AVES. 287 



The Iisart in ;ill Birds consists of four cliambers, and tlie two sides 

 of the heart are completely sepai'ated from one aiiotlier. 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 wliich has traversed the 

 Tiody, is returned Ly the great veins to the i-ight auricle. From 

 the right auricle it passes into the right ventricle, from which it is 

 driven bj' 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. Tlience it jjasses 

 into the left ventricle, liy which it is again propelled throughout the 

 whole body, to I'eturn again as venous blood to the right side of the 

 heart. The heart, therefoie, of Birds, ditfeis from that of Reptiles in 

 consisting of two sides, each composed of an auricle and ventricle, 

 the right side being wdiolly 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 thei-efore venous, the left side arterial. In all Reptiles, on the 

 other hand, the two circulations — namely, that thi'ough the lungs, 

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

 in the heart itself or in its immediate neighbourhood ; S(j that both 

 the lungs and the body ai-e supplied with a mixture of venous with 

 arterial blood. Though the heart of Birds resembles that (if 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 Bii-ds, in accordance with 

 their more extended respiration and their more energetic muscidar 

 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 tlie 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 exj)ulsion 

 from the bod)', by the process of " incubation " or " brooding " — a 

 process for which birds are especially adapted, in consequence of theii- 

 very high average temperature. The young liird, when ready f(jr an 

 independent existence, pei-forates the shell by means of a temporary 

 calcareous excrescence developed upon the point nf the uji])er 

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

 which live upon the ground, the young are aljle to i-un aliout and 

 look for food directly after they come out of the egg {Arcs prwcoce.i), 

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



