286 VERTEBRATE ANIMALS. 
The ear¢t in all birds consists of four chambers, and the 
two sides of the heart are completely separated from one 
another, In all essential details, as regards the structure of the 
heart and great vessels, and the course of the circulating fluid, 
Birds agree with Mammals. The impure venous blood which 
has traversed the body, is returned by the great veins to the 
right auricle. From the right auricle it passes into the right 
ventricle, from which it is driven by the pulmonary artery to 
the lungs. Having been submitted to the action of the air con- 
tained in the lungs, and having been thereby changed into ar-. 
terial blood, the blood is sent back to the left auricle by means 
of the pulmonary veins. Thence it passes into the left ventricle, 
by which it is again propelled throughout the whole body, to 
return again as venous blood to the right side of the heart. The 
heart, therefore, of birds, differs from that of reptiles in consist- 
ing of two sides, each composed of an auricle and ventricle, the 
right side being wholly concerned with sending the venous blood 
to the lungs, and the left side being entirely occupied with send- 
ing the arterial blood to the body. The right side of the heart 
is therefore venous, the left side arterial. In all Reptiles, on 
the other hand, the two circulations—namely, that through the 
lungs and that through the body—communicate with one an- 
other, either in the heart itself or in its immédiate neighbour- 
hood; so that both the lungs and the body are supplied with 
a mixture of venous with arterial blood. Though the heart of 
Birds resembles that of Mammals in general structure, its ca- 
vities are “relatively stronger, their valvular mechanism is more 
perfect, and the contractions of this organ are more forcible and 
frequent in Birds, in accordance with their more extended res- 
piration and their more energetic muscular actions” (Owen). 
The urinary organs of birds consist of two elongated kidneys, 
which open by means of their ducts (the ureters) into the 
cloaca, along with the termination of the intestine and the 
ducts of the reproductive organs. As a general rule, the female 
bird is provided with only a single ovary—that of the left side 
—and all birds, without exception, are oviparous. ‘The egg is 
always enclosed in a calcareous shell, and is developed after 
expulsion from the body, by the process of “incubation” or 
“ brooding ”—a process for which birds are especially adapted, 
in consequence of their very high average temperature. The 
young bird, when ready for an independent existence, perforates 
