In addition to the strength and elasticity of the wall of the arteries, which enables them to resist the pressure of the blood, they have the power of varying their calibre by the contraction or expansion of their muscular walls.  Many of the organs of the body function discontinuously, periods of activity alternating with comparative repose; during the period of activity a greater blood supply is demanded, and is furnished by relaxation of the muscle fibres which allows the calibre to increase, and with this the blood flow becomes greater in amount.  Each part of the body regulates its supply of blood, the regulation being effected by means of nerves which control the tension of the muscle fibres.  The circulation may be compared with an irrigation system in which the water supply of each particular field is regulated not by the engineer, but by an automatic device connected with the growing crop and responding to its demands.

[Illustration:  FIG. 12.—­THE VARIOUS CELLS IN THE BLOOD. (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.  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.