The Philosophy of Health; Volume 2 (of 2) / or, an exposition of the physical and mental constitution of man

by Southwood Smith

2ndly. The distribution of quantities is universally by proportions or multiples. Thus, of the air inspired, one measure is decomposed and three measures are returned unchanged: of the air decomposed at a single inspiration, there are always in store in the lungs precisely forty-eight measures; and so on in many other cases. The proportions are not arithmetical, but geometrical. When we compare arithmetical quantities with each other, we say that one quantity is by so much greater than another; when we compare geometrical quantities, we say that one quantity is so many times greater than another. From this adoption in the distribution of quantities of geometrical proportions it results that whatever be the size of the animal the ratios remain uniformly the same, and that thus one and the same law is adapted to the vital agencies of living beings under every possible diversity of magnitude and circumstance.

463. Such are the interesting and important properties and relations deducible from the phenomena of respiration. The disappearance of oxygen and azote from the air inspired, and the replacement of the oxygen that disappears by the production of carbonic acid, and of the azote by the exhalation of azote, in which, as we have seen, the great changes wrought by respiration on the air consist, are essentially the same in all animals, whatever the medium breathed, and whatever the rank of the animal in the scale of organization. In all, the proportion of the oxygen of the inspired air is diminished;—in all, carbonic acid gas is produced. Comparing, then, the ultimate result of the function of respiration in the two great classes of living beings, it follows that the plant and the animal produce directly opposite changes in the chemical constitution of the air. The carbonic acid produced by the animal is decomposed by the plant, which retains the carbon in its own system and returns the oxygen to the air. On the other hand, the oxygen evolved by the plant is absorbed by the animal, which in its turn exhales carbonic acid for the re-absorption of the plant.

464. Thus the two great classes of organized beings renovate the air for each other, and maintain it in a state of perpetual purity. The plant, it is true, absorbs oxygen during the night as well as the animal; but the quantity which it gives off in the day more than compensates for that which it abstracts in the absence of light. This interesting fact has been recently established by an extended series of experiments instituted by Professor Daubeney[2] for the express purpose of investigating this point.

465. From the general tenor of these experiments, it appears that, in fine weather and as long as the plant is healthy, it adds to the atmosphere an amount of oxygen not only sufficient to compensate for the quantity it abstracts in the absence of light, but to counterpoise the effects produced by the respiration of the whole animal kingdom. The result of one of these experiments will convey some conception of the amount of oxygen evolved. A quantity of leaves about fifty in number were enclosed in a jar of air; the surface of all the leaves taken together was calculated at about three hundred square inches; by the action of these leaves on the carbonic acid introduced into the jar, there was added to the air contained in it no less than twenty-six cubic inches of oxygen. As there was reason to conclude that the evolution of oxygen, in the circumstances under which this experiment was performed, was considerably less than it would have been in the open air, several plants were introduced into the same jar of air in pretty quick

succession: the amount of oxygen now evolved was increased from twenty-one to thirty-nine per cent., and probably had not even then attained the limit to which the increase of this constituent might have been brought. From the proportions of the constituent elements of carbonic acid gas ([442]) it necessarily follows that, by the mere process of decomposition, out of every eleven grains of carbonic acid gas eight grains of oxygen must be liberated, three grains of carbon being retained by the plant, and consequently that eight grains of oxygen must be restored to the atmosphere, less only by so much as the plant itself may absorb. How great, then, must be the production of oxygen by an entire tree under favourable circumstances; that is, when animal respiration and animal putrefaction present to it an abundant supply of carbonic acid on which to act!

466. This influence, says Professor Daubeney, is not exerted exclusively by plants of any particular kind or description. I have found it alike in the monocotyledonous and dycotyledonous; in such as thrive in sunshine and those which prefer the shade; in the aquatic as well as in those of a more complicated organization. How low in the scale of vegetable life this power extends is not yet exactly ascertained; the point at which it stops is probably that at which there ceases to be leaves.

467. From the whole, then, it appears that the functions of the plant have a strict relation to those of the animal; that the plant, created to afford subsistence to the animal, derives its nutriment from principles which the animal rejects as excrementitious, and that the vegetable and animal kingdoms are so beautifully adjusted, that the very existence of the plant depends upon its perpetual abstraction of that, without the removal of which the existence of the animal could not be maintained.

468. The changes produced upon the blood by the action of respiration are no less striking and important than those produced upon the air. The blood contained in the pulmonary artery, venous blood (fig. 140-7.), is of a purple or modena red colour: the moment the air transmitted to the blood by the bronchial tubes comes into contact with it, in the rete mirabile (fig. 140-10.), this purple blood is converted into blood of a bright scarlet colour. Precisely the same change is produced upon the blood by its contact with the air out of the body. If a clot of venous blood be introduced into a vessel of air, the clot speedily passes from a purple to a scarlet colour; and if the air contained in the vessel be analyzed, it is found that a large portion of its oxygen has disappeared, and that the oxygen is replaced by a proportionate quantity of carbonic acid. If the clot be exposed to pure oxygen, this change takes place more rapidly and to a greater extent; if to air containing no oxygen, no change of colour takes place.

469. The elements of the blood upon which a portion of the air exerts its action are carbon and hydrogen. The oxygen of the air unites with the carbon of the blood and forms carbonic acid, and this gas is expelled from the system by the action of expiration. The constituent of the blood which affords carbon to the air would appear to be chiefly the red particles. The other portion of the oxygen of the air unites with the hydrogen which is expelled with the carbonic acid in the form of aqueous vapour. The direct and immediate effect of the action of respiration upon the blood is then to free it from a quantity of carbon and hydrogen.

470. Physiologists are not agreed whether the union of the oxygen of the air with the carbon of the blood takes place in the lungs or in the system. Some experimentalists maintain that the oxygen which disappears from the air, and that which is contained in the carbonic acid, are exactly equivalent, so that no oxygen can be absorbed. According to this view, which has been clearly shown to be incorrect ([459]), the effect of respiration is merely to burn the carbon of the blood, just as the oxygen of the air burns wood in a common fire, the result of this combustion being the generation of carbonic acid, which is expelled from the system the moment it is formed.