We have followed the course of the blood as it is sent out from the heart by the great main artery-the aorta; we have seen it as it flows through the capillaries become polluted with carbonic acid, water, and other products of the oxidation; we have traced this polluted stream backto the heart, and then on to the lungs; and we know that it returns from the lungs cleansed and purified, and re- invigorated with oxygen. Our next business is to find out how this change takes place in the lungs. We must of course begin by examining the lungs themselves.
The lungs are the two large organs which, together with the heart, fill the entire cavity of the thorax or chest. This cavity is a sort of conical or beehive-shaped box, havingthe spinal column, the ribs,
and the breast-bone for its side walls, and a stout, strong, fiat muscle-the diaphragm-for its floor. It is a perfectly air-tight chamber, and both itsfloor and its walls are movable.
The ribs are attached to the vertebral column by joints, which allow them a limited up-and-down movement, and instead of passing horizontally round the body, they all slant downwards from their junction with the column behind. It is easy to see that with such an arrangement the up movement of the ribs must have the effect of enlarging the chamber, while their downward movement must have the opposite effect.
The lungs themselves are soft, spongy, elastic, and of a pinkish-gray color. They are securely attached both to the diaphragm (the floor) and to the sides of the thorax; so that there is absolutely no space between them and the walls of the chamber in which they work.
The consequence is that when the walls and floor of the thorax move, and so enlarge or diminish the size ofthe chamber, the lungs must expand or contract with them in order to accommodate themselves to the space provided.
The back of the mouth and the nasal passages open into a large cavity, the pharynx, and from the lower part of this a long, stout tube, the windpipe, extends downwards into the chest. This pipe, which is formed of stout rings of gristle, can be felt. The rings make it hard and resisting to the touch, and prevent it from collapsing with pressure. After entering the chest the windpipe branches into two-the bronchi; one bronchus going to the right, theother to the left lung.
In the lungs these bronchi divide and subdivide again and again, forming bronchial tubes, and end at la st in extremely fine branches whichspread themselves all through the substance of the organ. In fact theyactually form the substance of the lung itself, for each bronchial tube ends in a bunch of little, hollow, elastic bladders, called air-cells, and it is the mass of these air- cells which make up the entire lung. It is because each little air-cell itself is very elastic, that the whole lung has the power of expanding and contracting.
The lungs expand to take in more air; they contract to drive the air out. The air, inhaled at the mouth andnostrils, passes along the windpipe, bronchi, and bronchial tubes till it reaches the air-cells of the lungs. Let us now leave the air-passages and air-cells, and turn our attention to the great artery which brings the blood to the lungs to be cleansed.
This, the pulmonary artery, after leaving the right ventricle of the heart, divides into two branches-one for each lung. In the lung each branch divides again and again, until at last it forms the most delicate capillaries. These capillaries spread themselves through every nookand corner of the substance of the lung.
But as we have seen, the substance of the lung is really the thin, delicate, bladder-like walls of the air-cells, and it is round these little air-cells that the finest branches of the pulmonary capillaries spread themselves. These little vessels, like all other capillaries, eventually re-unite into tiny veinlets, and they in their turn continue to do the same, until at length they form four main pipes, the pulmonary veins (two from each lung) which proceed to the left auricle of the heart.
It is important to remember that the pulmonary arteries are the only arteries in the body which carry venous blood, and that it is this dark purple blood which flows through the capillaries of the lungs. In those delicate vessels it is separated from the air in the air-cells by the finestpossible membrane, and here the exchange takes place.
Oxygen from the air in the air-cells passes into the blood,and carbonic acid and water from the blood finds its way into the air-cells in place of it.
The double exchange takes place through the actualmembrane by osmosis. We have already explained and illustrated this passage of fluids through a membrane in one of our earlier lessons.
It is in this way that the blood is purified, and re- invigorated with oxygen; and when at length it is collected up by the pulmonary veins to be carried by them back to the heart, it is pure, bright, arterial blood. These are the only veins in the body that carry arterial blood.
