CHAPTER XII. 



AVES. * 



Warm-blooded oviparous bipedal animals, covered with feathers. 

 The chambers of the heart are completely separated. The right 

 aortic arch alone persists. There is a single occipital condyle, and 

 the anterior limbs have the form of wings. 



Birds are warm-blooded animals possessing a temperature, 

 which is generally higher than that of Mammals, reaching in 

 some cases, it is said, 112° F. and is maintained pretty con- 

 stantly irrespective of that of the surrounding air. This condi- 

 tion demands on the one hand a great energy of metabolism and 

 on the other a regulating mechanism by which the loss of heat is 

 controlled. The metabolism is undoubtedly favoured by the 

 respiratory arrangements, which ensure, in birds, a very com- 

 plete oxidation of the blood. Not only do the lungs by then- 

 complexity of structure expose a very large absorptive surface, 

 but the curious extensions of the bronchi into thin-walled air- 

 sacs, which extend among the viscera and into the bones, no 

 doubt assist in the oxidation processes by acting as reservoirs 



* C. L. Nitzsch. System der Pterylographie, Halle, 1840. Gray & Mitch- 

 ell, The Genara of Birds, London, 1841-9. C. E. SundevaU, Tentamen, 

 Stockholm, 1872-3 : English Edition, London, 1889. T. Huxley, On 

 the classification of Birds, Proc. Zool. Soc, 1867. Stejneger, Birds in 

 vol. 4 of the Standard Natural History, Boston, U.S.A. 1885. M. Fiirbringer, 

 Untersuchungen zur Morphologie u. Systetnatik der Vogel, Th. 1 and 2, 1888. 

 H. Gadow, Aves, BronrCs Thierreich, 1 and 2, 1891, 1893. A. Newton, 

 Dictionary of Birds, London, 1893-6. A. H.Evans, Birds, in the Cambridge 

 Natural History, 1899. W. P. Pycraft, Morphology and Phylogeny 

 of the Palaeognathae (Ratitae and Crypturi) and Neognathae (Cari- 

 natae), Trans. Zool. Soc, 15, 1900, p. 149. W. K. Parker, an impor- 

 tant series of memoirs on the anatomy and development of variovis birds, 

 a list of which will be found in the above-cited Dictionary of Birds, Intro- 

 duction, p. 80, note 2. 



