242 On the Ornithology of Wilts. 
cells or membranous sacs disposed for this purpose over the body: 
these sacs are situated in the chest, and among the muscles, and 
between the muscles and the skin; and in some birds are continued 
down to the wings, and extend even to the pinions, thigh bones, 
and other parts of the body: for the same purpose the feathers and 
especially the wing feathers also contain a large quantity of air. 
Now all these cavities and others not enumerated, such as the 
hollows of the bones, can be filled and distended with air at the will 
of the bird: by this means the strength and bulk of the bird is 
increased, without adding to its weight: and such a general diffusion 
of air throughout the body must be of infinite service in enabling 
it to fly, to poise itself in the air, and to skim far above the surface 
of the earth. Nor is that the only use of this wonderful provision 
of nature; I again quote Bewick, who says “it is likewise eminently 
useful in preventing its respiration from being stopped or interrupted 
by the rapidity of its motion through a resisting medium: were 
it possible for man to move with the swiftness of the swallow, the 
actual resistance of the air, as he is not provided with internal 
reservoirs similar to those of birds, would soon suffocate him.’ 
Another very remarkable peculiarity in the internal economy of 
birds, is their mode of digestion: the bill, is scarcely if ever, used 
for mastication, but solely as an instrument of prehension: it is the 
gizzard whose amazing strength and powers can scarcely be over- 
rated, that grinds down the grain and other food, and renders it fit 
for digestion. Experiments have been made, by which it has been 
incontrovertibly proved, that glass, nails, and the hardest substances 
have in a few hours been filed down by the action of the gizzard, 
without any injury accruing to it thereby: asa help to this digestive 
power small stones are often swallowed by birds, which are eminently 
useful in assisting this grinding process, thus rendering the food 
more amenable to the gastric juices. 
Now after this rapid glance at the general structure of birds, can 
we conceive anything more adapted for buoyancy and for rapid 
motion through the air, than their external and internal formation ? 
We cannot but be struck with their wonderful adaptation to the 
position which they were created to fill. Let us now push our 
