VASCULAR SYSTEM. 609 



The right auricle of the heart is larger than the left ; the right ventricle 

 has thin walls, and partly suirounds 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. 268) : — 

 (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 cceliac, 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. 268a) : — 

 (a) Two superior vena? cava;, 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 renal- 

 portal, 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. 

 The hepatic portal system is as usual,. — mesenteric veins from the 

 intestine combine in portal veins ; the blood filters through the liver ; 

 and is collected in hepatic veins, which unite with the anterior end of 

 the inferior vena cava. 



A functional renal-portal system is represented by small renal-portal 

 branches, which the femorals give off to the kidney. 



From the transverse vein formed between the two hypogastrics or by 

 the division of the caudal vein, a coccygeo-mesenteric arises, which 

 receives vessels from the cloaca and large intestine, and is continued 

 along the mesentery to join the hepatic portal system. 



As there are rarely any valves in the renal-portal veins, the blood 

 from the viscera and hind-limbs can pass freely either through the iliac 

 veins and thence to the inferior vena cava, or through the coccygeo- 

 mesenteric vein to the hepatic portal system. 



The epigastric vein of the bird takes blood from the fat-laden sheet 

 or great omentum which covers the abdominal viscera. It leads not 

 into the liver, but into one of the hepatic veins. 



Associated with the blood vascular system there is a 

 lymphatic system with a few lymphatic glands. 



39 



