CHAPTER XVII. 



CLASS. AVES 1 . 



BIRDS form a large and extremely homogeneous class of the 

 vertebrata, and are readily distinguished from all other animals 

 by the possession of an epidermal exoskeleton having the form 

 of feathers. Feathers differ from hairs in the fact that they 

 grow from papillae formed of both the horny and the Mal- 

 pighiaii layer of the epidermis, which papillae at first project 

 from the surface, and only subsequently become imbedded in 

 pits of the dermis. A dermal exoskeleton does not occur in 

 birds. 



The endoskeleton is characterised by its lightness, the 

 large bones being generally hollow ; but the pneumaticity 

 does not vary in proportion to the power of flight. The 

 cervical part of the vertebral column is very long and flexible, 

 while the postcervical portion is generally very rigid, owing 

 to the fusion of many of the vertebrae, especially in the 

 lumbar and sacral regions. The vertebrae are generally with- 

 out epiphyses to their centra. The cervical vertebrae in living 

 forms have saddle-shaped articulating surfaces, and many of 

 them bear ribs. The thoracic ribs in almost all birds have 

 large unciriate processes. The sternum is very large, and the 

 ribs are always attached to its sides, not as in many reptiles 



1 M. Fiirbringer, Untersuchungen zur Morphologic und Systematik der 

 Vogel, I. and II. Amsterdam, 1888. Cf. H. Gadow, Nature, xxxix. 

 1888, pp. 150 and 177. 



T. H. Huxley, "On the classification of birds." P. Z. S., London, 

 1867. 



E. Seleuka and H. Gadow, Vogel in Bronn's Classen und Ordnungen 

 des Thier-reichs 18691890. 



