Page:Popular Science Monthly Volume 20.djvu/772

750 and is termed tidal air. Besides the residual air which can not be expelled, the lungs ordinarily contain a large quantity which can be exhaled by a forced expiration. This is known as reserve air. And, in addition to these three kinds, a large quantity can be inhaled by a forced inspiration, called complemental air.

In this can be seen a fine adaptation to the requirements of varying circumstances; for, if the breathing of mammals were normally at the full capacity, there would remain no provision for varying the rapidity of respiration according to temperature and exercise. Moreover, the residual air gives uniformity and constancy to the respiratory change of the blood, and prevents sudden variations in the kind, amount, and temperature of the air in the lung-cells, which would be injurious to the blood and to the delicate tissues. The change of the residual air is slowly effected by the physical process of diffusion between it and the reserve air. When this purifying change is not sufficiently rapid, we are impelled to take a "deep breath," and so wholly replace the reserve air.

The relative and the absolute amounts of each of the four kinds of respiratory air in the human lungs may be tabulated thus:

The breathing-pump of mammals may be compared to a conical box with movable sides and base. By contraction of the muscles attached to and connecting the ribs, the sides of the chest are moved upward and outward; while at the same time the diaphragm, forming the arched base of the chest, is depressed or flattened by its muscular contraction. Thus the greater muscular effort in ordinary breathing is used to enlarge the cavity of the thorax or chest, producing inspiration—not, as in birds, to diminish the cavity, producing expiration. In other words, the air is forced into mammalian lungs by atmospheric pressure when, through muscular effort, the chest enlarges; and the air is expelled simply by the elastic reaction of the lungs and chest.

The lungs are freely suspended by the windpipe, and are distended by the atmospheric pressure in opposition to their elasticity. Consequently, an opening in the walls of the thorax is liable to produce suffocation, by giving the air a more direct and easy route to the vacuum of the chest than through the trachea and lungs.

A delicate membrane called the pleura closely invests the lungs, and is then reflected to line the cavity of the thorax. By the secretion of a serous fluid, it prevents friction, which would otherwise result from the constant movement.