rly imitated by the words "lubb, dup."
While the cause of the first sound is not fully understood, most
authorities believe it to be due to the contraction of the heart muscle
and the sudden tension on the valve flaps. The second sound is due to the
closing of the semilunar valves. These sounds are easily heard by placing
an ear against the chest wall. They are of great value to the physician in
determining the condition of the heart.

*Arteries and Veins.*--These form two systems of tubes which reach from the
heart to all parts of the body. The arteries receive blood from the heart
and distribute it to the capillaries. The veins receive the blood from the
capillaries and return it to the heart. The arteries and veins are similar
in structure, both having the form of tubes and both having three distinct
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
capacity than the system of arteries.

[Fig. 20]

Fig. 20--Vein split open to show the valves.

*Why the Arteries are Elastic.*--Th