rist as the _pulse_.

On inspection, between the fifth and sixth ribs on the left side of the
chest, a movement is perceptible, and, if the hand be applied, the
impulse may be felt. This is known as the throbbing, or beating of the
heart.

If the ear is placed over the region of the heart, certain sounds are
heard, which recur with great regularity. First is heard a comparatively
long, dull sound, then a short, sharp sound, then a pause, and then the
long, dull sound again. The first sound is caused mainly by the
tricuspid and mitral valves, and the second is the result of sudden
closure of the semilunar valves.

No language can adequately describe the beauty of the circulatory
system. The constant vital flow through the larger vessels, and the
incessant activity of those so minute that they are almost
imperceptible, fully illustrate the perfectness of the mechanism of the
human body, and the wisdom and goodness of Him who is its author.

Experiments have shown that the small arteries may be directly
influenced through the nervous system, which regulates their caliber by
controlling the state of contraction of their muscular walls. The effect
of this influence of the nervous system enables it to control the
circulation over certain areas; and, notwithstanding the force of the
heart and the state of the blood-vessels in general, to materially
modify the circulation in different spots. Blushing, which is simply a
local modification of the circulation, is effected in this way. Some
emotion takes possession of the mind, and the action of the nerves,
which ordinarily keep up a moderate contraction of the muscular coats of
the arteries, is lost, and the vessels relax and become distended with
arterial blood, which is a warm and bright red fluid; thereupon a
burning sensation is felt, and the skin grows red, the degree of the
blush depending upon the intensity of the emotion.

The pallor produced by fright and by extreme anxiety, is purely the
result of a local modification of the circulation, brought about by an
over-stimulation of the nerves which supply the small arteries, causing
them to contract, and to thus cut off more or less completely the supply
of blood.

* * * * *

CHAPTER VIII.

PHYSIOLOGICAL ANATOMY.

THE ORGANS OF RESPIRATION.

THE ORGANS OF RESPIRATION are the Trachea, or windpipe, the Bronchia,
formed by the subdivision of the trachea, and the Lungs, with the