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 tlie 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- F:a. t3.— Hemt of the uugmig, n fom-- 



, TVT .. J J! 1 1 1 chambered heart, the parts being more 



maiS. JNOt a drop OI blood separated than in higher animals: E, 



pan make the circuit of tllP "ght ventricle ; L, left ventricle ; D, 

 can make lue urouil OI lUe ,.jg^j „„ricle; F, pulmonary artery; 



body without passing througli ^ '^" »»"<='« ' a, aorta, 

 the lungs, the circulation to and from those organs being 

 as perfect as tlie distribution of arterial blood. The heart 

 fohg 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 



Fig. 74. —Theoretical Section of the . „„i • i • i • j.1 ui j 



Human Heart: a, right ventricle; Veutricle IS tO receive the blood 



/.inferior vena cava; c, tricuspid from the veius, and Send it to 



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



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

 pulmonary arteries: A, aorta; fc, left . i i i i j- i 



auricle;!,mitralTalve;m, left ven- receive tlie blOOd trom 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; 



d - 



