26 
SECOND BOOK. 
But, unlike Mammals, most birds have a number ot 
bags or ‘air-sacs’ in different parts of the body, 
which are open to the tubes that carry air to the 
lungs. Moreover, some of their bones are hollow, 
forming air-chambers which connect with the air- 
sacs. These air -sacs 
and hollow bones not 
only help to render the 
body light, thus fitting 
it for flight in the air, 
but they also aid the 
lungs in the work of 
keeping the blood pure. 
5. “Here is the skele- 
ton of a bird. Notice 
how firm and compact 
are the bones of the 
body. On the other 
hand the neck has many 
joints, so that the bird, 
when alive, could bend 
it in various ways. 
The neck is always long enough to allow a bird to 
plume every part of its body, and to reach the gland 
which gives out oil for dressing the feathers. 
6 . “A bird’s fore-limbs are not for grasping or 
picking up food, but generally for flying, and its 
mouth is supplied with a stiff, horny bill or beak. 
7 . “In some birds the bill is so shaped that food 
Skeleton of a Bird of Prey (Vulture). 
