6. Is it a good thing to try to bite hard candy or nuts with the teeth? Why? 7. Where are the fats of our food digested? The starch? The proteids? 8. What is the value of a vegetable salad in a dinner? 9. Why can we not consider alcohol a food? Summary. The digestive tract is a tube about 30 feet long in an adult. It is lined with mucous membrane. Glands take digestive liquids from the blood and store them for the digestion of the food. In the mouth food is broken into small fragments and mixed with the saliva. The "milk teeth are 20 in number; the permanent teeth, 32. Food is swallowed by the contraction of muscles of the throat and esophagus. Three sets of muscles act upon food in the stomach. The glands of the stomach secrete the gastric juice; this has an acid reaction, and contains the ferments pepsin and rennin. Food leaves the stomach through the pylorus, the gateway into the small intestine. In the small intestine the digestion of all three classes of foods: carbohydrates, fats, and proteids, is completed. A special ferment is provided for each. The juices containing these ferments are secreted by the liver and pancreas. Digestion in the intestine takes place under alkaline conditions. The digested food of the small intestine is absorbed through the villi, and then poured into the blood for transportation to the cells. Dietetics is the science of the diet. The body needs food of the right quality and in the right quantity. Alcohol cannot be called a food, because it has the effects of a poison. If patent medicines contain alcohol, they are as bad as liquors themselves. CHAPTER XXXVII HOW THE BLOOD IS CIRCULATED 374. What is the Use of Blood?- What have we learned thus far about the body? We have learned its general plan, have we not, and that it has a bony framework, to which its muscles and its soft organs are attached, and by which they are protected? We have also learned that all the living material of the body is composed of cells, and that the cells work together, each doing its share for the good of all. Then, finally, we have learned how our food is prepared for the use of the cells by the process of digestion. But how can the food get to the cells? Can the cells of the brain, or the lungs, or the muscles, go to 'the digestive tract for their food? Of course not; only when the food is actually inside the blood channels can it get to the cells. We have learned how the villi of the small intestine absorb the digested food, so that it can be transferred to the blood. Can you see now that what we call the "circulation of the blood" is also the "circulation of the food"? The taking up of the food by the cells is the last operation of digestion; we call it assimilation. There is another use of the blood system. What can the cells do with their waste products, the materials they have no further use for, and must get rid of? They turn these materials over to the blood. So the blood is also the great sewer of the body. Do the cells in the skin Is there a third need of the blood system? remain as warm as the cells inside the body? in cold weather they become much colder, and need heat from the inside of the body. They get this heat from the warm blood that bathes them. The blood thus regulates the body's temperature, so that the outside cells are kept warm; it also sees to it that the inside cells do not get too warm. The blood is carried throughout the body in tubes; these tubes, with the heart and the blood itself, make up the circulatory system. About of the weight of the body is blood. How much does your blood weigh? If we examine the blood with a microscope, we learn that the blood is made up of flat disks which are red, and are called red corpuscles, of colorless, round bodies called white corpuscles (see §353), and of a liquid called the plasma. Red 375. What are Red Corpuscles For? The red corpuscles (Fig. 273) make up about 45% of the weight of the blood. What is the weight of the red corpuscles in your body? Each corpuscle, when it is examined. under the microscope, is found to be a rounded plate, somewhat like a coin with rounded edges. The center of the plate is thinner than the edges. Each corpuscle, looked at alone, has a yellow color; but as they are usually in groups, one over another like coins in a stack, the group appears to be red. Each tiny plate contains a substance called hemoglobin White FIG. 273. Corpuscles of the blood, greatly magnified. The white corpuscles are the larger, and re semble the ameba. (hē'mō-glōb'in). This is a very active substance, and unites easily with oxygen to form a new substance called oxyhemoglobin. How is this word made? The red corpuscle is then carried along in the blood stream to the cells of the body, and gives up its oxygen to the cells. Thus the cells get their oxygen in convenient packages without having to go to the lungs for it. After the red corpuscle loses its oxygen it appears dark red or purplish. Bright red blood is rich in oxygen. } 376. What is the Work of White Corpuscles? — White corpuscles are not as numerous as the red, but they are much larger. They are one-celled bodies much like the ameba (see § 353), and have the power to move about much as the ameba has. They can go through the walls of the small blood vessels, and so get between the body's cells. They are the body's policemen, or soldiers, and attack and kill disease germs in the blood and in the tissues. When the skin is broken by an injury, white corpuscles gather at that point, and form a protecting wall, to prevent germs from entering the body through the break. 377. How is Bleeding Stopped? - We see that the blood is made up of solid bodies floating in a liquid, the plasma. As we should expect, most of the plasma is water. However, it contains, in solution, mineral matter and nutritive substances which go out into the tissues to feed the cells. The plasma is slightly alkaline (basic) in reaction (what does this mean?) and salty in taste. The carbon dioxide given off by the cells is dissolved in it, and then given off in the lungs (see § 387). Suppose that when you cut your finger, there was no way of stopping the flow of blood before a large quantity of the precious fluid leaked away. But, fortunately, when blood is exposed to the air, it forms clots, which close the wound, and prevent a great loss of blood. Of course if a large blood vessel is cut, the blood flows out too rapidly to form a clot, and other means must be used to stop the loss of blood. How does the blood form a clot? There is in it a wonderful substance called fibrinogen (fi-brin'ō-jen), meaning "fibrin-former." This becomes solid when exposed to the air and forms interlacing strings, or fibers, of fibrin. The blood is caught in the meshes of the fibrin, so that the corpuscles are able to hold on. The process by which a clot is formed is called coagulation (co'ǎg-u-la'shun) of the blood. We can help coagulation along by placing a little absorbent cotton in a wound; the fibers of the cotton hasten the formation of the fibrin and help to hold the clot in place. 378. How Does the Heart Pump the Blood? - The blood is interesting enough, if you think of it only at rest, but it is more interesting still, if you think how it is pumped through the body. What is the pump of the blood? The heart, of course. Blood flows through a system of tubes which are called, according to their size and the direction they are going (that is, whether toward, or from, the heart), capillaries, veins, and arteries. Which of these are the smallest? To enable the blood to flow freely through these tubes to every part of the body, force must be exerted. The heart, although only the size of your fist, is made of powerful muscles which can furnish the required force. A protective membrane surrounds |