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

THE COMPLEX BODY-MACHINE AND HOW IT WORKS

stomach is complex in its minute structure, the essential parts to notice being the inner layer, containing thousands of glands which make the gastric juice, and an outer layer of plain muscle-fibres, by means of which the stomach executes slow churning movements during digestion.

Food remains in the stomach for at least an hour and undergoes slow permeation by the gastric juice. At the beginning it is hardly altered from its original state; it has been coarsely shredded and mixed with a slimy lubricant and the break-down of some of the starch has begun, but it retains its cellular structure almost intact, and the proteins and fats have not yet been attacked. During its sojourn in the stomach it is reduced to a homogeneous texture, the cells being destroyed and all trace of structural organization eliminated ; at the same time the digestion of starch continues and that of proteins and fats is begun. The final result is a soft, slightly acid pulp known as chyme.

The gastric juice is a liquid made by a multitude of very tiny and simple glands situated in the inner layer of the stomach wall. It has several important characters : it is acid, containing a little hydrochloric acid, and it includes enzymes—pepsin, which attacks proteins, and lipase, which attacks fats. The glands begin to secrete this juice about five minutes after a meal is swallowed and it soaks slowly into the new

noted, however, that the extent of fat digestion in the stomach is very slight, for fats have a tendency to form comparatively large spherical droplets, into the interior of which neither hydrochloric acid nor lipase can penetrate, and the extent to which these substances can attack fats is limited by this

Fig. 30. A few of the cells which accumulate hoards of fat for use in time of need, greatly magnified.

The drops of fat are shown black.

factor. To the end the chyme contains little droplets of undigested fat.

Let us insert here a word or so about this characteristic of oils and fats and certain consequences that it entails. Amyone who uses brilliantine or who makes his own saladdressing knows how hard it is to mix oil with liquids like water or vinegar. Oil and fat are exclusive: they keep themselves to themselves. The fats which are stored in most fatty substances (such as meat-fats) are there in the form of droplets, and consequently when the cells containing them are broken down by pepsin they escape in droplet form. This handicaps the enzymes, which can only work at the surfaces of the drops and cannot get at the interior. But in some foods—egg-yolk, for example—the fats are already finely divided, and being therefore accessible to the enzymes are almost completely

fig. 29. A few of the muscle cells responsible for the digested in the stomach. This exclumovements of the digestive tube, greatly magnified. siveness of fats, we may remark, is

mass of food. As it does so it produces certain profoundly important changes. The pepsin begins the dissection of protein molecules. At the same time some of the simpler carbohydrates, such as cane-sugar, are attacked by the warm hydrochloric acid and broken down to glucose. A certain digestion of fats also occurs, partly under the influence of the hydrochloric acid and

partly under that of the lipase. It should be

probably the reason why fat is the chief stored fuel found in the body. In many of our tissues, and especially in the liver, there is storage of a _carbohydrate, glycogen, but only in limited amounts. Glycogen, being soluble in water, cannot be accumulated in a cell without affecting its chemical and physical processes, but a fat remains aloof and does not interfere with the processes about it until it is called into use. A being with fixed meal-times like Mr.

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