VASCULAR SYSTEM 749 



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), com- 

 posed 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. 449) : — 

 {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. 

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

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

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

 (c) The pulmonary arteries carry impure blood from right ventricle 

 to lungs. 

 The venous system consists of the following vessels (Fig. 450) : — 

 (a) Two superior venae cavae, each formed from the union of 

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

 from the breast. 

 (6) 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 

 this, passing upwards through the kidney, receives the sciatic, 

 and finally joins with the femoral and with the renal, 

 (c) The pulmonary veins carry pure blood from lungs to left auricle. 



