The venous blood as it issues from the right ventricle is loaded with carbonic acid. The oxygen present is insufficient to the whole of the hæmoglobin of the red corpuscles; much reduced hæmoglobin is present, hence the purple color of venous blood. As the blood-vessels pass through the capillaries of the lungs, this reduced hæmoglobin takes from the pulmonary air its complement of oxygen, all or nearly all the hæmoglobin of the red corpuscles becomes oxy-hæmoglobin, and the purple color forthwith shifts into scarlet. The hæmoglobin of arterial blood is saturated or nearly saturated with oxygen. Passing from the left ventricle to the capillaries, some of the oxy-hæmoglobin gives up its oxygen to the tissues, becomes reduced hæmoglobin, and the blood in consequence becomes once more venous, with a purple hue. Thus the red corpuscles by virtue of their hæmoglobin are emphatically oxygen-carriers. Undergoing no intrinsic change in itself, the hæmoglobin combines in the lungs with oxygen, which it carries to the tissues; these, more greedy of the oxygen than itself, rob it of its charge, and the reduced hæmoglobin hurries back to the lung in venous blood for another portion. Hæmoglobin combines loosely with carbonic oxide just as it does with oxygen, but the affinity with the former is greater than with the latter. While carbonic oxide readily turns out oxygen, oxygen cannot so readily turn out carbonic acid. This property of carbonic oxide explains its poisonous nature. Respiratory changes in the tissues. Arterial blood passing through the several tissues, becomes once more venous. A considerable quantity of the oxy-hæmoglobin becomes reduced, and a quantity of carbonic acid passes from the tissue into the blood. The blood which comes from a contracting muscle, is not only richer in carbonic acid, but also, though not to a corresponding amount, poorer in oxygen, than the blood which flows from a muscle at rest. A muscle is always producing carbonic acid, and when it contracts there is a sudden and extensive increase of the normal production. Oxygen is necessary for the life of the muscle. When venous blood instead of arterial blood is sent through the blood-vessel of a muscle, the irritability speedily disappears, and unless fresh oxygen be administered the muscle soon dies. Our knowledge of the respiratory changes in muscle is more complete than in the case of any other tissue; but we have no reason to suppose the phenomena of muscle are exceptional. On the contrary, all the available evidence goes to show that in all the tissues the oxidation takes place in the tissues and not in the adjoining blood. It is a remarkable fact, that lymph, serous fluid, bile, urine, and the other secretions contain no free or loosely combined oxygen, while the tension of carbonic acid in peritoneal fluid is as high as six per cent, and in bile and urine is still higher, etc.