750. 2. Albuminous Secretions.—All the close cavities, as the thorax, the abdomen, the pericardium, the ventricles of the brain, and even the interstices of the cellular tissue, are constantly moistened by a fluid which is termed serous, because it is derived from the serum of the blood. This serous fluid consists of albumen in a fluid form, and it differs from the serum of the blood chiefly in containing in equal volumes a smaller proportion of albumen. Membranes of all kinds consist essentially of coagulated albumen; and the albumen, as constituting these tissues, differs from albumen as existing in the serum of the blood only in being unmixed with extraneous matter, and in being in a solid form.
751. 3. Mucous Secretions.—As all the close cavities, or those which are protected from the external air, are moistened with a serous fluid, so all the surfaces which are exposed to the external air, as the mouth, the nostrils, the air-passages, and the whole extent of the alimentary canal, are moistened with a mucous fluid. Mucus does not exist already formed in the blood. It is always the product of a gland. Some of the mucous glands are among the most elaborate of the body; still the main action of the gland seems to be to coagulate the albumen of the blood, for the basis of mucous is coagulated albumen. The fluid that lubricates the mucous surfaces in their whole extent, the saliva, the gastric juice, the tears, the essential part of the fluid formed in the testes and in the ovaria, are mucous secretions. Hence the most complex and elaborate functions of the body, respiration, digestion, reproduction, are intimately connected with the mucous secretions: nevertheless, as far as regards their chemical nature, the mucous differ but slightly from the albuminous secretions; and it is probable that a slight change in the secreting organ is sufficient to convert the one into the other. By the irritation of mercury on the salivary glands, the saliva, properly of a mucous, is sometimes converted into a substance of an albuminous nature; and irritation in some of the serous membranes occasionally causes them to secrete a mucous fluid.
752. 4. Gelatinous Secretions.—The proximate principle termed jelly abounds plentifully in several of the solids of the body, and more especially in the skin; but jelly does not exist already formed in the blood. Yet it is not the product of a gland, neither is there any known organ by which it is formed. Out of the body albumen is capable of being converted into jelly by digestion in dilute nitric acid: this conversion is probably effected by the addition of a portion of oxygen to the albumen. Albumen contains more carbon and less oxygen than jelly; the proportions of hydrogen and nitrogen in both being nearly the same. According to MM. Gay Lussac and Thénard, the elements of albumen and jelly are,
| Carbon. | Oxygen. | Hydrogen. | Nitrogen. | |
|---|---|---|---|---|
| Albumen | 52.883 | 23.872 | 7.54 | 15.765 |
| Jelly | 47.881 | 27.207 | 7.914 | 16.988 |
The conversion of albumen into jelly is incessantly going on in the system; and the process accomplishes most extended and important uses. In the lungs at the moment of inspiration oxygen enters into the blood in a state of loose combination; but in the system, at every point where the conversion of albumen into jelly takes place, oxygen probably enters into a state of chemical combination with albumen; and the new proximate principle, jelly, is the result. The agent by which this conversion is effected appears to be the capillary artery: the primary object of the action is the production of a material necessary for the formation of the tissues of which jelly constitutes the basis, as the skin; but a secondary and most important object is the production of animal heat; the carbon that furnishes one material of the fire being given off by the albumen at the moment of its transition into jelly; and the oxygen that furnishes the other material of the fire being afforded to the blood at the moment of inspiration. This view affords a beautiful exposition of the reason why jelly forms so large a constituent of the skin in all animals. The great combustion of oxygen and carbon, the main fire that supports the temperature of the body, is placed where it is most needed, at the external surface.
753. 5. Fibrinous Secretions.—The pure muscular fibre, or the basis of the flesh, is identical with the fibrin of the blood. It contains a larger proportion of nitrogen, the peculiar animal principle, and is consequently more highly animalized than the preceding substances. It appears to be simply discharged from the circulating blood by the capillary arteries, and deposited in its appropriate situation; no material change in its constitution being, it would seem, necessary to fit it for its office.
754. 6. Oleagenous Secretions.—Fat of all kinds, which is found so extensively connected with the muscles, and with many of the viscera, and which is more or less diffused through the whole extent of the cellular tissue, marrow, milk, and nervous and cerebral matter, are essentially of the same nature. The basis of them all is oil; and oil exists already formed both in the chyle and in the blood.
755. 7. Resinous Secretions.—The peculiar substance forming the basis of bile, picromel; the peculiar substance forming the basis of urine, urea; the peculiar substance connected with the muscular fibre, and forming a component part of almost all the solids and fluids of the body, osmazome, consists of a common principle—a resin, which exists already formed in the blood, and more especially in the serosity of the blood.
756. 8. Saline Secretions.—The substances termed saline, namely, the acids, the alkalis, and the neutral and earthy salts, are disposed over every part of the system: they enter more or less into all the constituents both of the solids and fluids; they form more especially the phosphate of lime, the earthy matter of which bones are composed; and they all exist already formed in the blood.
757. From this account, then, it appears, that by chemical analysis, the blood is ascertained to contain water, albumen, fibrin, oil, resin, and various saline and earthy substances: it follows, that, with the exception of the absence of jelly, the constituents of the body and the constituents of the blood are nearly identical; and it is probable that they will be found to be perfectly identical when their analysis shall have become complete.