274 VETERINARY LECTURES 



408. It will thus be seen that the pulmonary artery carries venous 

 blood, and the pulmonary veins arterial blood. 



409. Inside the aorta and pulmonary artery, just as they leave 

 the heart, are the semilunar valves {Plate XXXIV., B, 10), 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. 



410. The Aorta (Plate XXXIV., B, 8) rises from the front and 

 upper part of the left ventricle, and is the main stem of the arterial cir- 

 culation. The common aorta is about two inches long, and divides 

 into two great branches, one — the anterior aorta (Plate XXXIV., 

 B t 6) — going to supply the head, neck, and fore extremities, while 

 the other — the posterior aorta (Plate XXXIV., B, 7) — proceeds to 

 the hinder parts of the body and limbs. 



411. Arteries (Plate XXXV., 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 

 coa ts — viz., internal, lined by endothelium ; middle, or 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. 



412. Capillaries (Plate XXXV., 3.3, 7.7, and 8.8).— This is a 

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

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

 about 30V0 °f an mc ^ * n diameter. It is through their thin walls 

 that the changes between the blood and 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, etc., to be thrown off by them. 



413. Veins (Plate XXXV., 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 



