Each lung is partially subdivided into separate portions called lobes. The right lung has three lobes, and the left lung has two. Each of these lobes, again, is composed of a large number of minute parts, called lobules. Each pulmonary lobule may be considered a lung in miniature, consisting as it does of a branch of a bronchial tube, air-cells, blood-vessels, nerves, and lymphatics, with a sparing amount of areolar tissue. The terminal portion of each lobule is composed of a group of pouches or air-cells, which communicate with the intercellular air passages. These cells are of various forms, according to the mutual pressure to which they are subject. Their cell walls are nearly in contact, and they vary from 1/50 to 1/90 of an inch in diameter. Outside the cells a network of pulmonary capillaries is spread out so densely that the interspaces or meshes are even narrower than the vessels, which are on an average 1/3000 of an inch in diameter. Between the atmospheric air in the cells and the blood in the vessels nothing intervenes but the thin membrane of the cells and the capillaries, and the delicate epithelium lining the former. And the exposure of the blood to the air is the more complete because the folds of membrane between contiguous cells, and often the spaces between the walls of the same, contain only a single layer of capillaries, both sides of which are thus at once exposed to the air. The enlargement of the capacity of the chest in inspiration is a muscular act; the muscles concerned in producing the effect being chiefly the diaphragm, the external intercostal muscles, etc. From the enlargement produced in inspiration, the chest and lungs return in ordinary tranquil expiration by their elasticity; the force employed by the inspiratory muscles in distending the chest and overcoming the elastic resistance of the lungs and chest wall being returned as an expiratory effort when the muscles are relaxed. The acts of expansion and of contraction of the chest take up, under ordinary circumstances, a nearly equal time, and can scarcely be said to be separated from each other by an intervening pause. The quantity of air that is changed in the lungs in each act of ordinary tranquil breathing is variable, but probably 30 to 35 cubic inches are a fair average in the case of healthy young and middle-aged men. The total quantity of air which passes into and out of the lungs of an adult, at rest, in 24 hours, has been estimated to be about 686,000 cubic inches. This quantity is largely increased by exertion; and it has been computed that the average amount for a hard-working laborer in the same time is 1,568,390. Breathing air is the quantity of air which is habitually and almost uniformly changed in each act of breathing. Complemental air is the quantity of air over and above this which a man can draw into the lungs in the deepest inspiration.