individual, are controlled by that part of the brain called the
_medulla oblongata_. The respiratory, or breathing process, is not
instituted for the benefit of man alone, for we find it both in the
lower order of animals and in plant life. Nature is very economical in
the arrangement of her plans, since the carbonic acid, which is useless
to man, is indispensable to the existence of plants, and the oxygen,
rejected by them, is appropriated to his use. In the lower order of
animals, the respiratory act is similar to that of the higher types,
though not so complex; for there are no organs of respiration, as the
lungs and gills are called. Thus, the higher the animal type, the more
complex its organism. The effect of air upon the color of the blood is
very noticeable. If a quantity be drawn from the body, thus being
brought into contact with the air, its color gradually changes to a
brighter hue. There is a marked difference between the properties of the
venous and the arterial blood.

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
the food are retained to promote the growth of the body, those which are
useless or injurious are in various ways expelled. There are, perhaps,
few parts of the body more actively concerned in this removal than the
skin.

[Illustration: Fig. 46: An ideal
view of the papillae. 1, 1.