proportionately the largest among animals. In some species the skin is quite as active as the lungs, and the latter can be removed without causing immediate death.
The lungs of birds are less developed structurally than those of mammals, but are much larger. Connected with them in various parts of the body are air-sacs, especially in the abdomen and beneath the skin of the neck and wings. Except in water-birds, the hollow 
Fig. 9.—General view of the Air Reservoirs of the Duck, opened inferiorly; also their Relations with the principal Viscera of the Trunk. 1,1, anterior extremity of the cervical reservoirs; 2, thoracic reservoir; 3, anterior diaphragmatic reservoir; 4, posterior ditto; 5, abdominal reservoir.—a, membrane forming the anterior diaphragmatic reservoir; b, membrane forming the posterior ditto.— 6, section of the thoraco-abdominal diaphragm.—d, subpectoral prolongation of the thoracic reservoir; e, pericardium; f, f, liver; g, gizzard; h, intestines: m, heart: n, n, section of the great pectoral muscle above its insertion into the humerus; o, anterior clavicle; p, posterior clavicle of the right side cut and turned outward. (From M. Sappey’s work.) bones also contain air, and by their connection with the lungs respiration can be continued through an opening in the arm or thigh-bone, although the windpipe may be tied.
The respiratory system is most developed in birds of n powerful flight, and doubtless aids in rendering them buoyant. Perhaps the air-sacs beneath the wings assist in holding the latter outstretched; and it has been suggested that the sacs might serve as a cushion to protect those which suddenly dive into water after prey.
The blood-capillaries in the lungs of reptiles and amphibians are exposed to the air on one side only, while those of birds and mammals are arranged on a different and superior plan, being exposed on two opposite sides. Lungs of birds consist of an aggregation of distinct globules or "lunglets." As the lungs are attached to the dorsal side of the chest and the diaphragm is imperfect, expiration is effected by an active effort—by pulling the bone nearer the spine, and so diminishing the cavity.
While reptiles can live in air too impure for mammals, birds will die in an atmosphere which to mammals is quite harmless. Birds bear
