layers, or coats, in their walls. The corresponding coats in the arteries
and veins are made up of similar materials, as follows:

1. _The inner coat_ consists of a delicate lining of flat cells resting
upon a thin layer of connective tissue. The inner coat is continuous with
the lining of the heart and provides a smooth surface over which the blood
glides with little friction.

2. _The middle coat_ consists mainly of non-striated, or involuntary,
muscular fibers. This coat is quite thin in the veins, but in the arteries
it is rather thick and strong.

3. _The outer coat_ is made up of a variety of connective tissue and is
also much thicker and stronger in the arteries than in the veins.

[Fig. 19]


Fig. 19—Artery dissected to show the coats.


Marked differences exist between the arteries and the veins, and these
vessels are readily distinguished from each other. The walls of the
arteries are much thicker and heavier than those of the veins (Fig. 19).
As a result these tubes stand open when empty, whereas the veins collapse.
The arteries also are highly elastic, while the veins are but slightly
elastic. On the other hand, many of the veins contain valves, formed by
folds in the inner coat (Fig. 20), while the arteries have no valves. The
blood flows more rapidly through the arteries than through the veins, the
difference being due to the fact that the system of veins has a greater