CIRCULATION. 



115 



and these discharge their contents into one common ventricle, 

 which propels the mixture partly into the lungs, and partly 

 through the body. la warm-blooded animals, the heart is 

 a completely double organ, consisting of two auricles and 

 two ventricles: the right auricle receives the dark blood 

 brought back from the tissues, and sends it into the right 

 ventricle, which propels it through the lungs; the left auricle 



Fig. 63. HEART AND GREAT VESSELS 

 OF FROG, a, Aorta ; 6, venous trunk 

 carrying dark blood to c, the right 

 auricle of the heart; d, left auricle 

 receiving aerated blood from the 

 lungs; e, ventricle receiving blood 

 from both auricles, and propelling 

 the mixed fluid up the 'truncus ar- 

 teriosus, both into the lungs and 

 the system; /, left lung. 



Fig. 62. HEART AND GREAT 

 VESSELS OF FISH, a, a, a, 

 Veins; b, b, right and left 

 extremities of the single 

 auricle of the heart; c, ven- 

 tricle of heart; d, bulbus 

 arteriosus ; e, e, branchial 

 arteries which convey the 

 dark blood from the heart 

 into the gills, to be purified 

 before passing on into the 

 branchial veins, and thence 

 into /, the aorta. 



receives the red blood returning from the lungs, and passes 

 the pure stream on into the left ventricle to be propelled into 

 the tissues of the body. In fishes, as well as in warm-blooded 

 animals, only red blood circulates through the body; but in 



