water. But the stomach and the long food tube need to exercise themselves on food that has bulk to it. Vegetables and fruit are especially useful in this line; so also is graham flour and grain with the hull on. Multitudes of people are intelligent enough to know that they can get all the proteid they need in other foods than meat. These people are called vegetarians.

1 Facts about teeth are given in Good Health. Here are the same facts in close compass. Microbes are the worst enemies the teeth have. They live and multiply on bits of food between the teeth, on the teeth, and on the gums. If they are left on the teeth long at a time, they work their way through the enamel, go ever deeper, finally reach the nerve at the center of the tooth, and then it is that toothache begins. We say the tooth is decaying, as indeed it is. To prevent decay, teeth should be washed with a toothbrush and water every night before going to bed; also after breakfast. Once a day something soft, like silk floss, should be drawn between the separate teeth to take out food fragments. Tooth powder or paste may be used once a day - even less often will do. Some teeth are more fragile than others, but both kinds decay if they are not kept clean. If the decay is not attended to, a chain of misfortunes may follow. Here is the chain. Badly decayed teeth cannot chew well. If chewing is not well done, the stomach has more work than it can manage. As a result the food goes onward half prepared, and the food tube is overtaxed. If this tube is overtaxed, the food is not ready to pass through its lining into the blood stream. If the blood stream does not receive rich supplies from the food, the whole body suffers for lack of the nourishment it ought to get from the blood. If the body is not well nourished, it is easily overcome by disease germs which attack it. For the sake of health, therefore, two things must be done: (1) Prevent decay by keeping microbes away from the teeth; (2) Go to a dentist once or twice a year. He will keep your teeth in good repair. Another point to bear in mind is that the teeth on the upper jaw and those on the lower jaw should be nearly opposite each other. They can chew best in this way. Then, too, if teeth are crooked, they should be straightened for the sake of good looks. See what Good Health says about a squirrel mouth and how to help it. Often the queer shape comes because the baby was allowed to suck its thumb or its finger. A dentist can give the jaw a better shape and compel crooked teeth to grow straight; he can make the teeth on the upper jaw grow opposite those on the lower jaw; but this work must be done while the jaw itself is young and growing. If a sound tooth is knocked out by accident, stick it back in place at once. The gums will grow about it and keep it firm and useful for many years. This is described in Emergencies.

CHAPTER XXI

CATS UNDER THE X-RAY

It is quite possible that the soldiers and athletes who shared in the eating experiments had no very definite notion about that which was to happen to the food which they swallowed, and it may easily be that some of them had not so much as heard about Dr. Cannon's experiments on cats.

These experiments were carried on in the laboratory of the Harvard Medical School, and the record of the work was published in 1898. Cats were chosen because they are easy to get hold of, ready to eat when they are fed, ready to sleep at almost any time, and easily controlled. Even among cats, however, Dr. Cannon had to choose carefully, for only those who were good-natured were useful to him.

Having made his choice he took bread, mixed into it a harmless chemical called bismuth,1 fed it to his cats, and waited for results. The bismuth was put in for this one reason, that its presence in the food made it possible to get a shadow of the shape of the stomach by means of Xrays. From shadows he hoped to discover, very definitely,

1 The exact chemical name is bismuth subnitrate.

how the stomach moves during the time that it is digesting its contents. Dr. Cannon was fortunate in the cats he chose, fortunate in his helpers, and fortunate in what he was able to learn through the X-rays; for he learned facts which had never been proved before.

After being fed the cat was put in place for its shadow picture. The particular cat which I have in mind was fed at fifty-two minutes after ten in the morning. At eleven o'clock work was well under way in the stomach, and once every half hour after that, until twelve minutes after six in the afternoon, the kindly cat consented to be put in place to have its shadow studied. Dr. Cannon traced the shadows one by one, so that an exact record was kept of the size of the stomach from the time of the hearty feeding until there was nothing left to be digested.

During this time there had been an interesting course of events. When first seen the stomach looked like a small leg of ham with a curled-up tail to it. But when six o'clock came the leg shape had disappeared entirely, leaving nothing but the tail to show where the food had been. Moreover, at this time the cat seemed hungry and called for food, with which it was promptly rewarded.

The diminishing size of the stomach was perhaps one of the smallest lessons learned that day; for while the cat slept, and while the X-rays were focused on its stomach, another fact was noted. It appeared that food

which had newly arrived stayed quietly in the upper end of the stomach as if it were in a reservoir. Here the saliva which had been swallowed with the food had a longer time to do its share in the work of digestion. But as fast as supplies were needed farther on this reservoir contracted itself and sent its contents forward a little at a time.

It was also seen that the firm walls of the lower part of the stomach had begun to contract in a series of wavelike movements. These waves started near the middle of the stomach and moved towards the smaller end of the elastic bag. They followed each other in regular succession. Once every ten seconds a new wave took its start from about the same spot and traveled the same course down to the smaller end.

Indeed, whenever the shadows were studied during that day these waves were seen to be following each other with unceasing regularity. Moreover, as time passed and as digestion progressed this middle part of the stomach grew gradually more and more

slender, like a neck, while the larger end stayed large for a longer time.

Through his study of shadows Dr. Cannon learned that within about fifteen minutes after food is swallowed a slender jet of softened food goes with a spurt through an opening at the lower end of the stomach and out into the tube which is the beginning of the small intestine.

For all animals, including man, this exit for the contents of the stomach is guarded by a strong muscle called the pylorus, or keeper of the gate. And well does this keeper do its work. Sometimes with every wave that rolls in its direction it opens wide enough to allow a spurt of digested liquid food called chyme to go through. But sometimes it stays persistently shut while wave after wave pushes in vain in its direction.

For the sake of getting an explanation of this uneven action of the pylorus Dr. Cannon induced the cat to swallow a small, specially prepared tablet made up of starch paste and of bismuth, the chemical substance already referred to. He then watched the progress of this pellet in the stomach. He saw it stay for a long time in the large, bulb-like end; saw it gradually make its way farther and farther down as it was sent forward by the waves of contractions; and finally saw that for forty-two minutes after the pellet reached the pylorus that diligent gatekeeper allowed nothing to pass onward.