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

THE COMPLEX BODY-MACHINE AND HOW IT WORKS

complexity, little is known of their constitution. The story of Mr. Everyman’s breakfast may now be summarized very briefly. It is handed from enzyme to enzyme, and stage by stage it is broken up. The digestive apparatus consists of a series of tubes and chambers through which the food passes, and a series of glands which make the disintegrating substances. The stages of the process are as follows :

The first workshop, in which the dismantling is begun, is the mouth.

In the mouth food is chewed—that is to say, it is broken up into smaller pieces and thoroughly mixed with

sort of shredded pulp, which 1s readily penetrated by the chemical substances responsible for digestion. The purpose of mastication is twofold. First and most important, saliva is a lubricant, and its slimi-

passage of food to the stomach. After it is swallowed, a well-chewed and therefore slippery mouthful akes about six seconds to glide from the mouth to the stomach ; a hard dry object (such as a cachet of bismuth carbonate) takes about fifteen minutes. The second function of saliva is to begin the digestion of starch, which is done, as we have seen, by the ptyalin which it contains. Ideally, when mastication is finished and the bolus is swallowed, the food should be reduced to a finely grained pulp and thoroughly mixed with saliva, but Mr. Everyman is generally too impatient for this to happen.

Saliva is made by the salivary glands, of which there are three on each side of the face. Each gland consists of a branching hollow tree of tubes of which the twigs end blindly while the main trunk, or salivary duct, carries the saliva away from the gland to the mouth. The end tubes are lined by thick cubical cells, and it is these cells that actually make the saliva. The. saliva contains a little salt, a little protein, some ptyalin, and a little mucin (the substance which makes it slimy). In manufacturing this juice the cells make special substances (ptyalin and mucin) ; moreover, in separating from the blood a fluid in which the proportion of dissolved substances to water

saliva. The teeth reduce food to a all bladder =

ness facilitates swallowing and the DPuodenum=

is very much lower than in blood, they do work and use up energy just as the tubulecells of the kidney do. The salivary glands of a man may turn out as much as ten times their own weight of saliva a day.

When the food has been masticated it is swallowed by the gullet (oesophagus), a muscular tube about an inch across and ten inches long, which leads down the

Oesophagus ————— = SSS —sf—————————————— SS 4 liver SS Oa Oey —Z SS SS _ees SOS Sy —SSsESQN SSS = . ——————— ——

Zs

Appendix

Rectum

Fig. 26. The organs of digestion.

neck and through the chest into the stomach.

At the back of the throat there are two other openings besides the oesophagus—the opening of the nose-passage, floored by the soft palate, and the opening of the wind-pipe. Swallowing is a complex act involving, among other things, the closure of these openings so that food cannot “ go the wrong

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