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.
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