36 ABSOEPTION ticular 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 fresh- ness 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 com- mencing decomposition, this activity dimin- ishes, and at last disappears altogether. An- other condition of some importance is that of pressure. An increased pressure upon the li- quid 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 tempera- ture, at least within moderate limits, is favor- able to it, and increases its activity. A state of complete liquefaction or solution of the ma- terial 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 dis- solved may both be absorbed, though in differ- ent 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 pulmo- nary membrane. The last and most important condition of the continued activity of absorp- tion is that by which the materials already ab- sorbed by the animal membrane are constantly removed from it, so that it is always ready to take up a fresh supply. If an animal mem- brane have on one side of it a liquid rich in absorbable materials, and on the other a li- quid which is poor in these materials or desti- tute of them, it will take up these substances from the first liquid, and the second liquid will again absorb them from it. Thus the mem- brane will not become saturated, but will re- tain 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 sub- stance of the membrane in its passage, but be- ing 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 ex- tent of surface over which the liquids recipro- cally come in contact with the membrane, since it is the absorptive power of the mem- brane itself which is the primary condition of the interchange of substances between them. The most favorable condition for continued and active absorption would be that in which the two liquids were kept in constant mo- tion and incessantly renewed, so that the first one should never be exhausted of its materials, nor the second saturated with the substances transmitted to it. If, at the same time, the in- tervening membrane maintained its freshness, unaltered by the changes of decomposition, the process of absorption would go on with the most continuous and uniform activity. These are precisely the conditions, in fact, which are present in the living body. In the alimentary canal, for instance, during digestion, there are constantly passing over the lining membrane of the intestine the nutritious fluids which have been extracted from the food. A portion of these are absorbed by the lining membrane ; but, on the other hand, they are immediately taken up from it by the blood in its minute vessels. This blood, in the incessant move- ment of the circulation, is instantly carried away to another part of the body, its place be- ing taken by other portions of the current fol- lowing each other without intermission. The living membranes themselves are maintained at the same time in their natural condition by the nutritive process, the temperature of the whole is constantly at or about 100 F., the superfluf ous materials are decomposed elsewhere, or discharged from the body by the excretory passages, and new supplies are incessantly furnished as the gradual digestion of the food is accomplished. Experiments have shown that absorption will take place in the living body with considerable rapidity even in non- vascular tissues, or where it is not directly as- sisted by the circulation of the blood. It has> been shown by M. Gosselin that if a watery solution of iodide of potassium be dropped upon the cornea of a rabbit's eye, the iodine passes into the cornea, aqueous humor, iris, lens, sclerotic coat, and vitreous body, in the course of eleven minutes; that it will pene- trate through the cornea into the aqueous hu- mor in three minutes, and into the substance of the cornea in a minute and a half. In the vascular tissues, however, the rapidity of ab- sorption is often much greater than this. Thus the absorption of oxygen by the blood in the lungs is apparently instantaneous ; the change of its color from blue to red, as soon as it ar- rives in the pulmonary vessels, showing the action of the gas which it has taken up from the atmosphere. This rapidity of absorption in the vascular tissues is due to the dissemina- tion of the blood in a vast number of minute channels, by which the vascular and absorbing surfaces are brought into intimate contact over a large surface ; and to the incessant motion of the fluid, by which its effect becomes per- ceptible at the earliest possible time. It is in some of the glandular organs that this absorp-
