306 
LIFE, IN ITS HIGHER FORMS. 
upon the air, would be comparatively powerless as a means 
of locomotion, if that limb were in the condition which it 
presents when the cook puts the bird on the spit. The 
breadth of the oar and its hold upon the element through 
which it is to move are, therefore, increased by a most ad- 
mirable contrivance. The quill-feathers, inserted along one 
edge of the arm, and radiating outwards and backwards, 
like a fan, answer the purpose proposed. J ust look at the 
quill-l'eather from a bird’s wing. With how small an ex- 
penditure of material is a broad surface obtained ! How 
slight and apparently feeble is the structure, when examined 
fibre by fibre ; and yet how firmly and compactly it binds 
together, and how strongly the expanded web resists the 
air ! Breadth, strength, and lightness were the requisites, 
and, incompatible as they might have appeared, they are 
here exquisitely combined. 
Even such instruments as these, however, would not 
avail to lift the animal from the earth, and to bear it with 
ease and rapidity through the thin air, were its body of the 
same density as that of a quadruped. It must, therefore, 
be made buoyant, and this buoyancy is secured by several 
concurrent ordinances. In the first place, the whole of the 
muscles are abundantly supplied with blood, which passes 
through a heart of four chambers, with a rapidity far greater 
than that which obtains in terrestrial animals. Secondly, 
to supply the oxygen which is required for the vitalising of 
this swiftly circulating blood, a peculiar system of respira- 
tion is required. The lungs aro very large — spongy masses 
of blood-vessels lying along each side of the back-bone, and 
bound down to it : through these the bronchi, or divisions 
of the windpipe, pass; and, opening into the general cavity 
