.OOO THE MECHANICAL FUNCTIONS. 



in the mechanism of this refined instrument of a 

 higher mode of progression. 



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 sur- 

 face upon the scapula ; thus admitting of the com- 

 plete 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 quadru- 

 peds, the movements are made from side to side, 

 instead of their consisting of flexion and extension ; 

 this variation from the usual structure being for the 

 purpose of folding down the joints of the wing, and 

 bringing them close to the body. The metacarpus 

 (m) consists originally of two bones, which soon 

 become united into one at the upper part. On the 

 radial side it has a process, derived perhaps from a 

 third metacarpal bone, which is anchylosed at a 

 still earlier period of ossification ; and to this pro- 

 cess a small pointed bone is connected, correspond- 

 ing to a rudiinental thumb (t). There are generally 

 two fingers, of which the first exhibits traces of 

 having been originally two bones : the inner finger 

 consists of two or three long phalanges, and the 

 outer one of a single phalanx : there is sometimes 

 also a rudiinental bone corresponding to a little 



