_Composition._—The structural composition of the blood, and the relation of its several constituents to each other, is best studied under the microscope. A thin transparent membrane in which blood is circulating through small vessels—the web between the toes of a frog’s foot, the mesentery, the membrane of a bat’s ear—affords an opportunity of observing blood in circulation. In any of the smaller vessels, whether artery or vein, a column of red corpuscles is seen moving in the axis of the stream. This column is surrounded by a layer of clear plasma. Amongst the red corpuscles a few leucocytes may be detected floating placidly down the current. Others are seen in the peripheral layer of plasma, tending to creep along the wall of the vessel rather than submit to be moved forward, as passive objects, by the current. If an irritant be applied to the membrane, the vessels dilate; yet, notwithstanding their wider calibre, the current becomes slower. The red corpuscles mass together. Apparently their constitution is slightly altered by this commencing inflammation, in such a manner that they cease to be clean, independent discs which slide past each other like small boats on a river; they exhibit a tendency to stick one to another. In the capillary vessels leucocytes may now be observed, not merely creeping along the inner surface of the endothelium, but squeezing themselves between its scales; making their way out of the vessel into the tissue-spaces through which the vessel passes. Such an observation gives the clue to the functions of the several constituents of the blood. The red corpuscles carry oxygen in chemical combination with their colouring matter. From them it passes into solution in the plasma; from the plasma through the walls of the capillary vessels into lymph; the tissues take it from the lymph as they require it. As fast as it is removed from lymph it is renewed from plasma. Carbonic acid excreted by tissue cells is dissolved in lymph. From lymph it is transferred to plasma. The reception of carbonic acid by these fluids is not quite so simple as the transference of oxygen from blood to lymph. It is aided by the presence of alkaline carbonates which are always ready to form “acid” salts: not acid to litmus-paper—the blood is always alkaline—but containing more than one unit of acid to one of base. Sodic carbonate has the formula Na₂CO₃. With an additional molecule of carbonic acid it becomes Na₂CO₃CO₂(HO)—bicarbonate. When in solution it can hold still more carbonic acid. If carbonic acid were merely dissolved in lymph and plasma, it would be impossible for the blood to carry it away with sufficient rapidity; just as it would be impossible for blood to bring sufficient oxygen were it not for the colouring matter (hæmoglobin) which forms a temporary, easily divorced union with it. But from a physical point of view it comes to the same thing. As the tension of oxygen in plasma falls, it dissolves more from the hæmoglobin.