The Red Corpuscles consist of a firm stroma with a substance in solution, which is partly composed of the paraglobulin already mentioned, but principally of a coloured substance, haemoglobin, which is an albuminoid with the property of being crystallizable, the form of crystal varying in different animals. The colouring matter can be entirely separated from the albuminoid, but not without chemical change, the product obtained being termed insoluble hśmatin, a substance remarkably distinguished by yielding more than 12 per cent, pure oxide of iron when burned. Iron is known in medicine as a most powerful tonic in debility caused by impoverishment or loss of blood, and this is in some measure explained by the consideration that for the production of blood, it is an essential ingredient.

Haemoglobin is principally remarkable as the substance which gives to the blood its power of absorbing oxygen.

81. Blood contains in its composition an amount of gas, which, when liberated, is nearly equal to half the volume of liquid from which it has been set free. This gas can be extracted by means of the air pump, part of it easily, and the rest with the aid of warmth. It contains a small quantity of nitrogen, probably introduced in the lungs from the external air, in accordance with ordinary physical laws, and not of any physiological importance. But the great bulk of the gas consists of carbonic acid and oxygen, which vary in their proportion in different parts of the circulation; the carbonic acid being, however, always in much larger volume than the oxygen.

It has already been pointed out that throughout the body chemical changes are constantly taking place, in which oxygen combines with organic matters, and that carbonic acid is among the products. This oxygen is introduced in respiration, and is carried by the blood in the arteries to the textures; while the blood which returns thence by the veins carries with it, back to the lungs, the carbonic acid resulting from the processes of oxidation which have taken place throughout the body. The blood going to the textures, or what is ordinarily known as arterial blood, has therefore more oxygen in it than that which returns by the veins, and the venous blood has more carbonic acid than the arterial. There is, however, a considerable amount of oxygen left in venous blood, except when the animal is killed by asphyxia, that is to say, stoppage of respiration; and the amount of carbonic acid given off by the lungs, is only a small proportion of the total amount contained in the blood.

82. The difference in the gaseous contents of the blood going to the textures, and that which returns from them, is accompanied with a great difference of colour. When blood is allowed to flow from a vein, it comes in a stream as dark as claret, while the blood which comes from a superficial cut is much lighter, and what spouts from a wounded artery is of a bright scarlet. The dark blood from a vein, when spilt on the ground, becomes bright in a few minutes, exposure to the oxygen of the air sufficing to enable it to part with carbonic acid, and take up oxygen; and scarlet blood exposed to carbonic acid becomes dark. If a test tube be filled to about a fourth from the top with defibrinated blood, such as can be obtained by breaking down clot, and be shaken up a few times so as to enable the air to mix with it, it will become bright scarlet, and when allowed to stand for some time it will get dark again; when shaken a second time, it will again grow bright; and this experiment may be repeated on the same specimen of blood day after day. The same changes may be exhibited with a solution of the colouring matter of the red corpuscles; for the corpuscles are destroyed by addition of water, and their fluid contents are set loose, and this solution altera its colour on exposure to oxygen and carbonic acid alternately.

It appears, therefore, in the first place, that the difference in colour of dark and scarlet blood depends, at least partly, on a chemical change in the coloured contents of the corpuscles; and this agrees with the results of spectral analysis, by which it is proved that the colouring matter, or cruorin as it is sometimes called, if arterial blood, is a different chemical substance from that of venous blood (Stokes). In the second place, it appears that the haemoglobin is the oxygen-carrier in the. blood. Indeed, it is proved by direct experiment that serum has little more power of absorbing oxygen than water has. With regard to the carbonic acid of the blood, although no doubt a large portion of it is known to be contained in the serum, it seems probable, from the effect of that gas on the colouring matter, that the portion which is removed in respiration belongs to the corpuscles.

83. Before leaving this subject, it may be well to notice an exception to the general rule, that blood returning from the textures is dark—not only is that sent to the heart from the lungs scarlet, but the blood returning from certain glands in action is of the same tint. Thus, in experiments on dogs it has been found, that while the blood in the veins coming from the sub-maxillary gland is dark when the gland is at rest, if the nerve (chorda tympani) which supplies the secreting structure be excited, and the gland thus irritated to secrete saliva, a much larger quantity of blood passes through the gland, and it escapes from it scarlet. The blood returning from the kidneys is also scarlet as long as urine is secreted, but is dark when, from any cause, the secretion ceases. In both these instances, it will be observed, that an enormously larger amount of blood basses through the organ than is required for the nourishment of its textures. In the same way, if the blood-vessels of a rabbit's head are paralysed by dividing the sympathetic nerve in the neck, in consequence of the great increase of blood allowed into the part, a portion returns unaltered, and the blood is found red in the jugular vein.