The functions of digestion ought normally to be carried on without any pain or inconvenience. On the contrary, satisfaction of the appetite should be a source of pleasure, and digestion should be productive of comfort, but in many instances this is not the case. Weight, distension, pain, flatulence, oppression, palpitation and other discomforts frequently follow the ingestion of food, and these discomforts are generally included under the name of dyspepsia. A great deal of dyspepsia is due to want of proper mastication. Either the teeth are bad and proper mastication is impossible, or the food is hurriedly swallowed without the person taking either the time or the trouble to chew it. The consequence is that little saliva is secreted, starchy foods go into the stomach unchanged, protein foods are imperfectly broken up and are thus more slowly acted upon by the gastric juice. The gastric juice itself is more sparingly secreted from want of the stimulus afforded by the saliva and of the nervous stimulus given by the sapid taste in the mouth. The first link in the process of mastication is thus imperfect and the others follow suit. Here it may be worth while to give a hint about the preservation of the teeth, because many people are under the impression that if they brush their teeth thoroughly they will save them from decay, but this is not the case, because a tooth-brush does not penetrate between the teeth, and it is between the teeth that particles of food stick, especially particles of meat. There they afford a lodgment to microbes and generate acid, and it is between the teeth that caries usually begins. This lodgment can only be removed by the use of a tooth-pick. The movements of the lips act as a brush to the external surface of the teeth, and the tongue does the same for the internal during the day, but during sleep their movements cease and time is afforded for the decomposition of food. A tooth-brush should therefore be used before going to bed, and if milk be drunk either by adults or children at bedtime the mouth should be well rinsed afterwards. But in many cases neither the lips, tongue nor a toothbrush can thoroughly remove the food from between the teeth, and a tooth-pick should be used. It may be quill or wood, and a very useful and cheap one can be made by simply cutting the end of a lucifer match into a wedge shape. Gobbling the food is very frequent amongst busy people or those who are of active habits, and whenever they think of something to be done, they are apt to eat more quickly than ever, and it is difficult for them to remember that they ought to eat slowly. Sometimes the only way to break them of the habit is to make them count the bites. The advice given by the late Sir Andrew Clark in this respect was very good : "Thirty-two teeth in one mouth, thirty-two bites to every mouthful, and for any tooth that is gone the number of bites must be proportionately increased." The conditions under which a meal is taken greatly affect its digestion. I have frequently had patients say to me : "It is very odd that if I eat a bit of plain mutton and stale bread at home I suffer dreadfully from dyspepsia, whereas if I go out to dinner and eat and drink everything that is put before me I have no trouble at all." The reason of this is evident in the light of the experiments of Pawlow, for when at home the nervous stimulus to the secretion of the digestive juices was absent, while it was present in the case of the dinner outside, and, consequently, in the first case dyspepsia occurred and not in the second.
Depressing emotions and fatigue also interfere with digestion. We have no experiments to show their effect upon the secretion by the stomach or intestine, but their effect upon the secretion of saliva is very marked. Thus, one plan of detecting a thief in India is to make all the suspected people sit round in a circle. Each one is then made to chew a little rice and again spit it out. This process is accompanied by some oaths, incantations or invocations, and the effect is that the fear of detection dries up the secretion of saliva, and the guilty person spits his rice out dry, while the others all put it out moist. Worry has a somewhat similar, though less marked, effect. It was pointed out to me by my friend, Dr. Rayner Batten, that a line of froth along each side of the tongue when it is first put out is an almost infallible sign of worry, and I have proved the truth of this upon many occasions. By attention to chewing and by resting a little while before and after a meal, and by refusing to think at meal times of worrying subjects, nervous dyspepsia may frequently be greatly lessened.
Many cases of dyspepsia depend on the presence of microorganisms in the alimentary canal which give rise to abnormal processes of decomposition in the food with the production of toxins and gases. Metschnikoff has proposed to cure those cases on the old principle of "setting a thief to catch a thief" and destroying these "wild" and harmful bacteria by the introduction of a large number of "tame" and harmless bacteria. To this latter class belongs the bacillus acidi lactici and Metchnikoff's plan is to give a pure cultivation of this bacillus either in the form of tablets or of milk already soured by its presence.
The dietaries appropriate either for general diseases or for diseases of special organs will be considered under the appropriate headings, but the dietary for a healthy man may be summed up very shortly in the words of Professor Chittenden : "The best dietary for a healthy man is a mixed diet, and not too much of it".