262 VERTEBRATE ANIMALS. 



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 Reptiles, on the other hand, the two cir- 

 culations namely, that through the lungs and that through 

 the body communicate with one another, either in the heart 

 itself or in its immediate neighborhood ; so that both the lungs 

 and the body are supplied with a mixture of venous with arte- 

 rial 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 ac- 

 cordance with their more extended respiration and their more 

 energetic muscular actions" (Owen). The urinary organs of 

 birds consists of two elongated kidneys, which open by means 

 of their ducts (the ureters) into the cloaca, along with the ter- 

 mination 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, with- 

 out 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 pro- 

 cess 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, often 

 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, as is seen in the common 

 Fowl. In most birds, however, the young are liberated from 

 the egg in a perfectly helpless and naked condition, and re- 

 quire to be fed by their parents for a longer or shorter time, 

 before they are able to take care of themselves. Most of these 

 birds, such as our common song-birds, reside in trees, and 

 build more or less elaborate nests. 



As regards their nervous system, the brain of Birds is rela- 

 tively larger than the brain of Reptiles, but it is destitute of 

 those folds or convolutions which form so marked a feature in 

 the brain of most Mammals. The organs of sense, with the 

 exception of touch and taste, are well developed in Birds, 

 vision especially being generally extremely acute. The eyes 

 are always well developed, and in no bird are they ever want- 

 ing or rudimentary. The chief peculiarity of the eye of Birds 

 is, that its anterior portion (cornea) forms the segment of a 



