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 Reptiles, 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 

 Yertebrates, Birds and Mam- FIG. 73. Heart of the uugoug, a four- 



Xr j. . , , cham bered heart, the parts beiug more 



mals. JNot a drop OI blood separated than in higher animals: E, 



1 4-1 ^ ^-.,'4- s^f 4-V. right veutricle; L, left veutricle; D, 



can make the circuit of the ri j ht auricle . F , pulmonary artery; 

 body without passing through K > left anricle ' A > aorta - 

 the lungs, the circulation to and from those organs being 

 as perfect as the distribution of arterial blood. The heart 

 J '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 



771 



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 veua cava; c, tricuspid f rorn the Veins, and Send it to 

 yalve ; a, right auricle ; e, pulmona- 

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

 pulmonary arteries ; h, aorta ; k, left . ,, , _ 

 auricle ; I, mitral valve ; m, left ven- receive the blood from the 



lungs, and propel it over the 

 The left ventricle has more to do than anv other 



body, 

 cavity. 



The two auricles contract at the same instant; 



