or supplemental,
air. These several quantities are easily estimated. (See Practical Work.)
In the average individual the total capacity of the lungs (with the chest
in repose) is about one gallon. In forced inspirations this capacity may
be increased about one third, the excess being known as the _complemental_
air (Fig. 43).

[Fig. 44]

Fig. 44--*Diagram* illustrating internal respiration and its dependence on
external respiration. (Modified from Hall.) (See text.)

*Internal, or Cell, Respiration.*--The oxygen which enters the blood in the
lungs leaves it in the tissues, passing through the lymph into the cells
(Fig. 44). At the same time the carbon dioxide which is being formed at
the cells passes into the blood. An exchange of gases is thus taking place
between the cells and the blood, similar to that taking place between the
blood and the air. This exchange is known as _internal_, or cell,
respiration. By internal respiration the oxygen reaches the place where it
is to serve its purpose, and the carbon dioxide begins its movement toward
the exterior of the body. This "breathing by the cells" is, therefore,
_the final and essential act of respiration_. Breathing by the lungs is
simply the means by which the taking up of oxygen and *the* giving off of
carbon dioxide by the cells is made possible.

HYGIENE OF RESPIRATORY ORGANS

The liability of the lungs to attacks from such dread diseases as
consumption and pneumonia makes questions touching their hygiene of first
importance. Consumption does not as a rule attack sound lung tissue, but
usually has its beginning in some weak or enfeebled spot in the lungs
which has lost its "power of resistance." Though consumption is not
inherited, as some suppose, lung weaknesses may be transmitted from
parents to children. This, together with the fact, now generally
recognized, that consumption is contagious, accounts for the frequent
appearance of this disease in the same family. Consumption as well as
other respiratory affections can in the majority of cases be _prevented_,
and in many cases cured, by an intelligent observation of well-known laws
of health.

*Breathe through the Nostrils.*--Pure air and plenty of it is the main
condition in the hygiene of the lungs. One necessary provision for
obtaining _pure air_ is that of breathing through the nostrils. Air is the
carrier of dust particles and not infrequently of diseas