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the increase of proteid food actually necessary during labour is probably smaller than in the annexed table :---1

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Age and sex.-Young animals require more food in proportion to their weight than adults, because they are growing in addition to maintaining metabolic processes. Aged persons require less food than those of middle age, and women less than men. The average minimum diets for different ages are thus given, in grammes:

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Climate.-Cold increases the appetite, increases the loss of heat, increases the desire and necessity for food with high heat-value; the fats have the highest heat-value of the proximate principles. Fat is even better than carbohydrates. Dr. McKendrick puts the matter thus: One gramme of stearin requires for its complete combustion over three grammes of oxygen, and it will only contribute 0·1 gramme of oxygen from its own substance; while one gramme of starch requires 1·68 gramme of

1 Taken from Sir L. Playfair, Lecture Royal Inst, 1865.

oxygen for its complete combustion, and it contributes about 0.5 gramme of oxygen from its own substance. Hence the combustion of stearin (as an instance of a fat) produces far more heat than the combustion of the same weight of starch (as an instance of a carbohydrate).

Digestibility of food.-In order to estimate the nutritive value of a food, it is not sufficient to calculate the percentage amount of its carbon, nitrogen, and other elements. An equally important consideration is whether the nitrogen, carbon, and other elements can be easily assimilated from that food, and since the first step towards assimilation is digestion, whether they are easily digestible.

We must, in considering this question, neglect, or almost entirely neglect, the idiosyncrasies of various people; what is easily digestible by one is difficult of digestion by another, and vice versa; diseases, too, especially of the alimentary canal, interfere considerably with the digestion of food. Looking, however, at the digestion of an average healthy man, the digestibility of a food depends on its source, and other extraneous circumstances, its bulk, its reaction, and the fact whether it is cooked or not.

The source of food. The proteids of animal origin are more easily and completely digestible than those from the vegetable kingdom (see p. 597); the same is true for the fats; and with regard to the carbohydrates, we know that starch from some plants is more digestible than that from others. Why this should be, it is at present difficult to say. Rubner gives the following table, which illustrates some of these statements:-

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With regard to meat, different kinds of flesh are more easily digested than others by artificial gastric juice; thus fish is more difficult to digest than mammalian meat; white flesh is more digestible than dark; raw beef than smoked. The presence of fat between the muscular fibres, as in lobster, increases the difficulty of digestion.

More valuable results have been obtained by Beaumont and Richet by observations in cases of gastric fistula; they have noted the actual

1 Chittenden and Cummins, American Chem, Journ. vi. 5.

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time the food stayed in the stomach, and constructed tables which show that

a. Meat remains from 2 to 5 hours in the stomach, the most digestible being lamb, then in order beef, mutton, veal, pork. Fish is equal to mutton.

b. Some starchy foods (rice, barley, tapioca) remain two hours or less in the stomach; others (beans, peas, potatoes) for 2 hours; white bread for three, and brown bread for four hours.

The bulk of the food.-A bulky food throws excess of work on the stomach, causes discomfort, and leads to diminished absorption, as all parts of the food cannot come so well into contact with the walls of the alimentary canal. The same amount of nutriment in a more concentrated form would be more readily digested. This is one great objection to vegetarian diet. The nitrogen is so diluted by great masses of insoluble cellulose and unnecessary starch that huge volumes have to be taken to obtain the requisite minimum of fifteen grammes of nitrogen daily. The carbohydrates, too, are apt to undergo fermentative changes, and the gases so formed give rise to flatulence, and increase the discomfort.

The reaction of food.-As a rule, it is slightly alkaline; this excites a flow of gastric juice; too much alkali neutralises the gastric juice, and thus hinders digestion. Too much acid (vinegar, lemon juice, &c.) is injurious and diminishes digestion, and may lead ultimately to serious disorders of the walls of the stomach.

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 tapeworm and trichina.

2. In the case of vegetable foods, it breaks up the starch grains, bursting the cellulose, and allowing the granulose to come in contact with the digestive juices.

3. In the case of animal foods, it converts the insoluble collagen of the universally distributed connective tissues into the soluble gelatin. 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. This may be hastened by hammering the flesh.

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 coagulated coat on its exterior, which keeps in the juices to a great extent, letting little else escape but the dripping (fat). Whereas in boiling, unless both bouillon and bouilli are used, there is considerable waste. Cooking, especially boiling, renders the proteids more insoluble than they are in the raw state, but this is quite counterbalanced by the other advantages that cooking possesses.

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Artificial digestion of food. This is now a most important branch of medical practice. In patients with feeble digestions, pepsin, papain, ox-bile, and other preparations are administered to help out the insufficient supply of digestive ferments supposed to be present. It is, however, much better to digest the food for the patient before administering it; indeed, in cases where a patient has to be fed by the rectum this is almost compulsory.

Artificial gastric juice, pancreatic juice, juice of the papaw plant, &c. are employed for the purpose. The two first named are generally made from the stomach and pancreas respectively of the pig. The food, such as milk, is warmed to the body temperature, and peptonised by the addition of a certain volume of the artificial juice. The peptonisation in these artificially digested foods seldom goes further than the formation of proteoses (albumoses); true peptone is formed, if at all, in very small quantities. The best commercial preparations of peptone sold are chiefly proteoses. Artificially digested foods are usually bitter. This is a great disadvantage they have. What the bitter substance is, is unknown. It is certainly neither albumose nor peptone.

Bread-making may be considered as a partial artificial digestion, the starch of the flour being largely transformed into dextrin and dextrose.

Stutzer has elaborated the system of artificial digestion to such a pitch of excellence that, both with regard to proteids and carbohydrates,' he is able to ascertain their digestibility with results which accord almost exactly with experiments which consist in the actual feeding of animals with the same foods, and subsequent examination of their fæces. The method of artificial digestion is by far the simpler of the two, and valuable statistics, relating chiefly to the fodder of domesticated animals, have been thus obtained. E. Pfeiffer has confirmed these observations. Sheridan Lea has recently attempted to imitate the conditions of normal digestion by placing the digestive mixture in a dialysing tube instead of a flask; the tube is kept constantly moving and suspended in a liquid into which the products of digestion pass out as they are formed. It was found by this means the amount digested and the rate of digestion are increased, and that intermediate products like dextrin (from starch) and anti-albumid (from proteids) are only discoverable in small quantities.

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1 M. Popoff, Zeit. physiol. Chem. xiv. 524.

3 Stutzer and Isbert, Ibid. xii. 72.

5 Journ. of Physiol. xi. 226.

2 Zeit. physiol. Chem. x. 301; xi. 207, 361. 4 Ibid. xi. 1.

CHAPTER XXVIII

THE DIGESTIVE JUICES AND THEIR ACTION

BEFORE proceeding to study in detail the character and composition of the digestive juices, and the way in which they act upon the various forms of food, it will be convenient to take a rapid survey of the whole process, and to mention the chief points in connection with each particular juice.

In the mouth mastication of the food takes place, and thorough admixture with the saliva. The saliva is an alkaline fluid containing inorganic salts and small quantities of organic matter. The two most important organic materials are ptyalin, an amylolytic ferment, i.e. a ferment by means of which starch is converted into sugar, and mucin, secreted chiefly by the submaxillary gland. The action of this slimy material is to lubricate the food, and it is in animals like the carnivora, unaccustomed to masticate their food well, or apt to swallow hard materials like bone, that mucin is most abundant, and thus the pharynx and œsophagus are protected during the process of swallowing.

The changes which the food undergoes in the mouth are chiefly mechanical, more or less fine subdivision by the teeth, partial solution, and lubrication by the saliva. There is, however, a chemical change brought about by the activity of the ptyalin, a conversion of starch into sugar (maltose). This change is, however, soon cut short, for when swallowed the food enters the stomach, and there finds an acid juice; in acid media, ptyalin is inoperative.

In some animals, however, namely, those which ingest a large quantity of starchy food, the process of insalivation is a longer one than in man. In some of these animals, such as the horse' and pig,2 the secretion of acid does not begin until the food has remained some time in the stomach, so that salivary digestion goes on unimpeded ; in other herbivora, the ruminants, there is a still more elaborate system of insalivation which is called rumination, the food returning to the Ellenberger and Hofmeister, Bied. Centr. 1887, p. 229; Goldschmidt, Zeit. physiol. Chem. x. 361. * Ellenberger and Hofmeister, Du Bois Reymond's Archiv, 1889.

3 Or it may be that the alkaline juice secreted by the stomach for the first hour or so, especially by its left half, is itself amylolytic.

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