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Congestion of the lungs is a frequent termination of pneumonia; and in that congestion the air-cells are easily ruptured and filled with blood. That blood assumes a black pulpy appearance, commonly indicated by the term of 'rottenness', an indication or consequence of the violence of the disease, and the hopelessness of the case.
On the right hand is the exterior of one of the lobes, and on the left hand are seen the branching tubes of the interior, through which the air we breathe passes to the exceedingly minute air-cells of which the lungs chiefly consist.
At no time do we breathe pure oxygen, but "air" i.e., a mixture of 21 parts of the former with 79 parts of an inert gas, nitrogen; and there is always in the air more oxygen than the blood actually takes from it in the air-cells. The intaking of air is termed by physiologists inspiration, and its expulsion expiration, the whole process being respiration.
In bronchitis inflammation affects the lining of the air-tubes, travelling from the larger towards the smaller, and in bad cases extending even to their termination in the minute air-cells. The inflammation leads to the pouring out of a secretion, which by degrees becomes thick like matter, or even very tenacious, almost as tough as though it were a thin layer of skin.
The lung-substance, however, with all its curious structure of air-cells and their network of minute vessels where, as in nature's laboratory, the blood receives its due supply of oxygen, is not reproduced.
The carbonic acid gas thus formed produces minute air-cells in the bread, or, as the cook says, makes it light. When this process is performed with exact attention to chemical laws, so that the acid and alkali completely neutralize each other, leaving no overplus of either, the result is often very palatable. The difficulty is, that this is a happy conjunction of circumstances which seldom occurs.
In Mr. Stokes' collection, I have seen a beautiful example of this structure, in a specimen of obsidian from Mexico, which is shaded and zoned, like the finest agate, with numerous, straight, parallel layers, more or less opaque and white, or almost perfectly glassy; the degree of opacity and glassiness depending on the number of microscopically minute, flattened air-cells; in this case, it is scarcely possible to doubt but that the mass, to which the fragment belonged, must have been subjected to some, probably prolonged, action, causing the tension slightly to vary in the successive planes.
All these have one and the same object, to give us the cooked particles of our flour separated by such permanent air-cells as will enable the stomach more readily to digest them. A very common mode of aerating bread in America is by the effervescence of an acid and an alkali in the flour.
One thing, however, may be borne in mind as a principle that the excellence of bread in all its varieties, plain or sweetened, depends on the perfection of its air-cells, whether produced by yeast, egg, or effervescence; that one of the objects of baking is to fix these air-cells, and that the quicker this can be done through the whole mass, the better will the result be.
It is the membrane which in this special sense covers or lines the respiratory tract from the very outlet of the nose to the terminal bronchi; in fact, to the very air-cells of the lungs themselves. Its function is that of supplying the involved passages with moisture, and it secretes a glairy or watery substance called mucus. Now, mark this well.
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