strain out from the fluid substances which might work great injury if
they passed into the blood. Between the capillary vessels and the
lymphatics is the tissue fluid, in which all the exchange takes place.
It is constantly added to by the blood, and returns fluid to the blood
and lymph; it gives material to the cells and receives material from
them.

[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.