the least is the mode by which their respiration is accomplished. This
is effected by means of air-vessels, which extend throughout the body,
and adhere to the under-surface of the bones. These, by their motion,
force the air through the true lungs, which are very small, and placed
in the uppermost part of the chest, and closely braced down to the back
and ribs. The lungs, which are never expanded by air, are destined to
the sole purpose of oxidizing the blood. In the experiments made by Mr.
John Hunter, to discover the use of this general diffusion of air
through the bodies of birds, he found that it prevents their respiration
from being stopped or interrupted by the rapidity of their motion
through a resisting medium. It is well known that, in proportion to
celerity of motion, the air becomes resistive; and were it possible for
a man to move with the swiftness of a swallow, as he is not provided
with an internal construction similar to that of birds, the resistance
of the air would soon suffocate him.