your windpipe, and is carried by two pipes, by one to the right and by the other to the left lung. These two pipes branch out into very small hair-like tubes, and carry the air into the little holes or cells in the sponge of the lungs. So there are an immense number of cells filled with air, and surrounded by tubes of blood, only separated from this air by the finest possible skin. The air contains a great deal of oxygen gas, and the blood a great deal of carbonic gas (which gives it that dark colour). These two gases cannot come so close together without rushing towards one another; so the carbonic gas rushes through the skin of its tube to get to the oxygen, and the oxygen rushes through the skin to get to the carbonic gas, and in their haste they rush past one another. The carbonic gas fills the air-cells, and the oxygen is caught by the blood, gets mixed with it, and is retained in the veins. This has all happened in a very little part of a second, and we breathe out again, sending the carbon out of our mouths into the air outside, and it is got rid of from the body, because it poisons it. The oxygen has quite changed the colour of the blood, which is now a bright red; but it has also changed its nature. It was dead matter (for we do not eat live flesh), dead and flat, like milk; it is now lifegiving, sparkling, and brisk, like ginger-beer, and in that state it returns to the heart. But we must now see what the heart is like, for it is the pump that keeps the blood moving all over the body. The heart of each person is about the size of his own closed fist. Every one knows the shape of a heart, and a bullock's or sheep's heart, lungs, and liver may be seen any day at a butcher's shop. But now let us see what it is like inside. Cut an orange in half, and you will see there is a white partition running across the middle, which divides each half into two portions, separating one from the other. The heart is very like that; there is a partition between the upper and lower half, only there are holes in each partition fitted with valves, so as to let the blood fall from the upper half to the lower, but which close, so that it cannot 2 Fig. 12. 5 3 4. return from the lower to the upper one. Paste a piece of thick drawing paper over each half of the orange, and then fit the two halves together and you will have a very good idea of a heart. These two thicknesses of drawing paper will make a wall between the two halves of the heart, so that nothing can pass from one half to the other. Blood can run from 1 to 2, and from 3 to 4, but none can go from either 1 or 2 into either 3 or 4; though if we were to get a curved pipe and put one end into 2, and the other end into 3, as is done with the pipe 5, we should then be able to get a flow of blood from 2 to 3. This is very nearly what is done in the case of the heart. A pipe, like 5, carries the blood out of 2 up into the lungs, into which it empties the blood, as we have seen, by a very great number of very little pipes, and then a great many others suck the blood up again, pour it into one large pipe, which conveys it into 3. Now let us put the two halves together again, turning the cut edge outwards, and we have a very good representation of the heart in its position. Insert a quill through the rind into 1, and we have the great vein that brings Fig. 13. the blood from the body into the heart. As soon as it has filled the cavity of 1, that cavity (which is called the right auricle, because it is covered with a piece of flesh, like one of your ears), begins to squeeze itself together, just as you do your hand when you double your fist. Put a sponge full of water into your hand and squeeze it. What happens? Why all the water runs out between your fingers. It is just so with the heart, when it squeezes the blood inside it, the blood is forced out through little holes in the wall between it and 2, pushing the valves open, until it has all run into 2, and filled it. Then 1 begins to open itself again, ready to receive some more blood, and 2 begins to squeeze itself, forcing the blood up a pipe (called the pulmonary or lung artery) out of one side of 2, like the pipe 5, (Fig. 12) into the lungs, from which 4 AORTA it is carried by the pulmonary vein to the other side of the heart, 3. As soon as 3 is full, it squeezes the blood into 4, just as 1 squeezed it into 2, and 4 squeezes it into another pipe from the bottom of it. This pipe is called the aörta, and carries the blood up towards the head for a little way, and then turns round, dividing itself into several branches, one going to each arm, one to each side of the head, and the main pipe going to the lower parts of the body. Let us now follow the course of the blood. Starting from the right auricle, No. 1, which we will suppose to be empty, blood is poured into it from the great veins, (lower vena cava and upper vena cava), the former of which brings back the worn out blue blood filled with carbonic acid, and the latter a mixture of waste blood, and the new chyle from the thoracic duct. Fig. 14. Septum Ventriculorum. Aorta descending. Fig. 15.--Theoretical Section of the Human Heart, seen from the front. As soon as the right auricle is full, it begins to squeeze itself, or contract, forcing the blood down into the right ventricle (so called, because it is shaped some thing like a man's stomach or belly). When this ventricle is filled, it contracts and forces up the blood into the right and left pulmonary arteries, and so into the lungs, where it is distributed by small capillaries, is purified by the air, and being sucked up again by similar capillaries, poured by the right and left pulmonary veins into the left auricle, which contracts in the same way as the right auricle did, forces the blood down into the left ventricle, which in its turn forces it up into the aorta. But why does not the blood at each contraction of an auricle or ventricle run back through the opening by Fig. 16. The Top of Heart, the Auricles being dissected off. which it entered? Because each of those little holes only opens forwards in the direction the blood is to take, like the valve of a pump. Suppose a reservoir were to burst and send all its water in a great bulk against the front door of a house which was left a little open, it would pour with a great flood into the house, dashing the door open against the passage wall. But |