starch, and then the alcoholic fermentation, due to the action of the yeast, begins. The bubbles of carbonic acid, burrowing passages through the bread, make it light and spongy. This enables the digestive juices subsequently to soak into it readily and affect all parts of it. During baking the gas and alcohol are expelled from the bread, the yeast is killed, and a crust forms from the drying of the outer portions of the dough. White bread contains, in 100 parts, 7 to 10 of protein, 55 of carbohydrate, 1 of fat, 2 of salts, and the rest water. COOKING OF FOOD The cooking of foods is a development of civilisation, and much relating to this subject is a matter of education and taste rather than of physiological necessity. Cooking, however, serves many useful ends :-- 1. It destroys all parasites and danger of infection. This relates, not only to bacterial growths, but also to larger parasites, such as tapeworms and trichinæ. 2. In the case of vegetable foods it breaks up the starch grains, bursting the cellulose and allowing the digestive juices to come into contact with granulose. 3. In the case of animal foods it converts the insoluble collagen of the universally distributed connective tissues into the soluble gelatin. The loosening of the fibres is assisted by the formation of steam between them. By thus loosening the binding material, the more important elements of the food, such as muscular fibres, are rendered accessible to the gastric and other juices. Meat before it is cooked is generally kept a certain length of time to allow rigor mortis to pass off. Of the two chief methods of cooking, roasting and boiling, the former is the more economical, as by its means the meat is first surrounded with a coat of coagulated protein on its exterior, which keeps in the juices to a great extent, letting little else escape than the dripping (fat). Whereas in boiling, unless bouillon and bouilli are used, there is considerable waste. Cooking, especially boiling, renders the proteins more insoluble than they are in the raw state, but this is counterbalanced by the other advantages that cooking possesses. Beef Tea. In making beef tea and similar extracts of meat it is necessary that the meat should be placed in cold water, and this is gradually and carefully warmed. In cooking a joint it is usual to put the meat into boiling water at once, so that the outer part is coagulated, and the loss of material minimised. An extremely important point in this connection is that beef tea and similar meat extracts should not be regarded as foods. They are valuable as pleasant stimulating drinks for invalids, but they contain very little of the nutritive material of the meat, their chief constituents, next to water, being the salts and extractives (creatine, hypoxanthine, lactic acid, &c.) of flesh. Many invalids restricted to a liquid diet get tired of milk, and imagine that they get sufficient nutriment by taking beef tea instead. It is very important that this erroneous idea should be corrected. One of the greatest difficulties that a physician has to deal with in these cases is the distaste which many adults evince for milk. It is essential that this should be obviated as far as possible by preparing the milk in different ways to avoid monotony. Some can take koumiss; but a less expensive variation may be introduced in the shape of junkets, which, although well known in the West of England, are comparatively unknown in other parts. The preparation of a junket consists of adding to warm milk in a bowl or dish a small quantity of rennet (Clark's essence is very good for this purpose) and flavouring material according to taste. The mixture is then put aside, and in a short time the milk sets into a jelly (coagulation of casein), which may then be served with or without cream. Soup contains the extractives of meat, a small proportion of the proteins, and the principal part of the gelatin. The gelatin is usually increased by adding bones and fibrous tissue to the stock. It is the presence of this substance which causes the soup when cold to gelatinise. ACCESSORIES TO FOOD Among these must be placed alcohol, the value of which within moderate limits is not as a food, but as a stimulant; condiments (mustard, pepper, ginger, curry powder, &c.), which are stomachic stimulants, the abuse of which is followed by dyspeptic troubles; and tea, coffee, cocoa, and similar drinks. These are stimulants chiefly to the nervous system; tea, coffee, maté (Paraguay), guarana (Brazil), cola nut (Central Africa), bush tea (South Africa), and a few other plants used in various countries all owe their chief property to an alkaloid called theine or caffeine (C,H10N4O2); cocoa to the closely related alkaloid, theobromine (C,H,N4O2); coca to cocaine. These alkaloids are all poisonous, and used in excess, even in the form of infusions of tea and coffee, produce over-excitment, 8 loss of digestive power, and other disorders well known to physicians. Coffee differs from tea in being rich in aromatic matters; tea contains a bitter principle, tannin. To avoid the injurious solution of too much tannin, tea should only be allowed to infuse (draw) for a few minutes. Cocoa is not only a stimulant, but a food as well: it contains about 50 per cent. of fat and 12 per cent. of protein. In cocoa, as manufactured for the market, the amount of fat is reduced to 30 per cent., and the amount of protein rises proportionally to about 20 per cent. Green vegetables are taken as a palatable adjunct to other foods rather than for their nutritive properties. Their potassium salts are, however, abundant. Cabbage, turnips, and asparagus contain 80 to 92 water, 1 to 2 protein, 2 to 4 carbohydrates, and 1 to 1.5 cellulose per cent. The small amount of nutriment in most green foods accounts for the large meals made by and the vast capacity of the alimentary canal of herbivorous animals. LESSON VII THE DIGESTIVE JUICES SALIVA 1. To a little saliva in a test-tube add acetic acid. Mucin is precipitated in stringy flakes. 2. Filter some fresh saliva to separate cells and mucus, and apply the xanthoproteic or Millon's test to the filtrate; the presence of protein is shown. 3. Put some 0.5-per-cent. starch solution into two test-tubes. Add some filtered saliva to one of them, and put both in the water-bath at 40° C. After five minutes remove them and test both fluids with iodine and Trommer's test. The saliva will be found to have converted the starch into dextrin and sugar (maltose). 4. The presence of potassium sulphocyanide (KCNS) in saliva may be shown by the red colour given by a drop of ferric chloride. This colour is discharged by mercuric chloride. 5. The reaction of saliva is alkaline to litmus paper. GASTRIC DIGESTION 1. Half fill four test-tubes A with water. B with 0-2-per-cent. hydrochloric acid. C with 0-2per-cent. hydrochloric acid. D with solution of white of egg (1 to 10 of water). 1 2. To A add a few drops of glycerin extract of stomach (this contains pepsin) and a piece of a solid protein like fibrin. To B also add pepsin solution and a piece of fibrin. To C add only a piece of fibrin. To D add a few drops of pepsin solution and fill up the tube with 0.2-percent. hydrochloric acid. 3. Put the tubes into the water-bath at 40° C. and observe them carefully. In A the fibrin remains unaltered. In B it becomes swollen, and gradually dissolves. In C it becomes swollen, but does not dissolve. 4. After half an hour examine the solution in test-tube B. (a) Colour some of the liquid with litmus and neutralise with dilute alkali. Acid-albumin or syntonin is precipitated. (b) Take another test-tube, and put into it a drop of 1-per-cent. solution of copper sulphate; empty it out so that the merest trace of copper sulphate remains adherent to the wall of the tube; then add the solution from testtube B and a few drops of strong caustic potash. A pink colour (biuret 1 Benger's liquor pepticus may be used instead of the glycerin extract of stomach. reaction) is produced. This should be carefully compared with the violet tint given by unaltered albumin. (c) To a third portion of the fluid in test-tube B add a drop of nitric acid proteoses or propeptones are precipitated. This precipitate dissolves on heating and reappears on cooling. 5. Repeat these three tests with the digested white of egg in test-tube D. 6. Examine an artificial gastric digestion which has been kept a week. Note the absence of putrefactive odour; in this it contrasts very forcibly with an artificial pancreatic digestion under similar circumstances. FERMENTS The word fermentation was first applied to the change of sugar into alcohol and carbonic acid by means of yeast. The evolution of carbonic acid causes frothing and bubbling; hence the term 'fermentation.' The agent yeast which produces this is called the ferment. Microscopic investigation shows that yeast is composed of minute rapidly growing unicellular organisms (torulæ) belonging to the fungus group of plants. FIG. 12.-Cells of the yeast plant in process of budding, between which are some bacteria. (Yeo's Physiology.') The souring of milk, the transformation of urea into ammonium carbonate in decomposing urine, and the formation of vinegar (acetic acid) from alcohol are produced by the growth of very similar organisms. The complex series of changes known as putrefaction, which are accompanied by the formation of malodorous gases, and which are produced by the growth of various forms of bacteria, also come into the same category. That the change or fermentation is produced by these organisms is shown by the fact that it occurs only when the organisms are present, and stops when they are removed or killed by a high temperature or by certain substances (carbolic acid, mercuric chloride, &c.) called antiseptics. The organisms produce fermentative effects. by shedding out soluble ferments or enzymes. The germ theory' of disease explains the infectious diseases by considering that the change in the system is of the nature of fermentation, and, like the others we have mentioned, produced by microbes; the transference of the bacteria or their spores from one person to another constitutes infection. The poisons produced by the growing bacteria appear to be either alkaloidal (ptomaines) or protein in nature. The existence of poisonous proteins is a very remarkable thing, as no profound chemical differences have yet been shown to exist between them and those which are not poisonous, but which are |