VASCULAR SYSTEM. 643 



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 (with chordae tendineae and 

 papillary muscles) into the left ventricle. Thence it is 

 driven through the arterial trunk into the carotids, the 

 subclavians, and the dorsal aorta. The bases of the aortic 

 and pulmonary trunks are guarded by three semilunar 

 valves. From the capillaries 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. 



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. 320) :- 



(a) The arterial trunk, as it rises from the heart, gives off on each 



side an innominate artery. Each innominate gives off a carotid 

 and a subclavian, and the subclavian immediately divides into 

 a brachial to the arm and a pectoral to the breast muscles. 



(b) The dorsal aorta, formed by a continuation of the arterial trunk 



bending round on the right side, gives off cceliac, mesenteric, 

 renal, femoral, sciatic, iliac, and other arteries. 

 (<:) The pulmonary arteries carry impure blood from right ventricle 



to lungs. 

 The venous system consists of the following vessels (Fig. 321) : 



(a) Two superior venre cavce, each formed from the union of 

 jugulars from the head, a brachial from the arm, and a pectoral 

 from the breast. 



(b) The inferior vena cava is formed from the junction of two iliac 



veins just in front of the kidneys. Each of these iliacs results 

 from the union of a femoral from the leg, an efferent renal 

 from the kidney, and a " renal-portal," or hypogastric, which 

 passes upwards through the kidney. To understand this hypo- 

 gastric, it is convenient to begin at the tail. A short caudal 

 vein divides anteriorly into right and left branches, each of 

 which receives an internal iliac from the sides of the pelvic 

 region. Thus the hypogastric is formed at each side, and 



