Our bodies, therefore, and the bodies of all other animals, are gas engines, which burn sugar by exploding it mixed with air. Most of our food, to be sure, isn’t sugar, but bread and potatoes and cookies and all sorts of nice things made of butter and flour, milk and the like. But as I have already pointed out, the most important portion of this food is either made over into our own life-jelly, or else it is changed into sugar and exploded in our muscles.

Sometimes when the automobile goes by, one of the things you notice is a very bad smell. This is largely the unburned and half burned gasoline. Gasoline, when it burns clean, changes to water and to the odorless and slightly tangy gas which we get in soda water, carbon dioxid as it is called. When you burn gasoline, then, you get the same products as if you boiled plain soda water.

Most things that burn, likewise, burn to carbon dioxid and water. Wood does it, and coal, all kinds of oil that we burn in lamps, and the gas that we burn for light and heat. So, too, do all sorts of candles—paraffin, for example, or wax; and so, too, do the old fashioned tallow candles which our great-grandmothers used to make. Tallow, however, is a fat, except for its taste, like the fat we eat. Practically, we eat nothing that we cannot also burn, when dry—tho we do burn a good deal that we cannot eat.

Most of these burnable foods explode in the body, a good deal as they burn outside. They form carbon dioxid and water—when you “see your breath” on a cold day, you merely see the water in it that came from your exploding muscles. If you eat largely only plain wholesome foods, bread and butter, fresh vegetables, fruit, candy and cookies and crackers, and all the various other foods that burn clean, they will burn clear in your bodies, and you will yourselves be clean and sweet as children ought to be. But if you have a taste for things you ought not to have, and get them, then instead of good clean water and carbon dioxid, you will explode to a lot of unwholesome, poisonous, and smelly things, that are not at all nice outside the body, and are still worse inside.

Unfortunately, we cannot live altogether on these clean-burning fats and starches and sugars, which explode to carbon dioxid and water, and leave nothing more behind than a wax candle when it burns. We can’t make our life-jelly out of these foods. So we have to eat also, eggs and milk and cheese and beans and peas and meat and fish, some parts of which we can build to our life-jelly. But only about a tenth part of our food needs to be of any of these life-jelly-making sorts. The other nine-tenths should be the clean-burning things with which we do most of our work and play.

But whatever the fuel with which we run our bodily engines, sooner or later it gets used up and the waste products have to be blown off into the air. Insects and automobiles, which take the air pretty directly into their cells, blow off their waste gases directly into the air again. The lowly creatures which breath the air in the water, send their carbon dioxid back into the water again. But we who have blood, use that to carry off our exploded sugar and other things.

The water of the burned up food is simply added to the watery part of the blood. The carbon dioxid becomes in the blood ordinary cooking soda; the blood carries the soda to the lungs, and there it changes to carbon dioxid again, exactly as it does when, as cooking soda, or baking powder, you add it to flour and use it to raise cake. Finally it comes out of the lungs with the breath, and that is the end of it so far as we are concerned.

Still, we are not through with it yet; because the plants take in thru their leaves the carbon dioxid that we animals breath out thru our lungs, take it apart again, mix it up with water and other things which they get thru their roots, and finally make it over into wood, and into starch and sugar and the like which we animals eat up once more. So if we eat the plants, the plants also eat us; and the same stuff keeps getting used over and over again. And a mighty convenient arrangement it is, too, since there is precious little stuff to make living things out of in the world at best. Most of the earth is just rocks.

However, I started to tell you something about the burnt up food and exploded muscle-sugar, While it is still in the body, before blood and lungs and skin and kidneys have combined to carry it away.

Did you ever stop to think why you are sleepy when night comes? You play hard all day, running about until your legs are tired enough to drop off. By and by, you begin to be sleepy, an hour or two it may be before your proper bed time. You are tired in your legs. But you are sleepy in your eyes. Your legs are not sleepy in the least; and your eyes are not tired. How did the eyes find out that the legs had been running hard and needed sleep?

It is these same waste matters in the blood. We run our legs off by day; and by night time a hundred thousand little explosions in our muscles have used up so much sugar and the rest, that the blood is filled with the waste material, and the lungs cannot carry it off.

So it stays in the blood and poisons us—not badly, but just about as much as if we had taken a small dose of laudanum or alcohol or any of the large number of sleepy poisons, which kill one by putting him to sleep so hard that he cannot wake up. Our “fatigue toxins” as we call them (which is simply Latin for poisons that we make by getting tired) poison us just enough to make us sleep. While we sleep we don’t do much; less of these toxins are formed; the lungs and kidneys have time to catch up with their work; pretty soon the blood is clear again, and we wake up in the morning ready to do it all over once more. We are made to stand a certain amount of poisoning, and get over it. The trouble comes when we poison ourselves With things that we put into our blood, that we might have kept out.

Did you ever think why, after you have been running hard for a long time, your legs ache? Or why they stop aching when you sit down to rest, but don’t stop at once? It is these same fatigue toxins. You explode your muscle cells faster than the blood can wash away the products of the explosions. So these accumulate. By and by, they begin to poison the muscle, and you begin to feel the pain. If you keep on working, as people have to sometimes in spite of weariness, the ache and the poisoning gets worse and worse, till the muscle simply refuses to work any longer.

If you stop to rest, the ache of weariness still continues. But after a little, the blood stream washes the muscle clean. Then the ache is gone, and you can get up and run again. Nevertheless, a whole day’s play or work will so load up the blood with toxins that it can no longer wash the muscle clean. Then you must take a longer rest, go to sleep, and give time for the blood itself to clean up.

Perhaps you have noticed (if you haven’t, try it—only don’t lie on the damp ground) that when your legs are tired, they rest and stop aching much more quickly if you put your feet up higher than your head. This is, of course, because the blood current coming from the tired muscles, can run down hill, and so most easily drain off the toxins which make the ache. So too, you can keep fresh much longer, whether you are working or playing with the muscles, or sitting still and working your brains over your lessons, if you stand up properly and don’t slouch. When you slouch, you cramp your lungs. The cramped lungs fail to clear the blood. The dirty blood fails to wash brain or muscles clean, and you get tired sooner than you ought. For the same reason, you tire more quickly in bad air. But if you give blood and lungs a fair chance, they will do a lot of resting for you while you are still at work.

But long before we get in the least tired, we get out of breath. Poisons as before, only this time it is largely the carbon dioxid that does the business. The muscle-sugar explodes, and forms the carbon dioxid. The carbon dioxid leaks out into the blood; and the blood, circulating thru the body, carries it to a certain nerve center high up in the back of the neck. This in a sense tastes the carbon dioxid, something as the tongue tastes it in a glass of soda water.

When the nerve center in the neck tastes a little carbon dioxid, it doesn’t say anything. But the moment the taste begins to get strong (which is in less than a quarter minute after one starts running hard) it telephones over the nerves to the lungs: “Here, here, here! What is the matter with you fellows. Get busy. Breathe hard. This blood is fairly sizzling with burnt up sugar!”

Thereupon the lungs get down to work. They breathe as hard and as deep as they can; while the heart, which has also been telephoned of the situation, beats harder and harder, to give the lungs all the blood they can clean, and the working muscles all the blood they can dirty.

If heart and lungs hold their own, nothing in particular happens. But if we keep running on too hard, so that muscles poison the blood faster than the lungs can un-poison it, then the nerve center which is in the back of the neck interferes once more. When it cannot make heart and lungs work faster, it calls off the muscle. Suddenly it gives us such a feeling of loss of breath and suffocation, that we simply cannot run another step. We have to stop. Then heart and lungs catch up on their work.

Curiously enough, getting one’s “second wind” as we say, when the lungs after pumping violently, settle down to working steadily once more tho we still keep on running, and “getting in training” so that we can do all sorts of exercises without getting winded, both these highly desirable conditions depend in part on teaching this “respiratory center” in the neck not to raise so much of a row when it smells a little carbon dioxid in the blood. We train our muscles to do their work; and we also train this nerve center not to get rattled and turn on that feeling of suffocation until it absolutely has to. We get it used to burnt muscle-sugar so that it doesn’t mind the taste as it did.

So, as I say, we live only by just escaping being mildly poisoned. But the curious thing about it is that among these various poisons which would certainly kill us forthwith, if we did not promptly get them out of our bodies, stands, of all things, sugar.

We eat a good deal of sugar in our food. We make a good deal more out of other sorts of food. If we did not make sugar, and have it always on hand in our blood, we could neither work nor live. And yet thousands of persons, every year, die of nothing in the world but sugar poisoning.

Sugar is so very poisonous that we have a special arrangement in our livers for keeping down the amount that at any one time gets into the blood. But for this, a box of candy, or a meal of bread and potatoes would inevitably kill us within three hours. The blood of a full grown man always contains about a quarter ounce of sugar, that is to say, two ordinary lumps. If he has less than two lumps, he begins to starve. If he has more than three or four lumps, his head feels heavy and he cannot keep awake. He begins, in short, to be poisoned. But any one who should get his blood half as sweet as he takes a cup of tea or coffee, would promptly drop into a sleep from which he would never wake up at all.

One thing then that the liver is for is to catch the sugar as it goes by, after a meal, and store it up where it will do no harm. Then it slowly feeds it out again, as the muscles use it up, always keeping the amount in the blood at two lumps. But if we eat or make more sugar than the liver can pack away, then the rest is changed into fat and stowed under the skin and around the muscles. So we store our food as fat, and use it as sugar—fat, luckily, being one of the few things we make in our bodies that are not poisons.