bstacle
to the flow has thus been greatly lessened, and the water flows out in
intermittent jets to correspond to the compression of the bulb.

Chapter VIII.

Respiration.

202. Nature and Object of Respiration. The blood, as we have learned,
not only provides material for the growth and activity of all the tissues
of the body, but also serves as a means of removing from them the products
of their activity. These are waste products, which if allowed to remain,
would impair the health of the tissues. Thus the blood becomes
impoverished both by the addition of waste material, and from the loss of
its nutritive matter.

We have shown, in the preceding chapter, how the blood carries to the
tissues the nourishment it has absorbed from the food. We have now to
consider a new source of nourishment to the blood, _viz._, that which it
receives from the oxygen of the air. We are also to learn one of the
methods by which the blood gets rid of poisonous waste matters. In brief,
we are to study the set of processes known as respiration, by which
oxygen is supplied to the various tissues, and by which the principal
waste matters, or chief products of oxidation, are removed.

Now, the tissues are continually feeding on the life-giving oxygen, and at
the same time are continually producing carbon dioxid and other waste
products. In fact, the life of the tissues is dependent upon a continual
succession of oxidations and deoxidations. When the blood leaves the
tissues, it is poorer in oxygen, is burdened with carbon dioxid, and has
had its color changed from bright scarlet to purple red. This is the
change from the arterial to venous conditions which has been described in
the preceding chapter.

Now, as we have seen, the change from venous to arterial blood occurs in
the capillaries of the lungs, the only means of communication between the
pulmonary arteries and the pulmonary veins. The blood in the pulmonary
capillaries is separated from the air only by a delicate tissue formed of
its own wall and the pulmonary membrane. Hence a gaseous interchange,
the essential step in respiration, very readily takes place between the
blood and the air, by which the latter gains moisture and carbon dioxid,
and loses its oxygen. These changes in the lungs also restore to the dark
blood its rosy tint.

The only condition absolutely necessary to the purification of the blood
is an organ having a delicate membrane, on one side of whi