16 INTRODUCTORY LECTURE. 



later secondary periods was so dank and dense, and overloaded with 

 irrespirable elements, as to need the precipitation of so much carbon 

 as has been consolidated in our coal-fields and chalk -hills, before it 

 was fitted for the full development and vital enjoyment of the warm- 

 blooded and quick-breathing classes? But these and other consi- 

 derations suggested by the successive introduction of water-breathing^ 

 and slow air-breathing Vertebrates, would lead us too far away from 

 the proper subject of the present elementary discourse. Suffice it to 

 say, that the oviparous class of animals which next makes its ap- 

 pearance in the order of Creation, is remarkably characterised by the 

 energy of the circulating and respii'atory functions, and by the high 

 temperature of the body. I allude to the class Aves, characterised 

 as accurately, as briefly, by the name of " feathered bipeds : " bipeds, 

 because the anterior members are exclusively organised for flight ; 

 feathered, because the body which is to soar in air must be lightly 

 clad, and yet warmly clad, — must be covered by most efficient non- 

 conductors, so as to retain that elevated temperature which is the 

 necessary consequence of the organic combustion of so much mus- 

 cular and nerv^ous fibre in the energetic actions of flight. But Birds 

 enjoy almost every kind of locomotion : a few (Apteryx) burrow in 

 the earth : some (Ostrich, Rhea) traverse its surface as swiftly as the 

 most rapid courser : many climb trees : an entire Order is aquatic, 

 swimming or diving with facility. The legs and feet of Birds are 

 accordingly variously modified for these different powers, and furnish 

 the Naturalist with excellent characters for the primary divisions of 

 the class. The lungs are now divided into very minute cells, pro- 

 ducing a vast extent of the vascular respiratory membrane ; they also 

 communicate with larger cells, forming capacious reservoirs of air, 

 which are continued through every part of the body, even into the 

 substance and cavities of the bones. The heart is divided into four 

 chambers, two muscular ventricles and two auricles ; a single artery 

 arises from each ventricle, and a complete double circulation is es- 

 tablished, — the left auricle and ventricle circulating the arterial blood, 

 the right auricle and ventricle the venous, transmitting to the lungs 

 the entire mass of the carbonized blood. The blood is of a deep but 

 bright vermilion red, and richly laden with the discoid cells, which 

 are elliptical, but smaller than in the Reptilia {Jig. 4. c). 



The jaws of Birds are always edentulous and sheathed with horn, 

 of divers configurations, adapted to their diflerent modes of life and 

 kinds of food. The head is small, and supported upon a long neck ; 

 the mandibles performing most of those purposes for which the 

 anterior members, by their conversion into Avings, are unfitted ; so 

 that the beak combines the functions of hand and mouth. The 



