THE f ORE-LIMB IN BIRDS. 249 



The fore-limb of a bird, when in a state of rest, exhibits a 

 great change of position if it be compared with that of an or- 

 dinary reptile ; and this change is of a character similar to, 

 but in some respects greater than, that which the arm of a man 

 presents, when compared with the fore-limb of a quadrupedal 

 mammal. The humerus lies parallel with the axis of the body, 

 its proper ventral surface looking outward. The forearm is in 

 a position midway between pronation and supination, and the 

 manus is bent back upon the ulnar side of the forearm, in a 

 position, not of flexion, but of abduction. 



In ordinary birds, the proximal end of the humerus is ex- 

 panded, and its articular head transversely elongated ; its ven- 

 tral face is convex, and provided with a strong preaxial ridge, 

 ivhich gives attachment to the pectoral muscle. The proper 

 dorsal face is concave from side to side, especially toward the 

 postaxial margin, where the pneumatic aperture occurs in those 

 birds which have the humerus hollow. The distal end is ex- 

 panded, and the articular surface for the radius is a convex 

 facet, directed obliquely inward, on its ventral face. In this 

 respect the bird's humerus exaggerates a feature of that of the 

 Lizards. 



In the MatitoB these peculiarities are very feebly, or not at 

 all, marked, the humerus being a slender, cylindrical, slightly- 

 curved, bone. In the Casuaridce, Dinornithidoe, and Aptery- 

 gidw, the fore-limb is extraordinarily reduced, and may become 

 rudimentary. In the Penguins and, to a less degree, in the 

 great Auk, the humerus becomes flattened from side to side ; 

 the proximal end is singularly modified, and, at the narrow 

 distal end, the articular surface for the radius lies completely 

 in front of, and rather above, that for the ulna. 



The ulna, which often presents a series of tubercles, indi- 

 cating the attachment of the secondary quill-feathers, is usu- 

 ally a stronger, and a longer, bone than the radius. There are 

 only two carpal bones, one radial and one ulnar. 



In the Apterygidce and in the Casuaridm, 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 StruthionidoB and JRheidce, and in all Carinatm, there 

 are three digits in the manus, which answer to the poUex and 

 the second and third digits of the pentadactyle fore-limb ; and 

 the metacarpal bones of these digits are anchylosed together. 

 As a rule, the metacarpal of the pollex is much shorter than 

 the other two ; that of the second digit is strong and straight, 

 that of the third is more slender and bowed, so as to leave an 



