Excerpt from Illustrated Natural History of the Animal Kingdom, Being a Systematic and Popular Description of the Habits, Structure, and Classification of Animals From the Highest to the Lowest Forms, Vol. 2

The organs of circulation and respiration in birds are adapted to their peculiar mode of life they 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 - se 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 opening 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 011 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 vertebrae 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 barbs or fibers, which form the webs on each side of the shaft. The two principal modifications of feathers are quills and plumes, 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 ordinary feathers. This downy coat fulfills the same office 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.

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