CLASS II. AVES. 
3 
this, the intestinal canal is enlarged into a third stomach, the gizzard, in which the process of diges- 
tion is carried still further. In the granivorous birds, the walls of this cavity are very thick and 
muscular, and clothed internally with a strong, horny epithelium, serving for the trituration of 
the food ; but in the predaceous species the gizzard is thin and membraneous. The intestine is 
rather short, but usually exhibits several convolutions; the large intestine is always furnished with 
two coeca. It opens by a semicircular orifice into the cloaca, which also receives the orifices of 
the urinary and generative organs. The liver is of large size, and usually furnished with a gall- 
bladder. The pancreas is lodged in a sort of loop formed by the small intestine immediately 
after quitting the gizzard. There are also large salivary glands in the neighborhood of the mouth, 
which pour their secretion into that cavity. 
The organs of circulation and respiration in birds are adapted to their peculiar mode of life : 
tliey are not, however, separated from the abdominal cavity by a diaphragm, as in the mammalia. 
The heart consists of four distinctly separated cavities — two auricles and two ventricles — so that 
the venous and arterial blood can never mix in that organ, and the whole of the blood returned 
from the different parts of the body passes through the lungs before being again driven into the 
systemic arteries. The blood is received from the veins of the body in the right auricle, from 
which it passes through a valvular openirg into the right ventricle, and is thence driven into the 
lungs. From these organs it returns through the pulmonary veins into the left auricle, and passes 
thence into the ventricle of the same side, by the contraction of which it is driven into the aorta. 
This soon divides into two branches, which by their further subdivision give rise to the arteries 
of the body. 
The jaws or mandibles are sheathed in a horny case, usually of a conical form, on the sides of 
which are the nostrils. In most birds the sides of this sheath or bill are smooth and sharp, but 
in some they are denticulated along the margins. The two anterior members of the body are 
extended into wings. The beak is used instead of hands, and such is the flexibility of the verte- 
bral column, that the bird is able to touch with its beak every part of its body. This curious and 
important result is obtained chiefly by the lengthened vertebrae of the neck, which in the swan 
consists of twenty-three bones, in the stork of nineteen, the ostrich eighteen, the domestic cock 
thirteen, the raven twelve. The vertebra of the back are seven to eleven ; the ribs never exceed 
ten on each side. 
The clothing of the skin of birds, consists of feathers, which in their nature and development re- 
semble hair, but are of a far more complicated structure. A perfect feather consists of the shaft or 
central stem, which is tubular at the base, where it is inserted into the skin, and the barhs or 
fibers, which form the wehs on each side of the shaft. The two principal modifications of feathers 
are quills and jflumes, the former confined to the wings and tail, the latter constituting the gen- 
eral clothing of the body. Besides the common feathers, the skin of many birds, and especially 
of the aquatic species, in which the accessory plumules rarely exist, is covered with a thick coat- 
ing of down, which consists of a multitude of small feathers of peculiar construction ; each of these 
down feathers is composed of a very small, soft tube imbedded in the skin, from the interior of 
which there rises a small tuft of soft filaments, without any central shaft. These filaments are 
very slender, and bear on each side a series of still more delicate filaments, which may be regarded 
as analogous to the barbules of the oi'dinary feathers. This downy coat fulfills the same oflBce as 
the soft, woolly fur of many quadrupeds, the ordinary feathers being analogous to the long, smooth 
hair by which the fur of those animals is concealed. The skin also bears a good many hair-like 
appendages, which are usually scattered sparingly over its surface ; they rise from a bulb which is 
imbedded in the skin, and usually indicate their relation to the ordinary feathers by the presence 
of a few minute barbs toward the apex. 
Once or twice in the course of the year the whole plumage of the bird is renewed, the casting 
of the old feathers being called moulting. In many cases the new clothing is very different from 
that which it replaces, and in birds inhabiting temperate and cold climates we can frequently dis- 
tinguish a summer and winter dress. This circumstance has given rise to the formation of a con- 
siderable number of false species, as the appearance of the birds in these diff'erent states is often 
very dissimilar, and it is only by an accurate study of the living animals, which is of course almost 
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