Absorption (Lat. absorbere, to suck up). I. The process by which nutritious and other fluids are imbibed by animal and vegetable tissues, to be appropriated for their growth, activity, or modification. All the organized membranes and tissues of the living body have the property of absorbing, to a certain extent and under favorable circumstances, the fluids which are brought in contact with them. This property continues to belong to the tissues in question even after the death of the body, or after they have been separated from all connection with the neighboring parts, until their natural structure and composition have begun to be altered by the effects of decomposition. Thus a dried ox bladder will absorb water in which it is immersed, and again become moist and supple; and even microscopic cells and fibres will absorb coloring matters with which the vessels of the tissue have been injected. This shows that the power of absorption resides in the substance of the animal tissue or membrane itself, and not in any property communicated to it from the rest of the system.

Nevertheless, although the capacity for absorption still exists in a separated membrane, it is much less active than in the same tissue durina: life, for the reason that after death it soon comes to an end by the saturation of the membrane by the absorbed fluid; while during life it is kept in a constant state of activity by the incessant renewal of the fluids and the movement of the circulating blood. In the process of absorption, as it takes place in animal organizations, the fluid does not penetrate the tissues mechanically, by openings or orifices, however minute. The existence of such orifices, or open absorbent mouths, was formerly taken for granted, as the most convenient way of explaining the phenomenon; but later and more complete microscopic examination has failed to show their existence, and takes away all reasonable grounds for the assumption. So far as we can decide upon a question of such delicacy, absorption consists in the imbibition of a fluid by the solid tissue in such a manner that the fluid and its ingredients unite, or combine directly with the substance of the tissue; so that the union which results is not simply a mechanical entanglement, but rather an intimate and complete molecular combination of the two. - It is found that different animal substances have the power of absorbing different liquids in different proportions.

Thus an animal membrane which will absorb in a given time 100 parts by weight of pure water, will absorb only 65 parts of a saline solution; and this difference will be greater, within certain limits, the stronger the saline solution is made. A tissue which will absorb 100 parts of a saline solution will take up under the same circumstances only 24 parts of an oily liquid. Thus the activity of absorption varies with the same membrane for different liquids, and with the same liquid for different membranes. Chevreul found the following results by measuring the exact quantities of different liquids absorbed by different membranes and tissues in the same time:

100 PARTS OF

WATER.

SALINE SOLU'N.

OIL.

Cartilage.....................

absorb in

24 hours

231

parts.

125

parts.

Tendon ........

178

"

114

"

8.6

parts.

Elastic ligament............

148

"

30

"

7.2

"

Cartilaginous do.

319

"

3.2

"

Cornea.......................

461

"

370

"

9.1

"

Dried fibrine.... ..........

301

"

154

"

Thus, if the same membrane be brought in contact with a liquid containing at the same time a variety of different substances in solution, some of these substances will be taken up in greater abundance than the others; and the membrane accordingly will appear to exercise a kind of discriminative power or selection between these different substances. This power of selection, however, is simply the property, dependent on the natural structure and constitution of the membrane, of absorbing particular substances in certain fixed proportions, which proportions vary for different materials. - The activity of absorption varies also with other conditions. One of these is the freshness of the animal membrane. While still connected with the neighboring parts, or but recently separated from them, the activity of absorption is great, and a comparatively large quantity of fluid is taken up in a short time. Afterward, when the natural constitution of the membrane is already impaired by commencing decomposition, this activity diminishes, and at last disappears altogether. Another condition of some importance is that of pressure. An increased pressure upon the liquid will enable the membrane to absorb it more rapidly. Pressure and motion combined are still more effective.

Thus a medicinal ointment or lotion acts more rapidly and powerfully upon the parts if it be made to penetrate the integuments by brisk rubbing than if it be simply laid in contact with the surface of the skin. Temperature also is of considerable importance. A low temperature is unfavorable to absorption; a high temperature, at least within moderate limits, is favorable to it, and increases its activity. A state of complete liquefaction or solution of the material to be absorbed is essential. A substance which is in the solid form cannot be absorbed; it must first be dissolved either in water or some other appropriate menstruum, after which the solvent fluid and the substance dissolved may both be absorbed, though in different proportions. Even the gaseous ingredients of the atmosphere, which are absorbed in the lungs, are first dissolved in the animal fluids which bathe the respiratory passages, and are then absorbed in the liquid form by the pulmonary membrane. The last and most important condition of the continued activity of absorption is that by which the materials already absorbed by the animal membrane are constantly removed from it, so that it is always ready to take up a fresh supply.

If an animal membrane have on one side of it a liquid rich in absorbable materials, and on the other a liquid which is poor in these materials or destitute of them, it will take up these substances from the first liquid, and the second liquid will again absorb them from it. Thus the membrane will not become saturated, but will retain its activity of absorption until the second liquid has approximated in composition to the first, In this way a large quantity of material may pass through the membrane, from the first to the second liquid, combining with the substance of the membrane in its passage, but being constantly taken up by it on one side and discharged on the other. This process will be more active and long continued, the larger the quantity of the two liquids and the greater the difference in composition between them. It will also be more active, the greater is the extent of surface over which the liquids reciprocally come in contact with the membrane, since it is the absorptive power of the membrane itself which is the primary condition of the interchange of substances between them.