BIRD. 101 BIRD. so that it act* as a sort of mill, and with ex- traordinary power. In these birds, also, the grinding down of the contents of the gizzard is assisted by the small rough pebbles or grains of sand constantly swallowed, as is well exem- plified in domestic fowls. The liver and pancreas are usually large, as also are the kidneys; but there is no urinary bladder, and the urine, which cnntains very little water, is at once poured into llie cloaca, an enlargeiuent of the terminal por- tion of the intestine. The intestine varies greatly in capacity and length, being 'short' in all purely frugivorous and insectivorous birds, and 'long' in those which live upon fishes, carrion, grain, and grass; its walls difl'er in structure from the mammalian and agree with the reptilian type. The diaphrapm is incomplete and differently arranged from that of mammals. See Alimen- T.RT System, Evolution of. Food. — The amount of food required to sustain the high temperature and great bodily activity of birds is excessive, when compared with that re- quired by other animals of proportionate size; and young, growing birds tax the food-getting abilities of their parents to the utmost. JIany facts might be quoted in support of this, such as that of Aughoy's confined plovers, which ate an average of '200 locusts and similar large insects each day; fiedglings habitually consume more than their own weight of food between sunrise and sunset — sometimes twice as much. Adult birds eat almost continuously, and digestion is extremely rapid, the process being completed in from one to two hours in small birds. This ability for enormous consumption makes the birds of vast value to man in the reduction of insect pests; it also limits the time birds may fast, so that unless they can fly with very great rapidity, long journeys cannot be undertaken, and the fact that most birds are able to make great speed is probably an achievement of natu- ral selection related to this very characteristic. On the other hand, birds show considerable abil- ity to adopt a new diet, and adapt themselves to" it healthfully. The writings of Dr. F. E. L. Beal. Prof. S. A. Forbes, and others, published in the documents of the United States Depart- ment of Agriculture, and by the governments of Illinois and other States, contain much informa- tion on this subject. Respiration in Birdu is rapid and copious, and the pulmonary system includes not only a pair of rather large lungs, but a system of 'air-sacs,' by which air is distributed throughout the body. These air-sacs are of two kinds. One consists of membranous expansions of the lungs, which lie among the tissues in the form of in- flatable hags controlled by muscles, so that they can be emptied and filled at the will of the bird; they occur in all parts of the body and penetrate beneath the skin and inside many bones, but in varying amount. It is popularly believed that all bird-bones are hollow, but this is far from true. "Generally, the skeleton is most pneumatic in large birds that fly well, like vultures, storks, swans, and pelicans; less so in small birds, and least in heavy or little-flying water-birds. However, there are many excep- tions. While, for instance, most of the bones of many Passeres. of swifts, divers, rails, the kiwi, and of terns, are solid, and air-cells are restricted chiefly to the cranium, many parts of the skeleton of the large Ratitse are very pneu- matic. The greatest development of pneumatic cells exists in the screamers and hornbills, in which even the fingers and toes, in fact, any part of the skeleton, are hollow. ... It is well known that a bird which has its humerus shattered by shot can for some time breathe, al- though its beak and nostrils be tightly closed, and thus be submitted to unnecessary, excruci- ating pain. Compression of the thorax and abdomen suffocates a wounded bird better than strangulation." See Respib.tory System, Evo- I-VTION OF; AkIMAL He.T. The functions of these ai)'-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- ]>iper (see Colored Plate of Shore-Birds). These inflations are characteristics of male birds only, and appear onh- in the spring. The air-sacs also assist in furnishing a continuous current of air enabling some singing birds to maintain a long strain of melody, such as the nightingales. Arterial Circulation. — Conformably with this copious aeration, and the habits of activitj', the circulation of the blood in birds is ra])id, 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 in 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 th.in round, as in most mammals. See Circulatory System, EVOLITTION OF. Heproductive System. — 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 back' 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 jiassages (oviducts) in which it receives, in certain 'uterine' expansions, its coverings of albumen and .shell, to the right side of the rectum, and thence is voided in a more or loss 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 Ef;G and Emrryology.) All birds lay eggs, some only one, 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
