THE VASCULAR SYSTEM 335 



the truncus. The left one of these arches is absent in the 

 bird. The subclavian arteries come off from the right 

 systemic arch in lizards (dorsal type of subclavian) ; in 

 Chelonia, crocodiles, and birds, the subclavian arteries are 

 given off from the carotids (ventral type). 



In the mammal, the right and left arteries of the 4th 

 (systemic) arch differ from those of the Sauropsidan reptiles 

 in that the aorta is undivided, instead of being split to its base. 

 The artery on the right side does not reach round to the dorsal 

 aorta ; it is given the name of innominate artery, and it leads 

 to the right carotid and subclavian arteries. That on the left 

 side forms the so-called aorta, gives off the left carotid and 

 subclavian arteries, and continues back as the dorsal aorta. 

 It is connected with the pulmonary arch by the ductus arteriosus 

 as already mentioned. 



The internal carotid arteries are the anterior prolongations 

 of the lateral dorsal aortae, and they enter the skull by passing 

 up between the trabecular, close to the pituitary body. The 

 external carotids are the anterior prolongations of the ventral 

 aorta, on each side of the thyroid. 



The proximal ends of the arteries and veins are joined at 

 the heart. The distal ends of the arteries are connected with 

 those of the veins by the capillaries, so that the whole vascular 

 system is a closed one. When a vein starts from capillaries 

 and breaks down into other capillaries again before reaching 

 the heart, it is known as a portal vein. The hepatic portal 

 vein occurs in all chordates, the renal portal appears in the 

 Cyclostomes and disappears in the amniotes. 



The blood of Amphioxus is colourless, but in all higher 

 forms, haemoglobin, a respiratory pigment, is present in 

 corpuscles, which become known as " red blood-corpuscles. " 

 In the adult mammal, these corpuscles are peculiar in being 

 non-nucleated. The white corpuscles of the blood play an 

 important part in the defence of the organism against invasion 

 by foreign bodies. In the embryo, the blood arises from blood- 

 islands, between the mesoderm and the endoderm in the 

 region of the yolk. In the adult, blood-corpuscles are formed 

 in the marrow of the bones, and in the lymphatic organs. The 



