human beings, they train the blood of animals to meet it; and, keeping their serum in stock, they can, when the critical moment arrives, reinforce the fighting strength of the patient with this mercenary aid. =The Spleen.=—The spleen is placed on the left side of the body, and rather towards the back. It rests between the stomach and the inner surface of the eighth, ninth, tenth, and eleventh ribs. It is quickly distinguished from other organs by its brown-purple colour, a sombre hue to which it owed its evil reputation with the humoralists. The liver’s yellow bile tinged man’s mental outlook, preventing him from seeing objects in their natural brightness; but the spleen made black bile, which, mounting to the brain, displayed its malign influence upon the action of that organ, as, or in, the worst of humours. The spleen is invested with a capsule of no great toughness. Inside the capsule is “spleen-pulp.” When the fresh organ is cut across, it is seen that, although most of the pulp is of the colour of dark venous blood, it is mottled with light patches. In some animals—the cat, for example—these whitish patches are small round spots, regularly arranged at a certain distance from the capsule. The distinction into “red pulp” and “white pulp” marks a division into two kinds of tissue with entirely different functions. The white pulp is lymphoid tissue, lymph-follicles developed in the outer or connective-tissue coat of the branches of the splenic artery. Its function is to make lymphocytes, of which, for reasons which will shortly appear, the spleen needs an abundant supply. The constitution of the red pulp is entirely different, and peculiar to the spleen. The branches of the splenic artery divide in the usual way into smaller and still smaller twigs until the finest arterioles are reached; but these arterioles do not give rise to capillary vessels. At the point at which in any other organ their branches would attain the calibre of capillaries, the connective-tissue cells which make their walls scatter into a reticulum. They are no longer tiles with closely fitting, sinuous, dovetailed borders, but stellate cells with long delicate processes uniting to constitute a network. The blood which the arterioles bring to the pulp is not conducted by closed capillary vessels across the pulp to the commencing splenic veins. It falls into the general sponge-work. The venules commence exactly in the same way as the arterioles end. Stellate connective-tissue cells become flat tiles placed edge to edge. The endothelium of an arteriole might be likened to a column of men marching shoulder to shoulder, three or four abreast; the connective tissue of the pulp, to a crowd in an open place. The column breaks up into a crowd. On the other side the crowd falls into rank as the endothelium of veins. The capsule and the red pulp are largely composed of muscle-fibres. These relax and contract about once a minute. By their contraction the blood is squeezed out of