WING OF BIRDS. 507 



The wing of a bird does not, at first view, 

 present much analogy with the fore extremity of 

 a quadruped : but on a closer examination we 

 find it to contain all the principal bones of the 

 latter, though somewhat altered in shape, and 

 still more changed in their functions. Yet still 

 the same unity of plan, and perfect harmony of 

 execution may be discerned in the mechanism of 

 this refined instrument of a higher mode of pro- 

 gression. 



The head of the humerus (h) has a compressed 

 form ; and in order to obtain great extent of 

 motion, it is made to play by a very small 

 cylindrical surface upon the scapula ; thus ad- 

 mitting of the complete descent of the wing, 

 unobstructed by any opposing process, but at 

 the same time limiting its motion to one plane. 

 It is connected below, by broad attachments, to 

 the radius and ulna, forming with them a hinge 

 joint. These latter bones are separate, and of 

 great length, but so firmly united together by 

 ligament as scarcely to have any motion on one 

 another. The carpus (w), consists of two bones 

 only, the one articulated with the radius, the 

 other with the ulna. They move together as one 

 piece ; but, contrary to what takes place in 

 quadrupeds, the movements are made from side 

 to side, instead of their consisting of flexion and 

 extension ; this variation from the usual struc- 

 ture being for the purpose of folding down the 

 joints of the wing, and bringing them close to 



