A Treatise on Anatomy, Physiology, and Hygiene (Revised Edition) by Calvin Cutter (read more books .txt) 📕

Here it is that the true business of digestion commences. For as soon as any substance except water enters the stomach, this organ, with involuntary movements, that seem almost like instinct, commences the secretion of the gastric juice, and by long-continued contractions of its muscular coat, succeeds in effecting a most perfect mixture of the food with this juice, by which the contents of the stomach are reduced to the softest pulp.

The gastric juice, in its pure state, is a colorless, transparent fluid; “inodorous, a little saltish, and perceptibly acid. It possesses the property of coagulating albumen, and separating the whey of milk from its curd, and afterwards completely dissolving the curd. Its taste, when applied to the 446 tongue, is similar to that of mucilaginous water, slightly acidulated with muriatic acid.” The organs of its secretion are an immense number of tubes or glands, of a diameter varying from one five hundredth to one three hundredth of an inch, situated in the mucous coat of the stomach, and receiving their blood from the gastric arteries. A chemical analysis shows it to consist of water, mucilage, and the several free acids—muriatic, acetic, lactic, and butyric, together with a peculiar organic matter called pepsin, which acts after the manner of ferments between the temperature of 50° and 104° F.

The true process of digestion is probably owing to the action of pepsin and the acids, especially if the presence of the chloro-hydric or muriatic be admitted; since we know, by experiments out of the body, that chlorine, one of its elements, is a powerful solvent of all organic substances.

The antiseptic properties of the gastric juice, as discovered by experiments made on Alexis St. Martin, doubtless have much influence on digestion, although their true uses are probably not yet known.

As soon as the food is reduced to a state of fluidity, the pyloric orifice of the stomach is unclosed, and it is thrust onwards through the alimentary canal, receiving in the duodenum the secretions of the liver and pancreas, after which it yields to the lacteals its nutrient portion, and the residuum is expelled from the body.

There have been many hypotheses in regard to the nature of the digestive process. Some have supposed that digestion is a mere mechanical process, produced by the motion of the walls of the stomach; while others, in later times, have considered it as under the influence of a spirit separate from the individual, who took up his residence in the stomach and regulated the whole affair; while others still would make it out to be a chemical operation, and thus constitute the stomach a sort of laboratory. But to all these ridiculous hypotheses Sir John Hunter has applied the following playful language: “Some will have it that the stomach is a mill; others that it is a fermenting vat; and others that it is a stewpan; but in my view of the matter, it is neither a mill, a fermenting vat, nor a stewpan, but a stomach, a stomach!”

At the present day this process is regarded as a complex, and not a simple operation. It seems to be a process in which the mechanical, chemical, and vital agencies must all act in harmony and order; for if one of these be withdrawn, the function cannot be sustained for any considerable length of time; and of the chemical and mechanical parts of the process, since the former is much more important, and, as a matter of course, the vital powers are indispensable, therefore digestion may be considered as a chemical operation, directly dependent on the laws of vitality, or of life; since the proper consistency of the food depends, in a great measure, upon the character of the solvents, while the secretion of these fluids, their proper amount, 447 together with the peculiar instinct—as it almost seems to be—necessary to direct the stomach in its many functions, are exclusively and entirely dependent on the laws and conditions of life.

As food is necessary to supply the waste and promote the growth of the body, it follows that that will be the best adapted to the system which contains the same chemical elements of which the body is composed; viz., oxygen, hydrogen, carbon, and nitrogen. These elements are found in greater or less quantity in all animal food, and in many vegetable products. Hence, that article of food which contains all these elements in a proper proportion will tend much more to the growth and strength of the body than those kinds which are deficient in one or more of them. Much experience on this point, and scientific research, seem to show that a reasonable amount of animal food in health tends to give greater strength of muscle, and a more general sense of fulness, than in ordinary cases a vegetable diet is able to do, owing to the presence of nitrogen in animal tissues. Yet there are examples of the healthiest and strongest men, who live years without a morsel of animal food; and the fact can only be accounted for, by supposing that the system has the power to make the most economical use of the little nitrogen offered to it in the food; or else that it has by some means the power to abstract it from the atmosphere, and transform it to the living animal substance.

The proximate principles, which are the most important in nourishing the body, are albumen and fibrin. These constitute the greater part of all the softer animal tissues, and are also found in certain classes of vegetables, such as peas, beans, lentils, and many seeds. Hence, in many cases, a vegetable diet, especially if embracing any of those articles, would be sufficient to sustain life, even if no animal food should be eaten. But no animal can exist for a long time if permitted only to eat substances destitute of nitrogen, as in the case of a dog fed entirely on sugar, which lived but thirty days. And owing to this fact, Baron Liebig proposes to call substances used for food, containing nitrogen, “elements of nutrition,” and those containing an excess of carbon, “elements of respiration;” since, according to his view, the food is necessary to support the growth of the body by replacing the effete and worn-out particles with new matter, and also to keep up the supply of fuel, in order to promote a sufficient degree of heat in the system. Accordingly, under the first division would be included all lean meats and vegetables, such as peas, &c.; while the fat of animals, vegetable oils, sugars, tubers, (as the potato,) and all other substances containing starch, would be included under the latter division.

This definition of exhalants is from the theory of Haller and others. It is now believed that the fluids exude through the thin coats of the blood vessels. This process is called exosmose, and is the exhalation of old physiologists.

It is a well-established fact, in animal and vegetable physiology, that membranes possess the property of allowing fluids and gases to pass through them in either direction, and also to permit two fluids to pass in opposite directions at the same time. This property is designated endosmose when a fluid passes from without a body inward; and exosmose when the reverse takes place. The first is called imbibition. One of the most striking instances of this, in the human system, is shown in the lungs, where carbonic acid and water pass out through the mucous membrane of the bronchial tubes and air-cells; and the oxygen of the air enters the blood through the same membrane. By this process of imbibition, the oxygenation of the blood is much more readily and faithfully accomplished; inasmuch, as by the immense number of bronchial tubes and air-cells a larger quantity of blood is exposed to a greater portion of air, than if the blood were directly laid open to the atmosphere in a mass, or the air were immediately transmitted through it.

Since the function of respiration is to free the system of superfluous carbon and hydrogen, by union with the oxygen of the air, it follows that the greater the amount of the products to be expelled, the larger the quantity of oxygen will be required to effect this purpose, as we find to be the case with those who consume large quantities of food.

The quantity of oxygen daily consumed through the lungs by an adult is about 32.5 oz., and the carbon in the food 13.9 oz. But in order to convert this whole amount of carbon into carbonic acid, which passes off through the lungs and skin, 37 oz. of oxygen are required; the remaining 4.5 oz. being absorbed by the skin. If the supply of food remain the same, while the amount of oxygen in the inspired air is diminished, the superfluous carbon will induce disease in the system, as is the case of those persons who are limited in their supply of air of a proper quality or quantity, and, consequently, have less appetite for food than those who are abundantly supplied with air of the proper standard of health; and in children, who proportionally consume more food than adults, and who are more active, thereby causing a more rapid circulation of blood, and, consequently, the removal of more superfluous particles of matter.

In children we notice the need of air, by their disposition to be much in the open air, and often inspiring more deeply than is common in older persons. 449 Also, if the carbon of the food does not have a requisite supply of oxygen from the air, or other sources, the body becomes emaciated, although nourishing food may be used. And on the other hand, if there be a diminished supply of food, but an abundance of atmospheric air, leanness and emaciation are sure to follow; owing to the fact that if the oxygen has no waste carbon from the body to unite with, it combines with the fat, and some other soft portions of the body, which the Author of nature seems to have provided for this very purpose; as is seen in the case of hibernating animals, who enter their places of winter abode sleek and fat, but crawl out in the spring not merely deprived of their fatty matter, but also with great diminution of all the softer parts, which have given up their share of carbon to supply animal heat. One important cause of emaciation in febrile diseases is the greater rapidity of the pulse and respiration, which consume more carbon than is afforded by the scanty supply of food that is taken, although profuse perspiration, which almost always occurs in some stages of fevers, greatly diminishes the full state of the body.

The theory of Baron Liebig concerning the change which the blood experiences in color, in its passage through the lungs, meets with the approbation of many physiologists, although there are some important difficulties in the way of fully receiving it. A chemical analysis of the blood shows it to be composed of albumen and fibrin, together with some other substances, in small proportions, and always perceptible traces of iron. He attributes the change in color to the iron, as this substance enters into combination with carbon and oxygen. For, as the blood passes through the trunks of the larger vessels and capillaries, it receives the carbon from the tissues which are continually transformed, and taking