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. 471. The theory of Dr. Crawford is essentially the same, which supposes that venous blood contains a peculiar compound of carbon and hydrogen, termed _hydro-carbon_, the elements of which unite in the lungs with the oxygen of the air, forming water with the one and carbonic acid with the other. Mr. Cooper, for many years past, has taught the same doctrine in his lectures, without any knowledge of the fact that Crawford had suggested a similar modification of his theory.