12 THE STRUCTURE AND LIFE OF BIRDS chap. 



"little finger") should be. The up-and-down move- 

 ment that the lizard has, and that we have in our 

 wrists, has almost disappeared : when the wing is 

 extended, it does not exist at all ; when it is flexed, 

 some movement of the kind becomes possible. The 

 fusion of one row of carpal bones with the meta- 

 carpals has no doubt helped towards this rigidity 

 which is so important to the wing. At the shoulder- 

 joint there is always the utmost free play. 



A bird's bones combine in a most remarkable way 

 lightness and strength. It is popularly supposed that 

 all birds, or at any rate those which fly much, have 

 all their chief bones hollow and marrowless. This is, 

 however, a fallacy ; some of the best flyers — e.g., the 

 Swallow — having even the Humerus solid. But whether 

 pneumatic or not, the bones are always fine in the 

 grain and strong. 



The chief results of the changes that the fore limb has 

 undergone may now be summed up. ( I ) It is of most re- 

 markable length. (2) It is at one time rigid, at another 

 flexible, according as rigidity or flexibility is required. 

 Contrast with a bird's hand a lizard's with its waggly 

 fingers. And how neatly and comfortably the wing 

 folds, when it is to be put to rest upon the body, in the 

 form of a Z- (3) There are broad surfaces of bone 

 to support the feathers. (4) Strength is combined 

 with lightness. (5) The loss of two metacarpal bones 

 and two fingers has been a gain, since the present 

 hand formed of three united metacarpals and two 

 united fingers (I am disregarding the insignificant 

 " thumb ") is more efficient for purposes of flight than 

 a hand with five fingers could well be. 



