VISCERAL SYSTEM. The digestive apparatus of birds resembles that of mammals; exhibiting, however, various modifications, according to the different kinds of food—some birds feeding on flesh, others on fish, others exclusively on in sects, others on seeds, others more indiscriminately on a variety of animal and vegetable sub stances. Few birds masticate their food in any degree; upon being swallowed, it enters the crop or craw, an enlargement of the (esophagus situ ated just before the breast-bone, where it is moistened by saliva and partly digested, so that its contents, easily regurgitated, form a suitable food for nestlings in sonic families, as, notably, the pigeons. The crop is wanting in the ostrich, and also generally in birds that feed on fish, and is of greatest size in those of which the food consists of seeds or grain. It is generally single, and on one side of the gullet; sometimes, as in pigeons, it is double. A second dilatation of the (esophagus, called the prorcn rieu/us, is gen erally largest in those birds in which the crop is wanting or small; and in this the food is further softened and chemically treated. The third and principal stomach is the gizzard (q.v.), which in birds of prey, fish-eating birds, etc., is a mere membranous sac, but in birds which feed on grain or seeds is very thick and muscular, so that it acts 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 contains very little water, is at once poured into the cloaca, an enlargement 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 differ in structure from the mammalian and agree with the reptilian type. The diaphragm is incomplete and differently arranged from that of mammals. See ALIMEN TARY 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. :Many facts might be quoted iu support of this, such as that of Aughey's confined plovers, which ate an average of 200 locusts and similar large insects each day; fledglings 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 Birds 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-saes are of two kinds. One consists of membranous expansions of the lungs, which lie among the tissues in the form of in flatable bags 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. hut 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 Ratitfe arc 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 RESPIRATORY SYSTEM, EVO LUTION OF; ANIMAL HEAT.