AYES. 287 



The heart 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 t"he 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 sub- 

 mitted to the action of the air contained in the lungs, and having 

 been thereby changed into arterial 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 

 consisting 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 

 sending the arterial blood to the body. The right side of the heart 

 is therefore venous, the left side arterial. In all Eeptiles, on the 

 other hand, the two circulations — namely, that through the lungs, 

 and that through the body — communicate with one another, either 

 in the heart itself or in its immediate neighbourhood ; 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 Mam- 

 mals in general structure, its cavities 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 respiration and their more energetic muscular 

 actions " (Owen). The urinary organs of Birds consist of two elon- 

 gated 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 mostly 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 the shell by means of a temporary 

 calcareous excrescence developed upon the point of the upper 

 mandible of the bill. In some Birds, mostly in the case of those 

 which live upon the ground, the young are able to run about and 

 look for food directly after they come out of the egg {Aves prcecoces), 

 as is seen in the common Fowl. In most Birds, however, the young 



