THE AVES. (',!) 



The blood is cold, and the majority of the blood-corpuscles are 

 red, oval, and nucleated. The bronchial tubes are not connected 

 at the surface of the lungs with terminal saccular dilatations, or 

 air-vesicles. When, as is ordinarily the case, the superficial 

 layers of the epidermis of Reptiles are converted into horn, the 

 corneous matter takes the form of broad plates, or of overlapping 

 scales, neither plates nor scales being developed within pouches 

 of the integument. 



The class of Aves consists of animals so essentially similar 

 to Reptiles in all the most essential features of their organization, 

 that Birds ma}^ be said to be merely an extremely modified and 

 aberrant Reptilian type. 



As I have already stated, they possess an amnion and a respi- 

 ratory allantois, and the visceral arches never develop branchial 

 appendages. The skull is articulated with the vertebral column 

 by a single condyle, into which the ossified basi-occipital enters 

 largely. Each ramus of the lower jaw, composed, as in Reptiles, 

 of a number of pieces, is connected with the skull by an os 

 quadratum, to which the hyoidean apparatus is not suspended. 



In no existing bird does the terminal division of the fore-limb 

 possess more than two digits terminated by claws, and the meta- 

 carpal bones are commonly anchylosed together, so that the 

 " manus " is of little use, save as a support for feathers. 



In the hind limb of all birds the tarsal and metatarsal bones 

 become more or less completely fixed, and the latter, anchylosed 

 together, so as to form a single osseous mass, the " tarso-meta- 

 tarsus." 



The greater and lesser circulations of birds are completely 

 separate, and there is only one aortic arch, the right. The right 

 ventricle has a muscular vah^e. The blood is hot, hotter on the 

 average than that of any other vertebrates, and the majority of 

 the blood-corpuscles are oval, red, and nucleated. The bron- 

 chial tubes open upon the surface of the lungs into air-sacs, 

 which differ in number and in development in different birds. 

 Lastly, the integument of birds is always provided with horny 

 appendages, which result from the conversion into horn of the 

 cells of the outer layer of the epidermis. But the majority of 



