THE FOKE LIMB IN BIEDS. 291 



but in some respects greater tlian, tliat wliicli the arm of a 

 man presents, when compared with the fore-limb of a quad- 

 rupedal mammal. The humerus lies parallel with the axis 

 of the body, its proper ventral surface looking oiitwards. 

 The fore arm is in a position midway between pronation and 

 supination, and the manus is bent back upon the ulnar side 

 of the fore arm, in a position, not of flexion, but of abduc- 

 tion. 



In ordinary birds, the proximal end of the humerus is 

 expanded, and its articular head transversely elongated ; its 

 ventral face is convex, and provided with a strong preaxial 

 ridge, which gives attachment to the jjectoral muscle. The 

 proper dorsal face is concave from side to side, especially 

 towards the postaxial margin, where the pneumatic aperture 

 occurs in those birds which have the humerus hollow. The 

 distal end is expanded, and the articular siu-face for the 

 radius is a convex facet, directed obliquely inwards, on its 

 ventral face. In this respect the bird's humerus exaggerates 

 a feature of that of the Lizards. 



In the Batitce these peculiarities are very feebly, or not 

 at all, marked, the humerus being a slender, cylindrical, 

 slightly cui^red, bone. In the Casuaridce, Dinornithidce, 

 and Apterygidce, the fore-limb is extraordinarily reduced, 

 and may become rudimentary. In the Penguins and, to a 

 less degree, in the great Auk, the humenis becomes flat- 

 tened from side to side ; the proximal end is singularly 

 modified, and, at the narrow distal end, the articular sur- 

 face for the radius lies completely in front of, and rather 

 above, that for the ulna. 



The ulna, which often presents a series of tubercles, 

 indicating the attachment of the secondary quill feathers, 

 is usually a stronger, and a longei', bone than the radius. 

 There are only two carpal bones, one radial and one 

 ulnar. 



In the Apterygidce and in the Casuaridce, there is but one 

 complete digit in the manus. It appears to answer to the 

 second of the pentadactyle limb, and is provided with a claw. 

 In the Struthionidce and Bheidce, and ia all Carinatoe, there are 



