Fig. 168.—Experiment to prove that
expired air contains carbon dioxide.
It is well known that the body of a living animal such as a rabbit or a man is always warm; and the experiment ([Fig. 168]) of passing through clear lime-water the air breathed out from the lungs shows, by the milkiness produced, that the animal is also constantly producing carbon dioxide during its life. Is life, then, always accompanied by a peculiar form of burning, in which the living substance of the body is the combustible material? It seems so, and the experiments of physiologists tend to confirm this view.
The necessity for breathing.—The energy which enables a muscle to contract is derived from the oxidation—the slow burning—of part of its substance, just as the energy which enables a steam-engine to move is derived from the burning of fuel in the boiler fires. The fires soon go out, and the engine stops, unless fresh fuel is added from time to time and a plentiful supply of air is available. Similarly, a muscle loses its power of contracting, a gland that of secreting, the brain that of thinking, unless the waste matters resulting from previous activities are cleared away and replaced by fresh food and fresh oxygen. Wherever vital action is taking place, whether in a contracting muscle, a secreting gland, or a thinking brain, there is continual consumption of oxygen and continual production of waste material, chiefly carbon dioxide. In the higher animals the renewal of oxygen and the removal of waste material are performed by the blood. Blood-vessels are to the body what rivers and canals are to a country: they act as highways for the transport of material. We may perhaps carry the analogy a little farther and find in the red corpuscles of the blood the rough equivalents of boats or canal-barges, for they carry with them tiny loads of oxygen. As the blood creeps along the narrow channels in an active tissue the red corpuscles relinquish their oxygen, and the fluid portion of the blood takes up carbon dioxide. The blood continues its course, and sooner or later arrives at a place where it can obtain a fresh supply of oxygen and get rid of its surplus carbon dioxide. In the rabbit this exchange takes place as the blood is passing through the capillaries of the lungs. There the blood is separated from the air by a membrane so delicate that gases can readily pass through it; and, hence, on leaving the lungs the blood has got rid of the waste carbon dioxide, and its red corpuscles are laden with fresh oxygen. This exchange of useless carbon dioxide for oxygen constitutes respiration or breathing.
The mechanism of respiration.—In active animals the air inside the lungs soon becomes vitiated, unless there is some means of changing it. Under ordinary conditions a man changes the air in his lungs from thirteen to fifteen times a minute. He does this quite automatically, and without thinking about it. Every four seconds or so a set of muscles contracts and pulls his ribs upwards and outwards; another muscular contraction pulls down the floor of his chest at the same time. As a consequence the cavity is much enlarged. The lungs follow the movements of the walls of the chest, and some thirty cubic inches of air are sucked in. Immediately the ribs fall back to their former position, the chest-floor rises, and air is driven out. Then after a short pause the process is repeated. It should be noticed that only about thirty cubic inches of air are changed at each respiration, although the capacity of the human lungs averages about 230 cubic inches. All mammals breathe in much the same way.
Plants and animals.—There are considerably more points of similarity than of difference between plants and animals. In every case the vital activities are accompanied by an oxidation of living substance, and from this fact arises the necessity for food and oxygen. The breathing of plants is essentially like that of animals, and consists in taking in oxygen and giving out carbon dioxide; though the mechanism of respiration is—except in the lowest plants and animals—entirely different in the two cases. It is in the sources from which they obtain their food that plants and animals are most unlike. An animal must be supplied with food which has already been prepared by some other living thing; and it is obvious that the food even of carnivorous animals can ultimately be traced back to plants, for the flesh-eater preys upon the vegetarian. Animals are therefore entirely dependent upon plants for food. In this sense the saying “all flesh is grass” is full of significance.
Green plants ([Chapter II.]) are quite independent of all other forms of life, and can build up their substance from water, mineral matter, and the carbon dioxide of the air. The taking in of carbon dioxide and giving out of oxygen by green plants has nothing whatever to do with respiration; it is part of their process of feeding. Green plants breathe in the usual manner—by taking in oxygen and giving out carbon dioxide. It should, however, be noticed that the peculiar method by which a green plant obtains its carbonaceous food is of the highest importance to animal life; for by this process the amount of injurious carbon dioxide in the air is considerably lessened, while the proportion of life-supporting oxygen in it is greatly increased.
Fungi ([Chapter XI.]) seem to be intermediate, as regards their method of feeding, between green plants and animals. They require their carbonaceous food in an organic form, that is, already prepared by other living things; but they can obtain the other elements of their food from mineral salts and water.
The thoughtful student will be increasingly impressed by the extent to which the plant and animal kingdoms are dependent upon each other, and by the manner in which each utilises the waste products of the other for carrying on its own life processes.