The functions of these air-sacs are not very evident. They serve to ventilate the rather in contractable lungs, and from them issues the air which inflates the `drums' in the neck of the prairie chicken, the gular pouches of the ad jutant stork, the 'horn' of the bell-bird (q.v.), and that extraordinary swelling of the throat which marks excitement in the pectoral sand piper (see Colored Plate of SHORE-BIRDS) . These inflations are characteristics of male birds only, and appear only in the spring. The air-sacs also assist in furnishing a continuous current of air enabling somo birds to maintain a long strain of melody, such as the nightingales.
Arterial Circulation.—Conformabl• with this copious aeration, and the habits of activity, the circulation of the blood in birds is rapid, the heart beating 120 times a minute at rest, and nearly doubling this rate at the first stroke of the wings; and its temperature (from 100° to 112° F.) is considerably higher than in mammals. The heart resembles that of the mammalia in its form and structure; but the right ventricle, in stead of a mere membranous valve, is furnished with a strong muscle to impel the blood with greater force into the lungs; and the carotid ar teries are peculiar. During incubation there is a congestion of blood ill the enormously dilated vessels of the abdominal wall, forming the 'brood-organ.' The red corpuscles are, on the average, twice as large as those of man, and elongated, as in reptiles, rather than round, as in most mammals. See CIRCULATORY SYSTEM, EVOLUTION OF.
Reproductive Systenz.—Birds are distinctly male and female, cock and hen. The females
possess a pair of ovaries (of which only the left one is functional as a rule) situated in the 'small of the hack' at the front end of the kidney. The ovary consists of a grape-like cluster of germinal eggs, which during the breeding season exhibit all stages of size and ripeness, but in winter are reabsorbed, so that the determination of sex by dissection at that season is difficult. The ripe egg passes down tubular passages (oviducts) in which it receives, in certain 'uterine' expansions, its coverings of albumen and shell, to the right side of the rectum, and thence fs voided in a more or less advanced state of development, which is completed by incubation or its equiva lent. The testes of the male are a pair of glands, situated as are the ovaries, from which the spermatic ducts lead to the cloaca. (See Eon and Emtinvoi.ony.) All birds lay eggs, some only ones others as many as twenty, the number seeming to depend upon the average chances of reaching sexual maturity possessed by each species. With rare exceptions, these eggs are cared for by the parents, and all birds of higher organization prepare receptacles (nests) for them, and furnish by brooding the continuous warmth necessary to their hatching—a process which requires from two to six weeks according to size and other factors. This necessity has developed a body of instincts, habits, and mental and structural characteristics, which include the most striking, significant, and interesting facts of bird life and history. See .NIDIFICATION,