ANIMAL CHEMISTRY.SECTION TWO.
Composition of the Digestive Organs and of their Secretions—Chemical Phenomena of Digestion.
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Mucus.—The living membrane of the alimentary canal is moistened with a liquid possessing many of the characteristics of vegetable mucus, but containing nitrogen. It is a thick, tenacious substance, which contains, dissolved in the water through which it is diffused, the ordinary salts of the serum of the blood; it swells up with water to a considerable mass, but without dissolving; it dissolves in alkaline liquors, and is precipitated therefrom on the addition of an acid and the tincture of galls; the mucus from different parts of the mucus membrane is, however, by no means identical in properties.
The liquid secreted by the internal surface of the stomach—the gastric juice—which exercises an important influence on digestion, differs essentially in its character from mucus. When the stomach is empty and contracted, it contains ordinary mucus; but if even indigestible substances are introduced, and still more, after taking proper food, a liquid is abundantlypoured out, which is colorless or pale yellow, and contains a very small quantity of solid matter (two per cent.), which consists principally of inorganic salts (common salts and sal ammoniac, with a trace of a salt of iron); it is specially characterized by the presence of a notable quantity of free muriatic acid, the proportions of which vary with the activity of the digestive powers at the time. This gastric juice possesses the remarkable property of softening down and dissolving fibrine and albumen, and thus converts the masses of food into the uniform pulp (chyme), from which the absorbing vessels of the small intestines take up the nutritious elements.
If we form an artificial gastric juice by mixing together the muriatic acid and salts in the right proportions, it is found to be totally incapable of dissolving the materials of the food, and, indeed to be quite inactive towards digestion. The organic material of the gastric juice, although its quantity be so minute, is, therefore, essential to its powers, and these may be perfectly conferred upon the previously inactive, artificial juice, by the addition of a little of the mucus of the stomach or by steeping in the acid liquor, for a short time, a small portion of a mucous membrane, and filtering the liquor; for this purpose it is not even necessary to use the mucous membrane of the stomach, for that of the bladder has been found to answer equally well. The substance which is dissolved out of the membrane in these cases has been termed pepsine. It has not been obtained in a truly isolated orpure form, but its properties are very remarkable. For its full activity it requires the presence of a free acid, as the artificial gastric juice becomes much less active in dissolving food when neutralized by an alkali, though it retains other properties, as that of coagulating milk-like rennet. If the artificial juice be precipitated by nitrate of lead, the precipitate washed, and then decomposed by sulphuret of hydrogen; the solution thus obtained possesses all the digestive powers of the juice. Hence, the pepsine and muriatic acid act together, by combining with oxide of lead.
Pepsine appears to be completely decomposed by contact with alcohol or boiling water; its powers are also destroyed by deodorizing substances; the solution of albumen and fibrine in gastric juice differs essentially from their solution in muriatic acid, as in the former case the quantity of acid is very minute, in relation to the quantity of material dissolved, and after solution the acid remains quite uncombined.
The action of the stomach in digestion, appears, therefore, so far as our actual knowledge extends, a purely catalytic fermentative action; one in which the active excitant is an organic substance (pepsine), secreted by the mucous surface, and whose properties are developed by the presence of muriatic acid, which is secreted at the same time. The new products into which the food, fibrine, albumen, gluten, starch, oils, sugar, etc., are converted, and which collectively constitute the white uniform pulp, termed by physiologistschyme, have not been made the subject of accurate chemical researches.
In the mouth the mass of nutritive material is acted on by a liquid which is secreted by the salivary glands, the saliva. It is alkaline, and holds in solution not one per cent. of solid matter, which contains some carbonate of soda and common salt, admixed mucous, and a peculiar organic body, termed salivary matter.
This last substance is soluble in water; its solution is not coagulated by heat, nor precipitated by tincture of galls, corrosive sublimate, acetate of lead, nor by acids. The pancreas, so similar in structure to the salivary glands, has a different secretion; it contains no salivary matter, but albumen and some salts; it is generally slightly acid.
Composition of the Bile.—The precise part which this remarkable secretion performs in the animal economy is not yet fully known; it has been the subject of repeated and accurate chemical examination, although, from the facility with which its elements are transferred into other bodies, by the action of the reagents employed, every succeeding analysis has led to different results.
The Coloring matter of the Bile.—is present during health but in small quantity, but in disease it sometimes accumulates so as to form solid masses. When pure, it is a reddish-yellow powder, which is scarcely soluble in water or in alcohol, but dissolves easily in a solution of caustic potash. This solution is of aclear yellow color, but when exposed to the air it becomes deep green, absorbing oxygen. This change is remarkably produced by nitric acid, and it is indeed the reaction by which the presence of the bile in the serum of the blood, in the skin, in the urine, and eyes, etc., may be shown in cases of jaundice.
Chyle and Lymph.—The nutritive materials extracted from the food by the absorbing vessels of the intestines, is thrown into the thoracic duct, where it meets with another fluid, which is transmitted to the same vessel from all parts of the body by the colorless veins or lymphatics. The fluid from the intestines is termed chyle; that from the body is generally termed lymph. It is the mixture of these that has alone been examined, for the vessels which carry either separately are too minute to allow of the extraction of their contents in a pure form.
When taken from the thoracic duct, a few hours after a meal, when, probably, the chylous element prevails, it is whitish, opaque, liquid, like milk, with generally a reddish shade; a short time after separation from the body, it coagulates; the clot is at first pale, but it soon becomes light crimson red; the milkiness of the serum is due to the presence of oil; it contains albumen, and coagulates by heat; except that it is more dilute, and that the hematosine is for the most part absent, the chyle and lymph have the same composition as the blood. It appears to vary, however, with the nature of the food, as Dr. Prout found the chyle of persons fed on vegetables to containa much smaller quantity of albumen than when they had had animal food.
Dr. Prout also indicates in chyle the presence of a substance which he terms incipient albumen, which is not coagulated by heat, except after the addition of acetic acid; the properties of this form of albumen, however, are not fully known.
Constitution of the Urine in Health and Disease.—The nature of this secretion has at all times been an object of considerable interest to the chemist, from the indications which changes in its composition give of diseases of important organs and from the number and interest of the different organic substances it contains. As in almost all other branches of animal chemistry, Burzelius first determined its composition, and lately Lecaner has ascertained with great care the limits to which the proportions of its ingredients may vary in health, and this established a correct basis of comparison for urine in the various conditions of diseases.
Of the Urine in Disease and after Death—Urinary Calculi.—To the chemist, the indications of disease of the urinary and digestive organs, formed by changes in the composition of urine, are most valuable. The majority of the substances which are taken into the circulation, but are incapable of assimilation to our organs, are thrown off by this secretion, and hence a variety of medicinal substances may be traced to it after having been ingested, sometimes quite unaltered,at others modified in their natures. Thus if alkaline salts of organic acids be taken into the stomach, the organic material is oxidized, probably during the action of respiration, while the alkali passes into the urine in the state of carbonate. If, however, the organic acid be taken uncombined, it escapes decomposition, and, passing into the urine, produces an abundant precipitate of salts of lime. In the case of the tartaric acid and oxalic acids, some organic bodies, as aspharagine and the oil of turpentine, are decomposed, and the products which they form are execreted, giving to the urine peculiar odors; in the latter case like that of violets.
The majority of coloring matters are thrown out of the system by this secretion, while others are not so given off.
The mineral acids—alcohol, camphor and most metallic salts—do not pass into the urine to any sensible degree.
Urine in Diabetes.—The most remarkable change in the nature of urine occurs in Diabetes Mellitus, it is voided in great quantity; it is found to contain a great quantity of grape, sugar, and very little urea.
It was supposed that in this disease urea ceased to be formed by the system, and was replaced by sugar; but it has been shown that, although the quantity of urea is very small in any one specimen of urine, yet the total quantity is so much increased that in twenty-four hours the natural quantity of urea is secreted; the secretion of sugar being an act of faulty digestion,and totally unconnected with the urea. These results have been fully confirmed by experience.
The diabetic urine sometimes contains albumen, which arises from complications of other forms of disease.
All that has been said in the former chapter about the solid and fluid constituents of the human body may, at first sight, and to a great many, seem to be superfluous and out of place in a work of this kind. It is true that the different modes of preserving bodies, as explained in this book, do not require this long dissertation on animal chemistry in order to be understood; still, when we consider that the chemicals used in these different processes have an object to accomplish, it must be granted that a thorough knowledge of the constituents of the body, their composition and chemical proportion, will, to a great extent, explain the reason why these same chemicals are used in preference to others.
The secondary object, which is not less important, consists in the fact that a thorough knowledge of the animal chemistry of the human organism is most necessary to understand the different changes which take place in the formation of the different juices and tissues of the body, when they enter into combination with the chemicals, the object of which is to render them imputrescible.
However, the study of these combinations affords a simple and clear explanation of the means resorted to in order to preserve bodies.