Chap. X.] THE HEART AND CIRCULATION. 195 



such as those beneath the skin and that at the back of the 

 pleuro-peritoneal cavity in the frog the latter communicatr j 

 by means of stomata with the coelom. 



In the frog, but not in the fowl or rabbit, there are four 

 lymph-hearts, whose function it is to pump the lymph back from 

 the lymphatic system into the veins. The anterior pair lie close 

 to and beneath the transverse processes of the third vertebra. 

 They communicate with the innominate veins. The posterior 

 pair lie one on each side of the urostyle about one-third of an 

 inch from its termination. They communicate with ^ie renal- 

 portal veins. In each case valves prevent the passage )f blood 

 from the veins into the rhythmically contractile lym^li-hearts. 

 In the bird and the mammal the lymph returns to the veins 

 through the .thoracic ducts or duct without the intervention of 

 lymph-hearts. 



Thus the course of the blood is, disregarding the lymph 

 overflow, heart, arteries, arterioles, capillaries, venules, veins, 

 heart. 



From what has been said in the last chapter, however, it 

 will be understood that the blood is no mere inert fluid which 

 returns to the heart in much the same state in which it left 

 it, but is throughout its course undergoing continuous meta- 

 bolism. Nor are the blood-vessels simple inelastic tubes. Each 

 artery and vein has, besides its smooth lining of endothelial 

 (epithelioid) cells, a triple coat of elastic tissue, in the mid layer 

 of which unstriped muscular fibres may be developed. In the 

 arterioles these muscular fibres are numerous. In the venules 

 the elastic coat is much reduced ; and in the capillaries both it 

 and the muscular coat are absent. So that we may say that the 

 arteries are very thick-walled and highly elastic ; the arterioles 

 are moderately thick-walled, elastic, and contractile ; the capil- 

 laries have only an epithelioid (endothelial) wall; the venules 

 are thin-walled and slightly elastic ; the veins are relatively 

 thin-walled and slightly elastic. The veins (of the rabbit, 

 e.g.) are, moreover, in some parts of the body, where they 

 are liable to pressure, provided with flap-like valves which 

 offer no resistance to blood flowing towards the heart, but close 



