We may now enquire into the nature of the changes that take place in the blood from the time that it leaves the lungs in an aerated state, or in the condition of arterial blood, till it is returned to the heart in a venous condition after having traversed the various organs of the body. The results that have been obtained by those chemists who are the best qualified to deal with this difficult subject show that from 100 volumes of blood about 60 volumes of gas can be abstracted. The gases consist of carbon dioxide, oxygen, and a little nitrogen, the proportion differing considerably according to whether arterial or venous blood is examined. In both kinds of blood the carbon dioxide is in great excess. Thus in 100 volumes of arterial blood there are: oxygen 20, nitrogen 1 to 2, carbon dioxide 40 volumes. In 100 volumes of venous blood there are: oxygen 8 to 10, nitrogen 1 to 2, and carbon dioxide 46 volumes. The nitrogen may be dismissed from consideration at once, for it is known that it presents no special affinity for any of the constituents of the blood, that it is absorbed as water would absorb it, and that its volume consequently remains unchanged. It is different with the other two gases. The oxygen introduced into the lungs by the process of inspiration rapidly diffuses through the delicate walls of the air-cells and the blood-vessels around them, and at once enters into a feeble chemical combination with the haemoglobin contained in the red blood corpuscles. By these it is distributed through the system, and in less than ten seconds reaches the capillary vessels, where the blood is brought into close relation but not in actual contact with the tissues. These have a stronger affinity for oxygen than the haemoglobin of the blood; it therefore leaves the red corpuscles, and, passing through the thin walls of the blood and lymph vessels, finally attacks or is seized upon by the tissues, muscle or nerve or gland, as the case may be.

The quantity of carbon dioxide that is generated in the tissues, to be taken up by the plasma of the blood and to be discharged from the body at the lungs, is dependent upon many circumstances. To liberate the force by which such an animal as the horse is able to accomplish its wonderful feats of locomotion, often under the disadvantage of having to drag a weighty load or to carry a heavy rider, something must be oxidized. At first sight it might reasonably be supposed that this something is the substance of the muscle itself, and this was long supposed to be the true explanation; but if this were the case, then nitrogen, which is an essential constituent of muscle proteid, ought to appear in the excreta after exertion in augmented quantity, either free or combined, as one of the products of the disintegration of that tissue. Careful chemical research, however, made both upon man and animals shows that exercise, even when violent and prolonged, does not materially increase the discharge of urea from the kidney, which is the chief channel by which nitrogen leaves the body. Hence the conclusion has been arrived at, that just as locomotion is effected in a steam-engine, with little wear-and-tear of the machine itself, the force being derived from the latent energy stored up in the fuel, so in the case of muscle the tissue itself is only the machine which utilizes the force set free by the combustion of some organic substance within it in which carbon is predominant. This substance, there is reason to believe, is glycogen, which is present in muscle at rest, but disappears after exercise, a substance that is in constant course of formation by the liver, and is stored up in the cells of that organ until it is again used up either by long fasting or after prolonged muscular exertion, and which, lastly, is a compound containing much carbon, whilst the oxygen and hydrogen in its composition are in the proportions to form water. During exercise more blood traverses the muscles, and therefore more oxygen is brought to them, the glycogen they contain is oxidized, carbon dioxide is formed, and water set free. The carbon dioxide is carried away by the blood, and is discharged partly by the lungs and partly by the skin; the water passes off by the kidneys. But other circumstances besides exercise affect the production of carbon dioxide. The quantity thrown off by the body is always increased after food. It is increased when the temperature of the surrounding medium is lowered, for since the temperature of the body is nearly 100° F., the oxidation of oil, of glycogen, and of proteids is necessary in order that it should be maintained. When, therefore, the external temperature falls, more of these substances must be ingested and oxidized to develop the required heat. The culmination of this is seen in the Esquimaux, who keep up their temperature during the severity of an Arctic winter by consuming large quantities of oily substances, the oxidation of which produces more heat in proportion to lbs. in weight than any other kinds of food.

In warm climates the amount of heat required to maintain that of the body is comparatively small; hence the propriety of reducing the quantity of food supplied under these circumstances, and using the carbohydrates rather than the hydrocarbons.

Young animals eliminate more carbon dioxide than old ones in proportion to their weight, their tissues being more watery, disintegrating more rapidly, and being more amenable to oxidation. Females, on the other hand, making, as a rule, little muscular effort, give off less than males of the same age and weight.

The proportion of carbon dioxide that is eliminated, as compared with the quantity absorbed, is termed the respiratory quotient, and in the horse is CO2/O = 0.9, whilst in carnivora the proportion is CO2/O = 077.

The amount of water that is expelled from the lungs during respiration is considerable. The average quantity of water contained in air at mean temperature and pressure is about 1 to 1.5 per cent, but the air which is expired is raised to a temperature only a degree or two below 100° F., and it is saturated, containing therefore about 7 per cent of water vapour. It has been estimated that in man about 600 litres of water vapour, weighing 720 grammes, are given off daily, and the quantity given off by a horse may be taken at at least five times that amount.