128 MORPHOLOGICAL TYPES 



arterial blood returns to the left auricle. The systemic "arch 

 therefore receives nothing but pure arterial blood from the 

 left ventricle, into which it has passed from the left auricle. 

 There are therefore two completely separate circulations in the 

 heart, and this is rendered possible by the fact that the heart 

 is four-chambered, both auricle and ventricle being completely 

 divided longitudinally. 



The right systemic arch gives off a pair of arteries which run 

 forwards for a short distance, known as the innominate arteries. 

 Each divides into two, forming the carotid arteries and the 

 subclavians. The carotids run forwards to the head ; the 

 subclavians supply the pectoral muscles and the wings. The 

 systemic arch runs up on the right side of the gut and reaches 

 a position dorsal to it where it is known as the dorsal aorta. 

 It gives off coeliac and mesenteric arteries to the alimentary 

 canal, and sciatic arteries to the legs, and then divides to form 

 the iliac arteries and the caudal artery which supply the hinder 

 regions of the body. 



The superior venae cavae receive the jugular veins, and these 

 are peculiar and interesting in that the right and left veins are 

 connected by a cross-channel at the top of the neck. In the 

 twisting of the long and flexible neck, it may happen that the 

 vein on one side is squeezed, and the flow of blood in it 

 interrupted. This blood can, however, return to the heart by 

 passing across the connexion just described, and down the 

 jugular vein of the other side. The superior venae cavae also 

 each receive a subclavian vein made up of a brachial vein from 

 the wing and a pectoral vein from the pectoral muscles. 



The inferior vena cava receives the hepatic veins, and is 

 formed by the junction of a pair of iliac veins. These receive 

 the femoral veins from the legs and the renal veins from the 

 kidneys. The blood in the hinder regions of the body is led 

 forwards in a caudal vein, which soon divides into three. Two 

 of these vessels represent the renal portal veins of the lower 

 vertebrates, but in the adult bird these veins connect direct 

 with the iliac veins and vena cava inferior, without breaking 

 down into capillaries in the kidneys at all. There is therefore 

 no renal portal circulation. The third vessel into which blood 



