A Handbook of Health by Woods Hutchinson (readnow TXT) π
CHAPTER II
WHY WE HAVE A STOMACH
WHAT KEEPS US ALIVE
The Energy in Food and Fuel. The first question that arises in our mind on looking at an engine or machine of any sort is, What makes it go? If we can succeed in getting an answer to the question, What makes the human automobile go? we shall have the key to half its secrets at once. It is fuel, of course; but what kind of fuel? How does the body take it in, how does it burn it, and how does it use the energy or power stored up in it to run the body-engine?
Man is a bread-and-butter-motor. The fuel of the automobile is gasoline, and the fuel of the man-motor we call food. The two kinds of fuel do not taste or smell much alike; but they are alike in that they both have what we call energy, or power, stored up
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That is why plants will, as you know, flourish and grow strong and green only in the sunlight, and why they wilt and turn pale in the dark. When the plant grows, it is simply sucking up through the green stuff (chlorophyll) in its leaves the heat and light of the sun and turning it to its own uses. Then this sunlight, which has been absorbed by plants and built up into their leaves, branches, and fruits, and stored away in them as energy or power, is eaten by animals; and they in turn use it to grow and move about with.
Plants can use this sun-power only to grow with and to carry out a few very limited movements, such as turning to face the sun, reaching over toward the light, and so on. But animals, taking this power at second-hand from plants by eating their leaves or fruits, can use it not merely to grow with, but also to run, to fight, to climb, to cry out, and to carry out all those movements and processes which we call life.
Plants, on the other hand, are quite independent of animals; for they can take up, or drink, this sun-power directly, with the addition of water from the soil sucked up through their roots, and certain salts[1] melted in it. Plants can live, as we say, upon non-living foods. But animals must take their supply of sun-power at second-hand by eating the leaves and the fruits and the seeds of plants; or at third-hand by eating other animals.
All living things, including ourselves, are simply bundles of sunlight, done up in the form of cabbages, cows, and kings; and so it is quite right to say that a healthy, happy child has a "sunny" disposition.
Plants and Animals Differ in their Way of Taking Food. As plants take in their sun-food and their air directly through their leaves, and their drink of salty water through their roots, they need no special opening for the purpose of eating and drinking, like a mouth; or place for storing food, like a stomach. They have mouths and stomachs all over them, in the form of tiny pores on their leaves, and hair-like tubes sticking out from their roots. They can eat with every inch of their growing surface.
But animals, that have to take their sun-food or nourishment at second-hand, in the form of solid pieces of seeds, fruits, or leaves of plants, and must take their drink in gulps, instead of soaking it up all over their surface, must have some sort of intake opening, or mouth, somewhere on the surface; and some sort of pouch, or stomach, inside the body, in which their food can be stored and digested, or melted down. By this means they also get rid of the necessity of staying rooted in one place, to suck up moisture and food from the soil. One of the chief and most striking differences between plants and animals is that animals have mouths and stomachs, while plants have not.
THE DIGESTIVE SYSTEMHow the Food Reaches the Stomach. Our body, then, has an opening, which we call the mouth, through which our food-fuel can be taken in. A straight delivery tube, called the gullet, or esophagus, runs down from the mouth to a bag, or pouch, called the stomach, in which the food is stored until it can be used to give energy to the body, just as the gasoline is stored in the automobile tank until it can be burned.
The mouth opening is furnished with lips to open and close it and assist in picking up our food and in sucking up our drink; and, as much of our food is in solid form, and as the stomach can take care only of fluid and pulpy materials, nature has provided a mill in the mouth in the form of two arches, of semicircles, of teeth, which grind against each other and crush the food into a pulp.
In this diagram the entire alimentary canal is shown enlarged, and the small intestine greatly shortened, in order to show distinctly the course of the food in the process of digestion.
In the bottom or floor of the mouth, there has grown up a movable bundle of muscles, called the tongue, which acts as a sort of waiter, handing the food about the mouth, pushing it between the teeth, licking it out of the pouches of the cheeks to bring it back into the teeth-mill again, and finally, after it has been reduced to a pulp, gathering it up into a little ball, or bolus, and shooting it back down the throat, through the gullet, into the stomach.
The Intestines. When the food has been sufficiently melted and partially digested in the stomach, it is pushed on into a long tube called the intestine, or bowel. During its passage through this part of the food tube, it is taken up into the veins, and carried to the heart. From here it is pumped all over the body to feed and nourish the millions of little cells of which the body is built. This bowel tube, or intestine, which, on account of its length, is arranged in coils, finally delivers the undigested remains of the food into a somewhat larger tube called the large intestine, in the lower and back part of the body, where its remaining moisture is sucked out of it, and its solid waste material passed out of the body through the rectum in the form of the feces.
THE JOURNEY DOWN THE FOOD TUBEThe Flow of Saliva and "Appetite Juice." We are now ready to start some food-fuel, say a piece of bread, on its journey down our food tube, or alimentary canal. One would naturally suppose that the process of digestion would not begin until the food got well between our teeth; but, as a matter of fact, it begins before it enters our lips, or even before it leaves the table. If bread be toasted or freshly baked, the mere smell of it will start our mouths to watering; nay, even the mere sight of food, as in a pastry cook's window, with the glass between us and it, will start up this preparation for the feast.
This flow of saliva in the mouth is of great assistance in moistening the bread while we are chewing it; but it goes farther than this. Some of the saliva is swallowed before we begin to eat; and this goes down into the stomach and brings word to the juices there to be ready, for something is coming. As the food approaches the mouth, a message also is telegraphed down the nerves to the stomach, which at once actively sets to work pouring out a digestive juice in readiness, called the "appetite juice." This shows how important are, not merely a good appetite, but also attractive appearance and flavor in our food; for if this appetite juice is not secreted, the food may lie in the stomach for hours before the proper process of digestion, or melting, begins.
The Salivary Glands. Now, where does this saliva in the mouth come from? It is poured out from the pouches of the cheeks, and from under the tongue, by some little living sponges, or juice factories, known as salivary glands.[2]
In this diagram are shown the three glands (G) of the left side. The duct (D) from the parotid gland empties through the lining of the cheek; those from the lower glands empty at the front of the mouth under the tongue (T). N, nerve; A, artery; V, vein.
All the juices poured out by these glands, indeed nearly all the fluids or juices in our bodies, are either acid or alkaline. By acid we mean sour, or sharp, like vinegar, lemon juice, vitriol (sulphuric acid), and carbonic acid (which forms the bubbles in and gives the sharp taste to plain soda-water). By alkaline we mean "soap-like" or flat, like soda, lye, lime, and soaps of all sorts. If you pour an acid and an alkali togetherβlike vinegar and sodaβthey will "fizz" or effervesce, and at the same time neutralize or "kill" each other.
The Use of the Saliva. As the chief purpose of digestion is to prepare the food so that it will dissolve in water, and then be taken up by the cells lining the food-tube, the saliva, like the rest of the body juices, consists chiefly of water. Nothing is more disagreeable than to try to chew some dry foodβlike a large, crisp soda cracker, for instanceβwhich takes more moisture than the salivary glands are able to pour out on such short notice. You soon begin to feel as if you would choke unless you could get a drink of water. But it is not altogether advisable to take this short cut to relief, because the salivary juice contains what the drink of water does notβa ferment, or digestive substance (ptyalin), which possesses the power of turning the starch in our food into sugar. As starch is only very slowly soluble, or "meltable," in water, while sugar is very readily so, the saliva is of great assistance in the process of melting, known as digestion. The changing of the starch to sugar is the reason why bread or cracker, after it has been well chewed, begins to taste sweetish.
This change in the mouth, however, is not of such great importance as we at one time thought, because even with careful mastication, a certain amount of starch will be swallowed unchanged. Nature has provided for this by causing another gland farther down the canal, just beyond the stomach, called the pancreas, to pour into the food tube a juice which is far stronger in sugar-making power than the saliva, and this will readily deal with any starch which may have escaped this change in the mouth. Moreover, this "sugaring" of starch goes on in the stomach for twenty to forty minutes after the food has been swallowed.
Starchy foods, like bread, biscuit, crackers, cake, and pastry, are really the only ones which require such thorough and elaborate chewing as we sometimes hear urged. Other kinds of food, like meat and eggsβwhich contain no starch and consequently are not acted upon by the salivaβneed be chewed only sufficiently long and thoroughly to break them up and reduce them to a coarse pulp, so that they can be readily acted upon by the acid juice of the stomach.
Down the Gullet. When the food has been thoroughly moistened and crushed in the mouth and rolled into a lump, or bolus, at the back of the tongue, it is started down the elevator shaft which we call the gullet, or esophagus. It does not fall of its own weight, like coal down a chute, but each separate swallow is carried down the whole nine inches of the gullet by a wave of muscular action. So powerful and closely applied is this muscular pressure that jugglers can train themselves, with practice, to swallow standing on their heads and even to drink a glass of water in that position; while a horse or a cow always drinks "up-hill." This driving power of the food tube extends throughout its entire length; it is carried out by a series of circular rings of muscles, which are bound together by other threads of muscle running lengthwise, together forming
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