THE SKELETON AND ANATOMY OF BIRDS. 



337 



number : they are usually from ten to twelve, but as many as 

 twenty-four are found in some birds. The caudal feathers are 

 called " rectrices," and have no aftershaft ; their bases are 

 covered by " tail-coverts." The body feathers cover the mass 

 of "down''; these plumulje have no hooks to the barbules, 



BW, 





u. 



n. 



1/ 

 B 



Fia. 174. — Wing of Bird. 



F, humerus; J2, TJ, radius and ulna; M.G, metacarpals; T, thumb; r.2, two-jointed 

 finger; B.IF, bastard wing; ^1-10, xtrimary feathers; B, secondaries. 



SO the barbs are quite free. Another form is found in the 

 so-called " filoplumes," which consist of simply a slender shaft 

 with a few barbs at the tip. If we move the feathers of a fowl 

 on one side, we shall observe that they are attached to the skin 

 in definite areas only ; the bare intervening spaces are called 

 "apteria,"' the feather-tracts "pterylffl.'' 



The Skeleton and Anatomy op Birds. 



The skeleton of the bird (fig. 175) is remarkable for two 

 things — extreme compactness and lightness. The former is 

 due to the excess of phosphate of lime in the bones, the latter 

 to the presence of hollow spaces in many of the bones. These 



