The science of life : fully illustrated in tone and line and including many diagrams

THE COMPLEX BODY-MACHINE AND HOW IT WORKS

carry the juices made by these glands into the intestine. Each has essentially the same structure as a salivary gland, but they are larger and more complicated.

The liver is by far the larger of the two organs. Everyone knows its shape and position. It secretes a greenish fluid called bile which trickles slowly down a tube, the bile duct, into the small intestine. If a meal is being digested, the bile runs straight down to the intestine ; if not, it is stored until required in a hollow sac, the gallbladder, which lies at the end of a side branch of the bile duct. The secretion of bile is more rapid during digestion.

The pancreas (the sweetbread properly so called of domestic animals, though other glandular matter is frequently dished up under this name) is a smaller gland, pink, about eight inches long, having an irregularly elongated shape, and lying in a horizontal plane behind the stomach. It secretes a colourless pancreatic juice, which flows directly along the pancreatic duct to the intestine.

Moreover there is yet another supply of ferments. ‘The intestine, like the stomach, has in its wall an enormous number of minute, simple glands, whose tubules open into the intestinal canal. These glands make the intestinal juice, or succus entericus, a fluid which plays a part of the utmost importance in the digestive process. These various juices act not only on the food, but also on one another. ‘The presence of bile makes some of the enzymes in the pancreatic juice more active, and the succus entericus converts inactive “‘ enzyme-precursors”’ in that juice into active enzymes.

The chyme is mixed with the secretions of all these glands as it enters the intestine, and so becomes exposed to the action of a much more vigorous team of enzymes than heretofore. They attack it simultaneously, continuing and finally completing its digestion. It is quite possible for a man to adapt himself to live without a stomach, but his small intestine is essential. Proteins —or, rather, the shattered results of the gastric digestion of these substances—are attacked by two enzymes, trypsin and erepsin, contributed by the pancreas and intestinal wall. ‘Trypsin specializes more in the earlier and middle stages of protein dissection, while erepsin is concerned with the splitting of the penultimate products. Between the two the protein molecules are completely taken apart into their constituent amino-acids. Carbohydrates also have been reduced to assimilable glucose—all, that is, that Mr.

Everyman will get out of his meal, for he may well have eaten more than his interior was prepared to cope with. Fats are fallen upon by certain substances present in bile, the so-called bile-salts, which overcome their stubborn tendency to form large droplets and disperse them into a state of minute division, called an emulsion. (In physical language, the bile salts lower the fat’s surface-tension.) When this has been done, the fats are readily assailed by lipase, an enzyme made by the pancreas, which splits them up into their last stage of fatty acids and glycerine. Thus for all the main classes of organic food-stuffs is the process of digestion completed.

But besides the various enzymes there is another important substance in this region, contributed by the pancreas. This is carbonate ofsoda. The enzymes just mentioned will not work in an acid medium, therefore the acidity of the chyme has to be neutralized before their operations can be performed. This is done by the alkaline carbonate of soda.

As in the stomach, digestion in the intestine is assisted by muscular movements. The writhings of the intestinal tube are of several kinds. There is, for example, a slow, regular swaying of whole loops of the intestine from side to side, and there is sudden chopping up of a straight length of intestine into segments by the appearance of a series of local contractions. ‘These movements can be studied by giving an experimental meal containing bismuth, and examining the subject with X-rays; the bismuth casts a dense black shadow, and so the movements of the food can be watched. The meal is seen to remain for a time in each loop of the intestine, undergoing various rockings and churnings which thoroughly mix the chyme with the digestive juices. After a little time a strong peristaltic wave runs along the loop, and sweeps the mixture into the next, where it suffers another period of mixing. Its progress is therefore irregularly rhythmical ; it pauses for a time, to be thoroughly churned, then sweeps on for a foot or so, then pauses again, and so on. In this manner, in some three hours, the meal is worried along the twenty odd feet of small intestine, and its entry into the blood-stream made possible.

Now the inside surface of the intestinal tube is given a velvety appearance by the presence of myriads of finger-like projections, the villi, each about a twenty-fifth of an inch long. But it is restless velvet; the villi wriggle about when a meal is being digested,

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