PART SIXTH.

DIGESTIVE SYSTEM.

I. OBJECT OF DIGESTION. - INANITION. - Foon.

THE aim of the digestive functions is to transform the substances borrowed from without, so as to enable them to pass into the system, to be absorbed and carried into the current of the circulation, in order to renew the organs, and keep up their functions.

These reconstructive substances are food.

By privation of food, animals are reduced to a state of inanition: the inevitable consequences of prolonged inanition are gradual loss of weight, cold, and death; animals die when they have lost of their original weight (Chossat.)1

This loss of weight is produced sooner in some animals than in others: cold-blooded animals will endure privation of food thirty times longer than the warm-blooded, and sometimes, even, for an almost incredible space of time: thus Cl. Bernard has known frogs go entirely without food for nearly three years, while a small bird dies of hunger after two or three days.

Inanition, as observed in persons subjected to a strict diet, not only affects the general temperature, but also the daily variations in temperature: even when there is no fever, this may vary 3°. This fact should be taken into account, in estimating the temperature of persons suffering from intermittent fever, who have long been on a low diet.

Some of the alimentary substances, intended to repair the incessant waste of the system, are immediately absorbed; while others, which are deposited on the surface of the

Chossat, "Recherches Expérimentales sur l'Inanition." Paris, 1843, in 4to.

digestive organs, must first undergo a change, from being subjected to the influence of the juices of these organs. This is because the food, received into the mouth, traverses the different parts of the digestive canal successively, and is subjected in its course to various mechanical influences, especially that of the different fluids which serve to liquefy and transform it. These modifications are not generally very striking; they appear to affect only the state of cohesion of the substances; insoluble elements being rendered soluble, and coagulable elements incoagulable, etc., while the unchanged parts are thrown off.

No aliment is complete, unless it contains all the elements of which the tissues of the body are composed.

1. Beside their organic principles the animal and vegetable matters which we consume contain various mineral products: such are the alkaline or alkaline-earthy salts, sulphur, phosphorus, iron, all elements necessary to every cell of our organs. Iron is administered to a chlorotic person as food, because iron, which is one of the indispensable elements of the economy, has been diminished in the blood. These mineral substances alone, are incapable of supporting life; and if those which are borrowed from the organic kingdom are found sufficient for this purpose, it is only because they contain in themselves a certain proportion of mineral matters.

The mineral salt that appears most indispensable to nourishment is chloride of sodium. Daily experience proved long ago that man cannot do without this salt, and the religious corporations which sought to subject themselves to the severest privations, tried in vain to banish chloride of sodium from their food. Physiological experiments on animals show (Wundt, Rosenthal, Schultzen) that this salt is indispensable to the system, and serious consequences have followed its suppression. Physiological chemistry explains these facts by showing that chloride of sodium enters into the composition of nearly every part of the organism, and is especially indispensable to the constitution of the blood serum and cartilages. It appears to assist in the process of the nutrition of the tissues, and is indispensable to the formation of the bile, pancreatic and gastric juices. Cattle-breeders are well acquainted with the favorable influence produced on the development of animals by administration of chloride of sodium; without asserting that mixture of this salt with the food produces increase of growth and fat, we must admit (Boussingault) that animals fed in this way have more

glossy and thicker hair, a more healthy appearance, are more sprightly and active, etc.

Attempts have been made, but without success, to substitute chloride of potassium for the sodium salt; it has been found, however, instead of possessing the useful properties of the latter, to produce serious injury.1

2. The principal aliments are those furnished by the animal kingdom, that is the different forms of albumen, designated under the common name of proteine substances, and several other similar elements classed together under the name of caseines. All these substances contain Oxygen (O), Hydrogen (H), Carbon (C), and Nitrogen (N), besides a certain quantity of Sulphur (S) and Phosphorus (P), mineral salts, etc. They also contain, probably, iron in small quantities, though this is not yet proved in all cases.

Some vegetable products supply the same aliment, such as gluten or vegetable fibrine, which is found in many seeds, particularly cereals; vegetable albumen, found in emulsive seeds and vegetable juices, and legumine or vegetable caseine, found in large quantities in the seeds of leguminous plants. These substances may all be classed under the name of albuminoids. The transformations undergone by the albuminoid substances contained in plants bear a striking resemblance to those which take place in the animal economy, and which we shall proceed to examine. During the germination of seeds, the albuminoid substances contained in plants give rise to digestive ferments bearing the essential features of some of the ferments furnished by the animal organs. Thus the diastase produced by the germination of cereals, closely resembles the fermentation which we shall see takes place in the saliva and in the pancreatic juice.

3. Next come the ternary, non-nitrogenous (or non-azotized) principles containing (C), (H), and (O), in the proportions required for the formation of sugar, starch, dextrine, gum, and various mucilages; all of these substances are incapable of directly forming globules, the prevailing matter of which is nitrogen. These substances are derived chiefly from the animal kingdom; they are also found in animal food, but in very small quantities. Sugar is found in milk, in the liver, and in the blood which flows from this organ; it has been discovered

1 See Cl. Champy, " Étude comparée de l'Action Physiologique des Sels Potassiques et Sodiques et de leurs Chlorures." Thèse de Strasbourg, 1870, No. 290.

in many epitheliums: in that of the cerebral ventricles are found white granules, some of which, in their behavior with the reagents, resemble amylaceous matter, and others dextrine; sugar also exists in the muscles and accumulates when they are not in action (as after long repose; after section of the motor nerves, and in the muscles of the fœtus), (Rouget). The integument of the non-vertebrated animals is formed of a glycogenous substance: this is the chitine of insects, the tunicine of the tunicata (animal cellulose), (Carl Schmidt). These substances are transformed into sugar by boiling with potash (Berthelot, Rouget). All these classes of alimentary substances become capable of being absorbed by contact with the digestive organs.

4. The last class of alimentary substances is the fats; these do not require to be digested, in the proper sense of the word; that is to say, the digestive juices produce no change in them; the fats are unchanged. They may, even, be absorbed by other surfaces than those of the digestive organs, as by the skin, for instance; we know that if fatty substances be rubbed on the skin, they will penetrate the epidermis: this is the only possible mode of nutrition by means of the external integument. The fatty substances are found in both the animal and vegetable kingdom.

Thus we see that nourishment may be derived, almost indifferently, from either the animal or vegetable kingdom: the amylaceous, glycogenous matters, forming almost the essential element of vegetables, are also found in animal products; thus we know that some savage tribes make fermented liquors (alcohol) with the sugar found in mares' milk. We have an instance, on the other hand, of an aliment which is apparently and essentially animal, though found in the vegetable kingdom: in the cheese which the Chinese make from legumine (caseine) derived from the fruit of leguminous plants.

It is especially important, however, to remark that the property of forming some of these substances does not belong to vegetables only, to the exclusion of animals: the formation of albuminoid substances evidently belongs to both kingdoms; the discovery of animal glycogeny (C. Bernard) proves that animals, as well as vegetables, can and do nat→ urally form amylaceous substances, and the same is true with regard to fatty substances: we owe to experiments by F. Huber, Milne-Edwards, and Dumas, the knowledge of the fact that bees, fed exclusively on sugar, still possess the

property of forming wax, which is a fatty substance. The possibility of an animal organism making any fatty substance used to be denied by many chemists and physiologists.

The animal and vegetable kingdoms also contain substances which resist the action of the digestive juices, and consequently pass through the intestinal canal only to reappear in the excrementitious products, separated from the alimentary principles accompanying them. These are, on the one hand, elastic and connective tissue, the digestion of which is very difficult and even impossible to some persons; and, on the other, numerous vegetable elements the most common form of which is the cellular or ligneous, forming the skeleton of most vegetables, the envelope of certain seeds, etc.

There is, finally, a peculiar class of substances, which must be considered as aliments, though they undergo little or no change in passing through the system and the interior of the tissues; they appear to produce the effect of diminishing combustion, or rather of rendering it more efficacious: in short, they promote the transformation of heat into force, and render the true alimentary substances previously ingested, more useful. Whence the name of reserve or economical aliments, bearers of force (dynamophorous). This singular class of substances which are not alimentary, and yet are aids to alimentation, has been the subject of numerous investigations, showing their number and the mode of action peculiar to each. Alcohol stands at the head of this class: according to many physiologists alcohol is burned in the system, serving thus to produce heat immediately (Liebig, Hepp, Hirtz, Schulinus); but recent investigations of Lallemand and Perrin show that if alcohol be received into the system it merely passes through it, and is always found again, as in the blood and tissues, especially the nervous tissue, in which it appears to take up its abode for some time: in short, it is not consumed, and its presence as an alimentary substitute only serves, by economizing combustion, to increase its utility. We can understand thus, that alcoholic drinks may be indispensable, in some degree, to a man who is obliged to perform severe labor, with insufficient nourishment; as to the fatal excess which so often succeeds a moderate use of these drinks, physiology shows us that our efforts should be directed less against this, than against the conditions which make the use of alcohol an imperious and fatal necessity for the working-man (Moleschott).