Respiration is a function by which the oxygen of the air is introduced into the blood, and by which part of the useless and hurtful materials are expelled, in a gaseous form, from the organism. It is divided into two parts: inspiration, during which the atmospheric air penetrates the pulmonary cells; and expiration, which expels the air which has been changed during its stay in the lungs. On reaching the cells of the lungs, the blood is separated from the air by their walls and those of the capillaries, which ramify over them. However thin these membranes may be, they suffice to confine the air and the blood in distinct cavities; but like the other organic tissues, they have the property of allowing themselves to be penetrated by endos-mosis and exosmosis. The oxygen of the air therefore passes through them in order to combine with the blood; while those gases contained in this fluid, which should be eliminated, separate from it and mingle with the air, which carries them away with it during expiration. It is an interchange of gases between the air and the blood, the air giving up oxygen to the blood, and receiving from it other gaseous fluids, among which carbonic acid gas predominates in volume. This being in excess in the venous blood is exhaled from the lungs, while the oxygen of the air combines with the blood which is carried to the heart by the veins, which has been deprived of a part of its nutritive elements, and has become unfit to support life. On coming in contact with the oxygen, the venous blood loses its dark colour, becomes a brilliant red, and returns to the heart transformed into arterial blood. This group of phenomena is called sanguification.

On the one hand the oxygen of the atmosphere burns carbon in the lung, on the other the lung exhales carbonic acid gas, nitrogen, and the vapour of water. From whence are these gases and this water derived? The carbonic acid gas is not produced in the lungs alone. The venous blood reaches the organ of respiration poor in oxygen, and charged relatively with the carbonic acid gas which it has received in its course from all the tissues; everywhere this acid has been produced by the combination of the carbon with the oxygen, which in the lung is borrowed from the air, but everywhere else it comes from the arterial blood. In a word, the oxygen combined with the blood by respiration, is separated from it little by little in the capillaries throughout the whole body in order to produce numerous products, among others carbonic acid gas. On leaving the heart and in the arteries, the blood contains 24 parts of oxygen per 1000, in the veins it contains only 11 per 1000. As for the nitrogen and the vapour of water, one is disengaged, and the other produced during this same process of nutrition, and both are drawn from the principles in the organism which are introduced into it by digestion or respiration.

Lavoisier was the first to demonstrate the absorption of oxygen by respiration, and to show by experiment the analogy existing between combustion and respiration. " Respiration," said he, " is nothing but a slow combustion of carbon and hydrogen, which resembles, in every respect, that which takes place in a lamp. ... In respiration, as in combustion, it is the atmosphere that furnishes the oxygen. . . . But since, in respiration, it is the substance of the animal itself, it is the blood which furnishes the combustible, if animals do not regularly repair by alimentation that which they lose by respiration the oil in the lamp will soon be wanting, and the animal will perish as the lamp will go out for the want of nourishment." Most physiologists have admitted Lavoisier's theory, and they consider respiration a slow combustion of the materials of the blood by the oxygen of the atmosphere, and as the source of animal heat We have just seen that this combustion takes place, not only in the lungs, but throughout the whole extent of the organs where the arterial blood carries the oxygen which presides over the phenomena of nutrition, that is, over the assimilation of the elements of which the blood is formed, and over the decomposition of some of these principles of which certain parts only remain in the system, while others return to be burned in the capillaries of the lungs, or to be exhaled in the form of gas or vapour of water.

Some authors again do not admit that combustion takes place in respiration, the phenomena of which, of quite a different order according to them, may be attributed to a reaction induced by the contact of organized substances. The decomposition and disassimilation of these are due to a series of acts of which very little is known, and which are compared to what has been termed by Berzelius "catalytic phenomena." But we confine ourselves to a mention only of this doctrine, which is not generally accepted.