THE CIRCULATION OF THE BLOOD. 



109 



one another, and at that point a small aperture brings the 

 two into communication. The venous and arterial cur- 

 rents are, therefore, mixed, 

 but not within the heart, as 

 in the other Eeptiles, nor so 

 extensively. In the structure 

 of the heart, as well as in that 

 of the gizzard, Crocodiles ap- 

 proach the Birds. 



The Highest Form of the 

 Circulating System is pos- 

 sessed by the warm-blooded 

 Vertebrates, Birds and Mam- FIG. 73. Heart of the Dugong, a fonr- 



, -vr i/ i i i j chambered heart, the parts being more 



malS. JNot a drop OI blOOd 8 eparated than in higher animals: E, 



fan mfllcp thf> firpnif of thp right ventricle; L, left veutricle ; D, 



TCUlt Or tne rjght au ,. icle . F, pulmonary artery; 



body without passing through K - left auricle ; A - aorta - 

 the lungs, the circulation to and from those organs being 

 as perfect as the distribution of arterial blood. The heart 

 / 9 * 9 consists of four cavities a 



right auricle and ventricle, and 

 a left auricle and ventricle. In 

 other words, it is a hollow mus- 

 cle divided internally by a ver- 

 tical partition into two distinct 

 chambers, each of which is 

 again divided by a valve into 

 an auricle and a ventricle. The 

 work of the right auricle and 

 ventricle is to receive the blood 



6 inferior vena cava; c, tricuspid f rom t he Veins, and Send it to 

 valve ; d, right auricle ; e, pulmona- ? 



ry veins; /, superior vena cava; g, the lungS ; while the Other tWO 

 pnl monary arteries; h, aorta; k, left . . . , , . , 



auricle ; I, mitral valve ; m, left ven- reCCl V6 the blOOd f rOUl the 



lungs, and propel it over the 



body. The left ventricle has more to do than any other 

 cavity. The two auricles contract at the same instant; 



