorsal surface is a 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 fibtoMS connective 

 tissue. 



Vascular system. The relatively large four-chambered 

 heart, the complete separation of arterial and venous blood, 

 the single aortic arch bending over to the right 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 

 venae cavae to the right auricle passes 

 into the right ventricle through the 

 auriculo-ventricular valve (which has 

 two muscular flaps without chordae 

 tendineae or papillary muscles). From 

 the right ventricle it is driven to the 

 lungs. From the lungs the purified * 

 blood returns to the left auricle, and 

 passes through two membranous valves F IG - 3^9- Diagram- 

 (with chordae tendineae and papillary matic section of clo- 

 v i \ ' i /% , i 



muscles) into the left ventricle. 



Thence it is _ driven through the 



arterial trunk into the carotids, the 



subclavians, and the t dorsal aorta. 



The bases of the aortic and pul- 



monary trunks are guarded by three 



semilunar valves. From the capil- 



laries the impure blood is collected 



anteriorly in two superior venae cavae (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. 



aca of male bird. 

 After Gadow. 



?., Upper region of clo- 

 aca into which rectum 

 opens; ud., median re- 

 gion into which ureter 

 (u.) and vas deferens 

 (vd.) open from each 

 side ; pd., posterior re- 

 gion into which bursa 

 Fabricii {B.F.) opens. 



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) : 



