352 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the coracoid and scapula to be inserted into the 

 humerus ; in other words, it works over a pulley. 



While a flying organ is developed in the Carinatse 

 (Fig. 148) by the elongation of the humerus and 

 of the radius and ulna, the latter of which is 

 marked by the impressions of the secondary wing- 

 feathers ; while both are so articulated with the lower 

 end of the humerus as to allow of little movement 



Fig. 148. Skeleton of Fore Limb of a Flying Bird. 

 Ji, Humerus ; r, radius ; u, ulna ; c c, carpal bones ; m, metacarpus ; 1, 2, 3, digits. 



on one another, or, in other words, aid in the forma- 

 tion of a rigid rod ; by the reduction of the carpus 

 to two bones ; and by the fusion of the second and 

 third metacarpals and the reduction of the digits . a 

 very different modification of the homologous parts 

 is found in the bat. Here (Fig. 149) all the meta- 

 carpals, save that of the first digit (thumb), are greatly 

 elongated, as are too the phalanges of the third, 

 fourth, and fifth digits ; between these and the body 

 there extends a fold of thin membrane (the "wing 

 membrane"), by the expansion of which these Mam- 

 mals are enabled to float and move through the air. 



