t nourishing food, and is made
to spend practically his entire time, including the sleeping hours, _out
of doors_. Not only is this done during the pleasant months of summer, but
also during the winter when the temperature is below freezing. Severe
exposure is prevented by overhead protection at night and by sufficient
clothing to keep the body warm. The abundant supply of pure, cold air
toughens the lungs and invigorates the entire body, thereby enabling it to
throw off the disease.

The success attending this method of treating consumptives suggests the
proper mode of strengthening lungs that are not diseased, but simply weak.
The person having weak lungs should spend as much time as he conveniently
can out of doors. He should provide the most ample ventilation at night
and have a sleeping room to himself. He should practice deep breathing
exercises and partake of a nourishing diet. While avoiding prolonged
chilling and other conditions liable to induce colds, he should take
advantage of every opportunity of exposing himself fully and freely to the
outside atmosphere.

*Summary.*--The purpose of respiration is to bring about an exchange of
gases between the body and the atmosphere. The organs employed for this
purpose, called the respiratory organs, are adapted to handling materials
in the _gaseous_ state, and are operated in accordance with principles
governing the movements of the atmosphere. By alternately increasing and
diminishing the thoracic space, air is made to pass between the outside
atmosphere and the interior of the lungs. Finding its way into the
smallest divisions of the lungs, called the alveoli, the air comes very
near a large surface of blood. By this means the carbon dioxide diffuses
out of the blood, and the free oxygen enters. Through the combined action
of the organs of respiration and the organs that move the blood and the
lymph, the cells in all parts of the body are enabled to exchange certain
gaseous materials with the outside atmosphere.

[Fig. 49]

Fig. 49--Model for demonstrating the lungs.

*Exercises.--*1. How does air entering the lungs differ in composition from
air leaving the lungs? What purposes of respiration are indicated by these
differences?

2. Name the divisions of the lungs.

3. Trace air from the outside atmosphere into the alveoli. Trace the blood
from the right ventricle to the alveoli and back again to the left
a