When the tension of oxygen in lymph is less than its tension in plasma, the former borrows from the latter. If the tension of carbonic acid in lymph is higher than in blood, it passes to the blood. The rapidly circulating blood at frequent intervals traverses the lungs. The whole blood of the body is exposed to air in the lungs once every minute. Oxygen tension being higher in pulmonary air than in venous blood, this gas is taken up. Carbonic acid tension being higher in venous blood than in pulmonary air, this gas escapes. The plasma in the capillary vessels which traverse the tissues exchanges gases with the lymph with very great rapidity. The specific gravity of blood varies from 1·056 to 1·059. The corpuscles are heavier than the plasma. Its reaction to test-paper is alkaline, owing to the presence of bicarbonate of soda and disodic phosphate. The alkalinity is greatest when the body is at rest; it is diminished by severe muscular exercise. Blood contains about 5,000,000 red corpuscles, and 7,000 or 8,000 leucocytes, to a cubic millimetre. Red blood-corpuscles are biconcave discs destitute of nucleus, and, so far as can be seen, devoid of any investing membrane. Seen in profile they appear biscuit-shaped, because the centre is hollowed out. Their largest diameter is 7·5 micromillimetres (¹/₃₂₀₀ inch)—a measurement of great importance to anyone who works with a microscope, because it serves as a standard by which to estimate the size of other objects. They are soft, but fairly tough and highly elastic. In circulating blood a corpuscle may occasionally be seen to catch on the point where two capillary vessels unite. It bends almost double under the pressure of the column of corpuscles behind it, and then springs forward. A red corpuscle is a vehicle for hæmoglobin. If blood is diluted with water, or if it is alternately frozen and thawed, the hæmoglobin separates from the corpuscles, which can then be seen as colourless discs. Hæmoglobin constitutes 40 per cent. of the weight of a moist corpuscle, or 95 per cent. of its weight after it has been dried. This is an enormous charge for a corpuscle to carry, and the question of how it carries it has been much discussed. It is not in a crystalline state. A corpuscle examined by polarized light is not doubly refractive. Microscopists know that if there were any crystals in the corpuscle it would appear bright on a dark ground when the Nicholl prisms are crossed. It cannot be in solution, since the water which the corpuscle contains would not suffice to dissolve it. It must be combined with some constituent of the corpuscle. But whether it is uniformly distributed throughout the disc, or in a semifluid form enclosed in spaces in a sponge-work; or whether the corpuscle is a hollow vesicle enclosing fluid hæmoglobin—a view which was long ago maintained, and has recently been revived—are questions which still await further evidence.