2 THE STRUCTURE OF THE FOWL 



(1) The Archceornithes (primitive birds) con- 

 tain only one known member, the fossil 

 Archaeopteryx. (2) The Neorniihes (modern 

 birds) are distinguished by shortness of tail 

 and fusion of the metacarpal bones of the palm, 

 and may be divided as follows : — 



{a) Ratitse. Running birds with rudiment- 

 ary wings and a flat breast-bone, e,g, 

 the ostriches. 

 {h) Odontolcse. Extinct fossil birds with 

 numerous teeth carried in long jaws, 

 e,g, Hesperornis. 

 (c) Carinatse. Flying birds with well- 

 developed wings and a breast-bone 

 provided with a keel to which the 

 powerful muscles of flight are attached. 

 It is in this division that the domestic 

 fowl is included. 

 Structurally, birds share with mammals the 

 distinction of being the most highly specialised 

 of vertebrates. Many of their modifications 

 are adaptations for flight. The fore-limb is 

 changed into a wing on which are large feathers 

 capable of offering a large elastic surface to 

 the resistance of the air. A wing-membrane 

 stretching between the arm and the forearm 

 increases the area of this surface. In order 



