THE CIRCULATION OF THE BLOOD. 



109 



Fig. 73.— Heart of the Dugoiig, a four- 

 chambered heart, the parts being more 

 separated than in higher animals: E, 

 right ventricle; L, left ventricle; D, 

 right auricle; F, pulmonary artery; 

 K, left auricle ; A, aorta. 



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 

 Vertebrates, Birds and Mam- 

 mals. Not a drop of blood 

 can make the circuit of the 

 body without passing through 

 the lungs, the circulation to and from those organs being 

 as perfect as the distribution of arterial blood. The heart 



consists of four cavities — a 

 e right auricle and ventricle, and 

 a left auricle and ventricle. In 

 other words, it is a hollow mus- 



- k , I 



i cle divided internally by a ver- 

 tical partition into two distinct 

 chambers, each of w T hich is 

 again divided by a valve into 

 an auricle and a ventricle. The 

 work of the right auricle and 



Fig. 74. — Theoretical Section of the j. • i • i, • j_i i i i 

 Human Heart: a, right ventricle; ventricle IS to receive the Mood 

 b inferior vena cava; «, tricuspid f rom the Veins, and Send it to 

 valve ; a, right auricle ; e, pulmona- 

 ry veins; /, superior vena cava; g, the lllUgS J while the Other two 

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



auricle ; I, mitral valve ; m, left ven- reCd Ve the blOOd I rom the 



tricle; n, septum. l i i •< j_i 



Jungs, 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; 



e ' 



d 



