IX. THE VESSELS. The aorta or artery from the heart, is about an inch in bore in us, and over a yard in the whale. After giving off branches for the head and arms it curves to the back of the chest, supplies the organs in the trunk, and ends at the loins in two branches for the haunches and legs. Such arteries break into smaller and smaller ones, which often join in loops, so that if one vessel is blocked at a point the blood may reach it by others joining lower down. How freely they join in this way may be seen from this picture of the neck and head. The heart throws the blood into arteries with a force equal to 3lbs. weight, yet the course of the blood is promoted by their being elastic. Being stretched they react on the blood and keep a steady flow like that of the fire-engine hose, which is made of India-rubber. Still, scarlet blood jets from a wounded artery, and unless some one has the presence of mind to press a finger on it, the loss may be fatal. Sudden strains of the limbs may crack arteries, and lead to swellings on them, which are called aneurisms. Capillaries are named after hairs, but they are fifty times finer, many only letting through a single blood cell. So closely woven are they that you cannot stick in a pin without wounding some. In the frog's web (here shown), the minnow's tail, or the bat's wing, which are transparent, you can see the wonderful circulation. of the blood. The cells rush in by the artery, pass slowly through the capillaries, staying to nourish the tissues and take their waste, and crowd away by the vein. This exchange sucks on the blood, and is the only force for moving the sap of plants. In animals the flow is controlled by the brain and nerves. If you anger a turkey-cock how full of blood his comb becomes. Veins collect the blood from the capillaries, and the smaller run together, until they make but two, which open into the heart. They are thin, knotted from valves, and often near the skin, as you can see on your hand as it hangs near the fire. Muscles when acting press onwards the blood of veins. Too much standing often swells and spoils the valves of the veins in the leg; if they burst the bleeding is fierce. Raising the leg and pressing on the finger will, however, stay it. A deep breath helps the flow in the veins, and if one is wounded it may suck in air with fatal effects. Health and life depend so much on the due motions of the blood that we should never check them. A tight necktie, or holding the breath, during a long violent effort, has caused many a fit, for thus the vessels of the brain easily burst. Many a lady has got a weak heart, fainting fits, short breathing, a big liver, a waist and a temper like a wasp, or, what she might think worse of, a red nose, from tight stays. X. THE LUNGS. If we were to stop breathing for five minutes we should die; this act then clearly concerns life, and as clearly concerns health, if we breathe too little or impure air. The mouth, or better the nose, for then it is warmed, lets in air which passes to the wind-pipe. This, shown on page 33, is a flexible tube six inches long, and nearly one wide, and it ends in a branch The for the root or fixed part of each lung. From thence the tube breaks in twos and twos till they get as fine as hairs. air-cells are pits leading off these final tubes, and round them and between them are the capillaries with blood to be aired. The space around the air-cells is filled with spongy tissue, and the lung thus made up is clothed with a smooth membrane, the pleura. Another part of the pleura lines the chest, so that the play of the lungs within it is made easy. The air passages are lined by mucous membrane, which is often the seat of colds and bronchitis. About 150 millions of little hair-like bodies on its surface waft the air to and fro, and expel dust as well as the slimy matter we cough up. The figure on page 33 will give some idea of the lungs, and those of the sheep, the lights, may be easily studied. They are pink, however, while ours get grey from dust and smoke in the air. Once lungs have breathed they get so light and springy that nothing can squeeze out air, and they always float on water. We draw in air by the midriff or diaphragm, a fleshy partition between the chest and belly, going down and by the ribs, which are slanting, being raised and turned out. The chest being then enlarged the outer air rushes in to fill the space. To fill the whole lungs, which |
