108 COMPARATIVE ZOOLOGY. 
that there are really four cavities—two auricles, and two 
ventricles. But both ventri- 
cles send off aortas which 
cross ore another, and at that 
point a small aperture brings 
the two in 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 
Fic. 72.—Heart of the Dugong, a typical of the heart, as well as @iz- 
four-chambered heart, the parts being ; 
more separated than in higher ani- zard, Crocodiles approach the 
mals: E, right ventricle; L, left ven- Bird 
tricle, D, right auricle; F, pulmonary 1rds. 
artery; K, left auricle; A, aorta. The Highest Form of the 
Circulating System is possessed by the warm-blooded 
Vertebrates, Birds, and Mammals. 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 abv igh eae ate 
of arterial blood. The heart 
consists of four cavities —a 
right auricle and_ ventricle, 
and a left auricle and yentri- 
cle. In other words, it is a 
hollow muscle divided inter- 
nally by a vertical partition 
into two distinct chambers, 
each of which is again di- 
vided by a valve into am an- Pig, i> Rhenetst sion of ti 
ricle and a ventricle. The b, inferior vena cava; ¢, tricuspid 
valve; d, right auricle; e, pulmona- 
work of the right auricle and _ ry veins; Ff, superior vena cava; 9, 
: ; m : pulmonary arteries; h, aorta; &, left 
ventricle is to receive the auricle; 7; mitral valve; m, left ven- 
blood from the veins, and ‘1: ™ septum. 
send it to the lungs; while the other two receive the 
