A rude apparatus to illustrate the mechanism of breathing in mammals is easily made by suspending the lungs of some small animal in a glass bell-jar closed below by an elastic membrane—a sheet of rubber, for instance—the only access for air to the interior being through the windpipe. Then the forced enlargement of the cavity, by pulling down the membrane, causes the inflation of the lungs. This apparatus is deficient in mobility of the walls.
Fig. 11.—Air-Cells of Lung, with Intervening Tissues. a, epithelium; b, elastic trabeculæ; c, membranous wall, with fine elastic fibers.
The rapidity of the respiratory movements in man is about one inspiration to four heart-beats, or fifteen to twenty-five per minute; greatly varying, however, according to age, sex, and circumstances. In animals with high temperature, breathing is much faster, becoming almost a tremor in birds. In the w T hale, on the contrary, breathing is suspended while the animal is under water; it being provided with reservoirs of pure blood. When the latter is exhausted, the creature comes to the surface and puffs and "blows" to obtain air and refill the reservoirs.
The difference in color of the blood of vertebrates is chiefly due to the varying amount of oxygen in chemical combination with the hæmoglobin of the red corpuscles—the brightness of color being proportionate to the oxygen. An essential part of the hæmoglobin is iron; and it has been supposed that the change in color is due to a chemical change from a ferrous to a ferric salt. But this simple and plausible explanation is now denied by eminent physiologists, who, however, admit that the iron has some essential but unknown influence. A
