12. THE STRUCTURE AND LIFE OF BIRDS cuap. 
“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—eg., 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 besummed up. (1) 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. 
