CLASS II. AVES. 3 



this, tlie 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 ccEca. 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 : 

 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 — so that 

 the venous and arterial blood can never mix in that organ, and the whole of the blood returned 

 from the dift'erent pails 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 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 winffs. The beak is used instead of hands, and such is the flexibilitv 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 oi 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 harbs 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 ''mall tuft of soft filaments, without any central shaft. These filaments are 

 very slender, and b',ar on each side a series of still more delicate filaments, which may be regarded 

 as analogous to +ue barbules of the ordinary feathers. This downy coat fulfills the same ofllice 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 diff"erent 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 dift'erent states is often 

 very dissimilar, and it is only by an accurate study of the living animals, which is of course almost 



