ch is a thin
sheet of blood, while the other side is in such contact with the air that
an interchange of gases can readily take place. The demand for oxygen is,
however, so incessant, and the accumulation of carbon dioxid is so rapid
in every tissue of the human body, that an All-Wise Creator has provided a
most perfect but complicated set of machinery to effect this wonderful
purification of the blood.

We are now ready to begin the study of the arrangement and working of the
respiratory apparatus. With its consideration, we complete our view of the
sources of supply to the blood, and begin our study of its purification.

[Illustration: Fig. 84.--The Epiglottis.]

203. The Trachea, or Windpipe. If we look into the mouth of a friend,
or into our own with a mirror, we see at the back part an arch which is
the boundary line of the mouth proper. There is just behind this a similar
limit for the back part of the nostrils. The funnel-shaped cavity beyond,
into which both the mouth and the posterior nasal passages open, is
called the pharynx. In its lower part are two openings; the
trachea, or windpipe, in front, and the oesophagus behind.

The trachea is surmounted by a box-like structure of cartilage, about
four and one-half inches long, called the larynx. The upper end of
the larynx opens into the pharynx or throat, and is provided with a lid,--
the epiglottis,--which closes under certain circumstances (secs. 137
and 349). The larynx contains the organ of voice, and is more fully
described in Chapter XII.

The continuation of the larynx is the trachea, a tube about three-fourths
of an inch in diameter, and about four inches long. It extends downwards
along the middle line of the neck, where it may readily be felt in front,
below the Adam's apple.

[Illustration: Fig. 85.--Larynx, Trachea, and the Bronchi. (Front view.)

A, epiglottis;
B, thyroid cartilage;
C, cricoid-thyroid membrane, connecting with the cricoid cartilage below,
all forming the larynx;
D, one of the rings of the trachea.
]

The walls of the windpipe are strengthened by a series of cartilaginous
rings, each somewhat the shape of a horseshoe or like the letter C, being
incomplete behind, where they come in contact with the oesophagus.
Thus the trachea, while always open for the passage of air, admits of the
distention of the food-passage.

204. The Bronchial Tubes. The lower end of the windpipe is just
behind the upper part of the s