The venous blood is carried, as we have previously described, to the
right side of the heart and to the lungs, where it is converted into
arterial blood. It is now of uniform quality, ready to be distributed
throughout the body, and capable of sustaining life and nourishing the
tissues. Man breathes by means of lungs; but who can understand their
wonderful mechanism, so perfect in all its parts? Though every organ is
subservient to another, yet each has its own office to perform. The
minute air-cells are for the aeration of the blood; the larger bronchial
tubes ramify the lungs, and suffuse them with air; the trachea serves as
a passage for the air to and from the lungs, while at its upper
extremity is the larynx, which has been fitly called the organ of the
human voice. At its extremity we find a sort of shield, called the
_epiglottis_, the office of which is supposed to be to prevent the
intrusion of foreign bodies.

* * * * *




CHAPTER IX.

PHYSIOLOGICAL ANATOMY

THE SKIN.


Through digestion and respiration, the blood is continually supplied
with material for its renewal; and, while the nutritive constituents of