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Fig. 4.—Red Blood-Corpuscles presenting, some the Surfaces, others the Edges, of their Discs, together with Single Representatives of Four Types of Leucocyte.

A, the most common type, highly amœboid and phagocytic. Its protoplasm is finely granular, its nucleus multipartite. B, a leucocyte closely similar to the last, but larger, and containing an undivided nucleus. It is shown with a cluster of particles of soot in its body-substance. C, a young leucocyte, or “lymphocyte.” D, a coarsely granular leucocyte. Its granules stain brightly with acid dyes—e.g., eosin or acid fuchsin.

Na₂CO₃Na₂CO₃CO₂(HO)

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.