the
systemic, or greater, by the left side.

As the blood is forced through the heart by forcible contractions of its
muscular walls, it has the action of a force pump, and gives the impulse
at each beat, which we call the pulse--the dilatation of the arteries
throughout the system. The contraction of the auricles is quickly
followed by that of the ventricles, and then a slight pause occurs; this
takes place in regular rhythmical order during health.

[Illustration: PLATE XX.

INTERIOR OF CHEST SHOWING POSITION OF HEART AND DIAPHRAGM.]

[Illustration: PLATE XXI.

Heart.

Right half, red.
Left half, blue.
Auricles, at upper end.
Ventricles, at lower end.
Arteries, red.
Veins, blue.

1. Left carotid artery.

2. Left jugular vein.

3. Portal system.

4. Vessels of the liver.

5. Arteries of the stomach. (Caeliac axis).

6. Vessels of the large intestine.

7. Vessels of the small intestine.

8. Artery of left kidney.

CIRCULATORY APPARATUS.]

The action of the heart is governed and maintained by the pneumogastric
nerve (tenth pair of cranial nerves); it is the inhibitory nerve of the
heart, and regulates, slows, and governs its action. When the nerve is
cut, the heartbeats increase rapidly, and, in fact, the organ works
without control. When the nerve is unduly irritated the holdback, or
inhibitory force, is increased, and the heart slows up in the same
measure. The left cavities of the heart, the pulmonary veins, and the
aorta, or systemic artery, contain red or florid blood, fit to circulate
through the body. The right cavities of the heart, with the venae cavae,
or systemic veins, the pulmonary artery, contain dark blood, which must
be transmitted through the lungs for renovation.

The arteries, commencing in two great trunks, the aorta and the
pulmonary artery, undergo division, as in the branching of a tree. Their
branches mostly come off at acute angles, and are commonly of uniform
diameter in each case, but successively diminish after and in
consequence of division, and in this manner gradually merge into the
capillary system of blood vessels. As a general rule, the combined area
of the branches is greater than that of the vessels from which they
emanate, and hence the collective capacity of the arterial system is
greatest at the capillary vessels. The same rule applies to the veins.
The effect of the division of the arteries is to make the blood move
more slowly along their branches to t