SEC. 1 DEFINITION OF BIRDS 93 



nourished before hatching by the great quantity of food-yolk in the 

 egg. There are no mammary glands to furnish the young Avith 

 milk after birth. The generative, urinary, and digestive organs 

 come together behind in a common receptacle, the clvaca, or sewer, 

 and their products are discharged by a single orifice. The kidneys 

 of the early embryo, called Jl\>lffian bodies, are soon replaced func- 

 tionally by permanent kidneys, and structurally by the testes of the 

 male and the ovaries of the female. The cavity of the alxhrnien, or 

 belly, is not separated from that of the thonu; or chest, by a com- 

 plete muscular partition, or diaphragm. The great lateral hemi- 

 spheres of the brain are not connected by a transverse commissure, 

 or corpis cullosum. Air is always breathed by true lungs, never by 

 gills. The blood, which may be cold or hot, has red oval nucleated 

 corpuscles ; the heart has either three or four separate chambers, — 

 the latter in birds, in which the circulation of the hot blood is com- 

 pletely double, i.e. in the lungs and one side of the heart, in the 

 body at large and the other side of the heart. The aortic arches are 

 several ; or if but one, as in birds, it is the right, not the left as in 

 mammals. The centra, or bodies, of the vertebrae are ossified, but 

 have no terminal ejn^jhyses. The skull hinges upon the backbone by 

 a single median protuberance, or cundyJe, and the bone {jxisioccipitid) 

 bearing this condyle is completely ossified. The lower jaw consists of 

 several separate pieces, the articular one of which hinges upon a mov- 

 able cjuadrate bone; and there are other peculiarities in the formation 

 of the skull. The ankle-joint is situated, not, as in Mammals, between 

 the tarsal bones and those of the leg, but between two rows of tarsal 

 bones. The skin is usually covered with outgrowths, in the form of 

 scales or feathers. Difterent as are any living members of the class 

 of Birds from an};- known Eeptiles, the characters of the two groups 

 converge in geologic history so closely, that the presence of feathers 

 in the former class, and their absence from the latter, is one of the 

 most positive differences we have found. The oldest known birds 

 are from the Jurassic rocks of Europe, and the Cretaceous l^eds of 

 North America. These birds had teeth, and various other strons: 

 pecixliarities of structure, Avhich no living members of the class have 

 retained. 



AVES, or the Class of Birds, may be distinguished from other 

 Sauropsida, for all that is known to the contrary, by the following 

 sum of characters : The body is covered with feathers, a kind of 

 skin-outgrowth no other animals possess. The blood is hot ; the 

 circulation is completely double ; the heart is perfectly four- 

 chambered ; there is but one (the right) aortic arch, and only one 

 pulmonary artery springs from the heart ; the aortic and the pul- 

 monary artery have each three semilunar valves. The lungs are 

 fixed and moulded to the cavity of the chest, and some of the air- 



