Illustration: FIG. 11.--A DIAGRAMMATIC VIEW OF THE BLOOD VESSELS. An
artery (_a_) opens into a system of capillaries, (_c_) and
after passing through these collects into a vein (_b_). Notice
that the capillaries connect with other vascular territories at
numerous points (_d_). If the artery (_a_) became closed the
capillaries which it supplies could be filled by blood coming from
other sources.]

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 aver