The reason is, that the grass which they eat is not at all like their flesh. It must therefore go through a great change to fit it to make the blood and flesh of such animals. And this cannot be done without considerable machinery. The flesh-eating lion has a single stomach, and the length of its intestines is only three times that of its body. But the grass-eating sheep has really four stomachs, and the length of its intestines is twenty-eight times that of its body. Fig. 16 represents the four stomachs of the sheep. In man FIG. 16. Esophagus Orifice of 3d Stomach Intestine... Pylorus. 4th Stom. 2d St 1st Stom. there is but one stomach, and the length of his întestines is about six times the length of the body. 25. In birds that eat grains and seeds there is a peculiar arrangement of the digestive machinery. They have no teeth, and their mill for grinding their food, instead of being in the mouth, is in the stomach. The gizzard, which is the stomach, is truly a mill for crushing the food to pieces. It has on the inside two How long are the intestines in the lion? In the sheep? In man! How many stomachs has the sheep? very hard surfaces, which are rubbed and pressed together by stout muscles. The grain is thus broken up just as it is done between two mill stones. While this is going on the gastric juice comes down from above, and dissolves and digests the broken grain. This arrangement is seen in Fig. 17, which represents What is there peculiar in the digestive machinery of grain-eating birds! the stomach of a turkey. At b is the gizzard cut open, showing the two hard grinding surfaces, and at a above is the part from which oozes the gastric juice. In those birds that live on flesh or fish there is no such grinding machinery, but the stomach is a thin bag, just as it is in all animals that live on such food. CIRCULATION OF THE BLOOD. 1. In the last chapter you saw how the supply of blood is kept up in the body. In this chapter I shall show you how the blood is circulated everywhere, in order that it may be used in building and repairing. The machinery that thus circulates the blood is called the circulating system. It has its pipes everywhere. There is no part of the body where the blood does no go. And this machinery keeps the blood everywhere in motion. It nowhere rests for a single moment. 2. This circulating machinery has a great central organ, the heart, situated in the chest. This forces the blood out all over the body through the arteries. It receives it back again by the veins. It forces the blood out through a large artery, called the aorta, and from this go branches in every direction. These Describe the arrangement of the digestive organs in the turkey. What kind of stomach have birds that eat flesh or fish? What is the machinery that circulates the blood called? Is the blood ever still any where? What are the different parts of the machinery? Through what does the heart send out the blood? Through what does it receive it back branches divide more and more, just like the branches of a tree, till the extreme branches are exceedingly small. 3. These small arteries end in a network of vessels that are so small that they are called capillaries, from the Latin word capilla, hair. They are really smaller than any hair. When you prick or cut your finger you wound a large number of these capillaries, and they let out their blood. 4. The heart acts like a forcing and suction pump. It pumps out the blood through the arteries, and by suction it draws the blood back by the veins. It forces out the blood by contracting itself, or making itself smaller. It draws in the blood by dilating itself, or making itself larger. 5. I will make these two actions of the heart plain to you by certain comparisons. When you press the two sides of a pair of bellows together by the handles, as represented in Fig. 18, you contract the bellows FIG. 18. that is, you make the room in it smaller. A part of What are the capillaries? Like what does the heart ast? How does it force out the blood? And how does it draw it in? Illustrate by comparison with a pair of bellows. the air is therefore forced out through the nose of the bellows. It is in the same way that the blood is forced out of the heart through the aorta. The only difference is that the heart contracts itself, instead of having it done, as in the case of the bellows, by hands and handles. Again, when you move the handles of the bellows apart, as represented in Fig. 19, you Fig. 19. enlarge the room in the bellows, and so the air rushes in to fill the vacant space. In like manner, when the heart dilates, or enlarges itself, there is more room in it, and the blood rushes in to fill it up. 6. Another comparison, to illustrate the contraction and dilatation of the heart, is this. Fasten a tube to the neck of an india- rubber bottle, and fill it up with water. Put the end of the tube in a vessel of Illustrate the action of the heart by the comparison of the indiarubber bottle. |