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 µ) 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.