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 which 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.