If we consider, therefore, the arterial tint to be due to the material combination of the coloring matter with the saline constituents of the serum, this will be darkened, when, by passing through the capillary system, the blood takes up an excess of carbonic acid; and again, in the lungs, when the carbonic acid is replaced by oxygen, the vermilion color is restored, not by any active agency of the oxygen, but by the natural tint of saline hematosine becoming evident.
Although this theory of the change of color is by no means free from objection, it still appears to be better founded than any other that has been proposed.
Animal Heat.—The phenomena of respiration consisting mainly in the conversion of carbon into carbonic acid by union with oxygen, the heat which is developed in the body of all red blooded animals has been naturally referred to that source; and as we know that the change from the arterial to the venous condition of the blood occurs at every point of the system, the almost complete equality of temperature throughout the body in health is explained. That the great source of heat is the respiratory process, is abundantly proved by the temperature being highest in those animals, and in the same animal, at those periods when the circulation is most rapid and the quantity of air consumed the greatest. But it has been calculated that the heat evolved by the combustion of the quantity of carbon thrown off from the body in twenty-four hours is no more than eight-tenths of the quantity generated in the body during that time, and the origin of the remainder must be found in the action of the muscles and the nervous power, which appears of itself to be a distinct source of animal heat.
ANIMAL CHEMISTRY.
SECTION TWO.
Composition of the Digestive Organs and of their Secretions—Chemical Phenomena of Digestion.
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 abundantly poured 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 or pure 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.