BIRDS. 
307 
of the chest, admit the air freely into every part of the 
interior : nay, more ; the atmospheric air bathes every 
blood-vessel of any considerable size throughout the body, 
passes into the hollow bones of the limbs, and even pene- 
trates between the muscles, and into great membranous 
cells beneath the skin.* 
The consequence of this copious supply of oxygen to the 
blood, not only in the lungs, but in every part of its course, 
is a great increase of its heat, which far exceeds that of the 
most warm-blooded quadruped. The heat pervading the 
whole of the animal tissues is communicated to the air, 
which, as we have just seen, is so extensively distributed 
about the body ; and thus the bird is not only rendered 
light by being blown out with air, but that air is brought 
up to a very high temperature, and so rarefied, and made 
very buoyant. 
The animal heat thus generated must not be allowed to 
escape too rapidly ; and hence a body-clothing is provided, 
which of all substances is perhaps the most effective non- 
conductor of caloric. A feather is in itself a study. When 
we look at the whole covering of a bird, we caunot help 
observing how soft, how light, how smooth, how compact, 
how warm it is; and if we examine each feather separately, 
there is not less to admire in the details of its structure. 
It consists of two parts ; a light but firm shaft formed of a 
* This peculiarity was once brought strongly under our own observation in 
the case of a pelican (Pdeeanua fuacus), which we wore dissecting. Tbe whole 
inner surface of the skin on tbo trunk was cellular, especially on the breast, 
forming an immense congeries of membranous bladders, inflated with air. As 
an example of the free in toreom m u nioati on that exists between the tissues of 
the body, it may bo mentioned that, in this specimen, the great gular pouch, 
when filled with water (to the amount of seventeen pints), allowed it, to escape 
by dripping from a wound in the outer joint of the icing. 
