The blood consists of a fluid, the blood plasma, in which numerous cells are contained. The most numerous of these are small cup-shaped cells which contain a substance called _haemoglobin_, to which the red color of the blood is due. There are five million of these cells in a cubic millimeter (a millimeter is .03937 of an inch), giving a total number for the average adult of twenty-five trillion. The surface area of all these, each being one thirty-three hundredth of an inch in diameter, is about thirty-three hundred square yards. The haemoglobin which they contain combines in the lungs with the oxygen in the inspired air, and they give up this indispensable substance to the cells everywhere in the body. There are also eight thousand leucocytes or colorless cells in a cubic millimeter of blood, this giving a total number of four billion in the average adult, and these vary in character and in relative numbers (Fig. 12). The most numerous of these are round and slightly larger than the red cells; they have a nucleus of peculiar shape and contain granules of a definite character. These cells serve an important part in infectious diseases in devouring and destroying parasites. They have power of active independent motion and somewhat resemble certain of the free living unicellular organisms. The blood plasma, when taken from the vessels, clots or passes from a fluid into a gelatinous or semi-solid condition, which is due to the formation within it of a network of fine threads termed fibrin. It is by means of the clotting of the blood that the escape of blood from ruptured vessels is arrested.