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cells give off something which should be removed from the body, the process is called excretion. The cells excrete carbon dioxide and certain minerals, as well as compounds called uric (yū'rik) acid and urea (yū'rē-ă These are the main excretions of the body, with the exception of the solid particles of the food which are not digested in the digestive tract. Most of the carbon dioxide is removed when the blood is in the lungs. The other waste materials are in solution, and the blood is freed from them as it passes through the kidneys. These are two bean-shaped organs, situated in the abdomen ; they are attached, one on each side, to the backbone. The wastes collected by the kidneys are stored in the bladder, until they are expelled from the body. The skin (see $$ 71 and 392) is another means of getting rid of the waste material of the cells.

384. Exercises. 1. Why do we divide the circulation into body circulation and lung circulation? What happens in each?

2. What part of the heart pumps the blood to each kind of circulation? What part of the heart receives the blood from each?

3. Why does the heart beat more rapidly when we are exercising the muscles? Do the contracting muscles help in the circulation of the blood ? How?

4. Suppose that the heart had only one compartment, and that it had the power of contracting and relaxing as now, could there be a circulation of the blood ? Would it be rapid ? Would the blood travel in any one direction?

5. Review § 135, especially the valves of the force pump. Does the heart have valves corresponding to the “ cylinder valve ” and the

discharge valve ”? How do these help the blood to go in one direction ?

6. How does a dog cool itself when it is hot? Explain.

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7. Which class of blood vessels has the thickest walls? Which compartments of the heart, the auricles, or the ventricles, need the thickest walls? Why?

8. When a boy or girl is growing very rapidly, the heart may not grow in proportion. What effect do you think too severe exercise will have upon the heart at such a time?

9. What bodies in the blood carry oxygen to the cells? Do they carry it as gas bubbles, or dissolved in the water of the blood, or combined with some substance in the blood ?

Summary. — The blood is the circulating liquid of the body. It carries oxygen and food to the cells, and waste materials away from the cells. It also regulates the temperature of the body.

Blood consists of a colorless liquid (plasma) with solid bodies (corpuscles) floating in it. It coagulates, or clots, on exposure to the air.

The organs of circulation are the heart, arteries, veins, capillaries, and lymph spaces.

The heart has 2 auricles and 2 ventricles.

Auricles receive blood into the heart; ventricles force blood away from the heart.

Veins carry blood to the heart; arteries carry blood away from the heart. Capillaries carry blood from the arteries, through the tissues, and back to the veins.

When an artery is cut, the bleeding should be stopped by a tourniquet placed between the wound and the heart. In the case of a vein the tourniquet should be on the side of the wound away from the heart.

The cells are bathed in a liquid - the lymph — which is much like blood, but has no red corpuscles.

The waste materials of the cells are removed from the blood (excreted) by the skin and the kidneys.

CHAPTER XXXVIII

RESPIRATION

. 385. How Do Animals Breathe ? — Do plants breathe ? Review $ 300 to recall the way in which the green cells of the leaf are open to the air which enters through the stomata. Do any animals breathe without lungs? As you think of the question, you will decide that if such lowly creatures as the ameba and the simpler animals breathe at all, they must get oxygen through their cell walls, and not in a special breathing apparatus. Even in an animal so complex as the earthworm, there is no special organ for breathing, but respiration is carried on only through the cells of the skin. In most higher animals, particularly water animals, the skin is still used as a help in respiration, although there are breathing organs (gills or lungs) to do most of the work (see 46). There must be some way of putting oxygen into the circulating fluid, and of taking carbon dioxide out of it.

Do you know the two gill coverings at the sides of the head of our ordinary fishes, and the “ arches" with pink “fringe” that make up the gills (Fig. 277)? In the tadpole, or polliwog, stage of the frog, the gills are easy to see; but as the tadpole becomes an adult frog, it develops lungs, and must come to the water's surface every little while for air. Do you remember that the mosquito wiggler has breathing tubes, and must come to the water surface? See $ 350. The adult mosquito and other insects also get their air supply through breathing tubes.

What is the difference between gills and lungs? One difference is that gills are really outside of the body cavity, and are bathed in

the water in which the creature lives. The gills take up some of the oxygen that is dissolved in the water, and give off the waste carbon dioxide to the water. In lungs we have a large surface of breathing cells in a cavity inside the body and the air enters only as a gas. But the air must be moist, or the

delicate lung cells will be injured (see Fig. 277.— The gills of a fish, with the gill cover removed.

$ 52).

In Chapter VII we learned something of how we breathe. We take in air, because the muscles of the chest relax, and the chest cavity becomes larger. As the cavity grows larger, the air pressure in the lungs is less than it is outside of the

therefore air rushes in from outside, and we say we have inspired, or inhaled.

body;

386. How Does Inhaled Air Get to the Lungs? Through what passage must the air go first on its way to the lungs? Of course through the nose. Like all the other tubes through which the air goes, the nostrils are lined with mucous membrane (see $ 363). Because of the length of the nose passage, there is a large surface of membrane. The membrane always has on it a considerable amount of liquid mucus. This moistens the passage, and catches dust particles, preventing them from going to the lungs. Cilia (sĩl'1-ă), small, threadlike extensions of the cells, are found in all the tubes of the breathing organs. They act like tiny brushes, and protect the lungs by sweeping back the dust particles.

A second reason for nose breathing is that the air we inhale is warmed in passing through the nose. • Ought we, then, to breathe through our mouths ?

Trace the passage of air from the nose to the lungs. The nose passages open into the pharynx, or throat, the same compartment into which food passes from the mouth. But while the food goes into the esophagus, the air goes through the larynx (lăr’inks), or voice box, into the windpipe, or trachea (trāk’ēcă). The lid which covers the larynx while food is being swallowed is called the epiglottis (ep'i-glot'is). The epiglottis does not go up and down like a real box lid, but it remains in its place, and the larynx is drawn up against it to close the passage into the windpipe. When swallowing is over, the larynx drops down a little way, and allows breathing to go on. Feel the larynx, or Adam's apple,” in your throat rise when you swallow.

If the larynx could not be closed, food would enter the windpipe, and cause us to choke. Does this ever happen? What do we mean by saying that food has gone down our "Sunday throat"? The larynx is a box made of pieces of cartilage.

The trachea, or windpipe, is a tube about 4į inches long, and about 1 inch in diamteer. It is composed of about 20 pieces of cartilage, bent around in the shape of a letter C. The pieces are held together by tough connective tissue. At its lower end the trachea is divided into two tubes, called bronchi (brðn'kī). One of them goes to each lung. The bronchi divide and subdivide a countless number of times to form bronchial tubes. The smallest of these tubes end in the air cells of the lungs.

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387. What are the Lungs Like? - Have you ever

seen the lungs of a chicken? They are two organs of pink, spongy tissue. The tissue is a mass of air sacs and

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