(_a_) The red blood cells, single and forming a roll by adhering to one another; (_b_) different forms of the white blood cells; those marked "1" are the most numerous and are phagocytic for bacteria.] 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.