174 



406. It will thus be seen that the pnhii^iuaiy artery carries venous 

 blood, and t\\c puhnonary veins, arterial blood. 



407. Inside the aona and pulmonary artery, just as they leave the 

 heart, are the semi-lnnav valves (Plate XXXI., B. /Oj, three in number; 

 on the free edges of these are small fibrous bodies, called the corpora 

 Arantii ; these valves, like the other valves of the heart, are to prevent 

 regurgitation of the blood. 



408. The Aorta (Plate XXXI., B. 8) rises from the front and upper 

 part of the left ventricle, and is the main stem of the arterial circulation. 

 The common aorta is about two inches long, and divides into two 

 great branches, one — the anterior aorta (Plate XXXI., B. 6) — going 

 to supply the head, neck, and fore extremities, while the other — the 

 posterior aorta (Plate XXXI . B. 7 ) — proceeds to the hinder parts of the 

 body and limbs. 



409. Arteries (Plate XXXII., 2 and 5). These are the vessels which 

 convey the blood from the left side of the heart to the various portions 

 of the body. They are very dense, and elastic, having three coats — 

 \\z., internal, lined by endothelium; middle, ox contractile, consisting 

 of non-striated muscular fibre, and elastic tissue ; and external, of 

 areolar structure. The arteries anastomose frequently with one 

 another, and finally terminate in the capillaries. 



410. Capillaries (Plate XXXII. , 3.3, 7.7, and 8.8J. This is a 

 system, or network, of minute vessels, constituting the connecting 

 medium between the arteries and veins. They are very small, being 

 about goVo of ^" inch in diameter. It is through their thin walls that 

 the changes between the blood and the tissues take place ; the nutrient 

 material is given out, and the effete products are taken up into the 

 blood stream and carried to the various excretory organs, such as 

 lungs, kidneys, skin, &c., to be thrown off by them. 



411. Veins (Plate XXXII., 4 and 9) are the vessels which return 

 the blood to the right side of the heart. They, like arteries have three 

 coats, which, however, are not so dense, strong, or elastic, and they 

 are, moreover, provided with pouch-shaped valves, to prevent the 



