with those of the other and diffusion takes place. If certain solids containing fluids are brought in contact with other liquids, some of the liquid passes into the solid and absorption takes place. Gases may also be absorbed similarly. Diffusion acting through an animal or vegetable membrane is called osmosis. Much of what is termed absorption in physiology is really osmosis. Most of the tissues of living bodies have the power of absorbing fluids — a property that often continues after death and until decomposition. Animal substances differ in absorbing power according to differences in the liquid, notably if they differ in specific gravity and if the fluids in the substances brought in contact are miscible. The following table from Chevreul shows the amounts of liquid absorbed by different substances in twenty-four hours:
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Parts of
Cartilage Tendon Elastic ligament Cartilaginous ligament Cornea Dried fibrin Parln of Saline ^Vater. Solution,
178 148 31!) 4(il 301
30
154 OU. .6 7.2 3.2 9.1
Activity of absorption, or osmosis, varies with the freshness of the membrane, being great soon after separation from the principal parts; and varies also with pressure, motion, and temperature. Absorption of oxygen by the blood in the lungs is apparently instantaneous, the change in color from dark red to bright red as goon as it arrives at the pulmonary vessels, showing the action of the gas it has taken from the atmosphere. This rapidity of absorption is due to the fact that in the circulation of the lungs the blood is spread out in the fine capillaries over a very large area, and to the incessant motion of the blood in the capillaries. Claude Bernard found that if a solution of iodide of potassium were injected into the duct of the parotid gland on one side of a living animal, the saliva discharged by the corresponding gland on the other side almost instantly afterward contained iodine. In a measureless instant, therefore, the iodine was taken up by the glandular membrane on one side, absorbed by the blood, carried to the heart, absorbed from the blood by the glandular membrane on the other side and furnished to the saliva. It is by this process of absorption that the elements of nutrition are taken from the intestines and conveyed to the tissues they are to nourish; the bones absorb much calcareous matter from the blood, cartilages less, and muscles less still; the brain takes more water than does muscle, and muscle more than bone. The active principles of drugs and poisons are dissolved by the juices in the stomach, and by osmosis pass, unchanged or slightly modified, into the circulation. (See Lacteals; Lymphatics.) Opium dissolved by the liquids of the stomach is absorbed by the membranous lining, taken away by the blood and distributed well through the body: at the brain it acts on the brain cells and produces sleep or narcotism or insensibility. The quickness of absorptive action is shown in using hypodermic injections; a few moments after the syringe has punctured the skin of the forearm a severe pain in the foot is sensibly relieved.
ABSORPTION, In Plants. The process by
which substances are taken into the body. A
few plants only, being devoid of any external
cover to the protoplasm, are able to engulf par-
ticles of food, which may then be digested. The
most prominent of these are the Myxomycetes
(q.v.), or slime molds, which in the period of
their vegetative activity consist of a mass of
naked protoplasm (called a plasmodium), some-
times as large as one's two hands. These plas-
modia, like huge Amœba (q.v.), creep about
and envelop particles of decaying organic matter,
etc., on which they feed. The zoöspores, or
reproductive bodies, of some Algæ and Fungi
are also microscopic bits of naked protoplasm,
but they probably do not ingest solid food during
this period. Inasmuch as the protoplasm of
most plants forms on its surface, as the first
step of development, a thin jacket of cellulose
or some similar material, the taking up of solid
substances is thereby absolutely prevented.
Whether the body consist of one cell or many,
it presents to the surrounding medium a contin-
uous membrane with no visible openings.
Through these cell-walls, therefore, neither solid
nor gaseous substances can pass without pre-
viously undergoing solution. The materials
whose absorption is to be explained are (1)
dissolved substances or solutes, and (2) the
solvent, water.
(1) Solutes. The protoplasm itself and its surrounding membrane (the cell-wall) contain a large amount of water (50 to 98%). This water may be conceived of as lying between the particles of which the subslanees named are composed, much as it stands between the close- set stalks of plants in a marsh. Since water always pervades the structures of plants, sub- stances in order to enter the plant body must be soluble in water. When so <lissolved they behave essentiall,v as gases: their molecules, being then free to move apart, tend to distribute (hemselves equally throughout the solvent. But the diffusion of solufes is greadv retarded by the molecules of the water, so that it is much slower than the similar diffusion of gaseous bodies. It is also retarded somewhat by the particles of cell-wall when these also are encoun- tered in the water. But the distances between the particles of the cell-wall are relatively so great that most solutes are able to pass freely between them. The structure of the protoplasm, however, is such that many sil)s(ances cannot readily pass (hrough it. Consequently, some materials which can enter the i>laiit body may travel only through the cell- walls and m;iy never enter the living protoplasm. Tho protoplasm permits at some periods substances to pass through it which at other times are excluded: probably due to ability to alter its structure on occasion. Such substances as can pass through the invisible spaces in cell-wall and protoplasm are therefore free to travel to any part of the plant body. If any such substances be removed from solution through use or storage, they will continue to be supplied from the regions of greater abundance, and consequently of greater pressure, to the regions of lesser pressure, i.e.. where the,v are being used. The fact that different amounts of a. given compound enter plants growing in the same soil is explic-
