Such spaces would
seriously interfere with the circulation, since the heart pressure could
not then reach all parts of the blood stream. This is prevented by the
contracted state, or "tone," of the blood vessels, due to the muscular
coat.

In the second place, the muscular coat serves the purpose of _regulating_
the amount of blood which any given organ or part of the body receives.
This it does by varying the caliber of the arteries going to the organ in
question. To increase the blood supply, the muscular coat relaxes. The
arteries are then dilated by the blood pressure from within so as to let
through a larger quantity of blood. To diminish the supply, the muscle
contracts, making the caliber of the arteries less, so that less blood can
flow to this part of the body. Since the need of organs for blood varies
with their activity, the muscular coat serves in this way a very necessary
purpose.

[Fig. 21]

Fig. 21--Diagram of network of capillaries between a very small artery and
a very small vein. Shading indicates the change of color of the blood as
it passes through the capillaries. _S._ Places between capillaries
occupied by the cells.

*Capillaries.*--The capillaries consist of a network of minute blood
vessels which connect the terminations of the smallest arteries with the
beginnings of the smallest veins (Fig. 21). They have an average diameter
of less than one two-thousandth of an inch (12 mu) and an average length of
less than one twenty-fifth of an inch (1 millimeter). Their walls consist
of a single coat which is continuous with the lining of the arteries and
veins. This coat is formed of a single layer of thin, flat cells placed
edge to edge (Fig. 22). With a few exceptions, the capillaries are found
in great abundance in all parts of the body.

[Fig. 22]

Fig. 22--*Surface of capillary* highly magnified, showing its coat of thin
cells placed edge to edge.

*Functions of the Capillaries.*--On account of the thinness of their walls,
the capillaries are able to serve a twofold purpose in the body:

1. They admit materials into the blood vessels.

2. They allow materials to pass from the blood vessels to the surrounding
tissues.

When it is remembered that the blood, as blood, does not escape from the
blood vessels under normal conditions, the importance of the work of the