ABSORPTION.

Absorption, by which fluids are introduced into the vascular system, depends on the laws of capillary attraction. The phenomena called endosmose and exosmose are also examples of the same widely applied principle, for the pores through which the fluids interchange are but short capillary tubes. If a fine tube be placed in water, that fluid will rise within it to a height proportional to the fineness of its calibre; and if the tube be broken off just where the water has risen, it will not overflow, but if removed by any means, will be succeeded by a fresh portion. This form of attraction depends, it would seem, on conditions of electric or chemical affinity, as mercury will not rise within the tube, and oil will rise very much higher if the tube be previously wetted with bile, which is therefore said to promote absorption. Tubes will also take up fluids more easily if they have been carrying the same fluid before and are still wetted with it, and this may explain what is often regarded as a power of selection. The wick of the oil lamp illustrates many of the facts we have been stating; the spaces between the threads are so many capillary tubes, which when filled do not overflow; but if the oil be removed, as by burning when the lamp is lighted, a continuous current is produced. Again, if the wick be soaked with water or oil, and placed in both liquids, one above the other, it will only take up that with which it was previously wetted, thus showing a kind of selective absorption. Water will pass through tubes or crevices so fine, as not to be perceived by the microscope even those ʊʊʊ in diameter; but if the water be impregnated with various substances, its ready transmission will be prevented. Albumen, gelatin, sugar, alcohol, salts, &c., act in this way, in degree according to the order mentioned. It was Dutrochet who first found that

fluids would permeate animal membranes, in both a current inwards and outwards, which he respectively termed endosmose and exosmose. He likewise con

structed an instrument which indicates and measures the passage of the fluids. This endosmometer consists of a glass tube closed at one end by animal membrane, or any porous matter, as plaster of Paris; the tube was filled with one liquid and then placed in another of dif ferent specific gravity, when the rapidity of the interchange was a measure of the "endosmotic force." He thought that the two liquids having different specific gravities, the process went on till they became equalized, by the rarer one passing out and the denser in; but if alcohol be placed internally and water externally, the latter, the heavier fluid, will pass in, which it will also do if it become the lighter fluid, gum solution being substituted for alcohol. Graham regards endosmose as rather dependent on electro-chemical action, and it is so powerful that it will occur in resistance to great pressure, which, however, may be sometimes an aiding force. The size of the pores, the nature, thickness, and physical states of the separating membrane, as well as the nature of the fluids, produce great variety in the process of endosmose. That distinguished philosopher has applied these facts to a method of separating bodies in solution, which he terms "". 'dialysis." Those substances which readily pass through animal membranes he calls "crystalloids"-such are hydrochloric acid and common salt; those which refuse to dialyse he terms "colloid," and most of the plastic materials of which the body is built, as albumen and gelatin, belongs to this group. It is possible to ascertain the time which any given substance will take to diffuse, and in the words of Prof. Galloway, "such a constant must enter all chronic phenomena of physiology, and that it holds a place in vital science not unlike the time of the falling of heavy bodies in the physics of gravitation." Most physiologi

cal processes, as secretion, nutrition, respiration, as well as the introduction of poisons, the action of purgatives, &c., depend on the principles we have been endeavouring to explain.

Absorption may be either venous or lymphatic, and will be most conveniently described under the heads of Alimentary and Lymphatic. Ulcerative, progressive, and interstitial absorption, as described by Hunter, are morbid processes, and will be hereafter alluded to. We have already seen that most of the nutritive matters of the food, when converted into the soluble form of peptone, are removed from the cavity of the stomach by the veins which so abundantly ramify in its mucous membrane. The rapidity of this removal varies with a replete or an empty state of the blood vessels and stomach. Erichsen found that it took 39 minutes for ferro-cyanide of potassium to appear in the urine, if the stomach into which the salt was introduced was full of food; whereas it appeared in the almost incredible space of 1 minute if the stomach was empty. He performed his well-known experiments on an individual whose bladder opened on the anterior abdominal wall (ectropia vesica), as he could examine the urine the moment it trickled from the ureters. As intimately connected with stomachic absorption, we shall briefly allude to the actions of medicines, which have been admirably investigated by Headland, whose results are shown in the following propositions :

1. "That the great majority of medicines must obtain entry into the blood or internal fluids of the body before their action can be manifested." A vascular connexion is therefore necessary for the passage of the medicine, and will suffice when nerves and other tissues are severed. The enormous rapidity of the action of some poisons will be understood, when it is remembered that Blake found chemical substances could be transferred through the circulation of a dog in 9 seconds.

2. "That the great majority of medicines are capa

ble of solution in the gastric or intestinal secretions, and pass without material change, by a process of absorption, through the coats of the stomach and intestines, to enter the capillaries of the portal system of veins." Saline purgatives would form an exception, if, as Poisseuille stated, they acted by making the serum of the blood ooze out as the saline solution of higher specific gravity passed along. Headland has opposed this theory with many ingenious arguments and experiments, and shown that dense solutions will pass through a membrane to a lighter one (as we have stated with regard to water and alcohol), and that solutions of low specific gravity will produce purging.

3. "That those medicines which are completely insoluble in water, and in the gastric and intestinal juices, cannot gain entrance into the circulation." Esterlen, however, states that finely powdered charcoal, if given to rabbits, will pass through, and can be found by the microscope in the blood of the porta. No such results have been obtained after carefully conducted experiments. The structureless basement layer of mucous membrane would seem to be impermeable to solids, however minutely pulverized.

4. "That some few remedial agents act locally on the mucous surface, either before absorption or without being absorbed at all," as irritant emetics or cathartics, and superficial stimulants, sedatives, or astringents.

5. "That the medicine when in the blood must permeate the mass of the circulation, so far as may be required to reach the parts on which it tends to act."

6. "That while in the blood the medicine may undergo change, which in some cases may, in others may not, affect its influence"-such as combination, as when an alkali unites with acid of the gastric or intestinal juices; reconstruction, as when tannic acid changes into gallic;

benzoic into hippuric acid; or decomposition, as when the salts of vegetable acids are decomposed, rendering the urine alkaline, as explained by Wöhler.

Dr. Headland classifies and indicates the action of the most important medicines under the following heads:

I. Hæmatics. They act while in the blood, which they influence permanently-either by restoring a deficient material (restoratives, as iron in chlorosis) or by counteracting a morbid material or process (catalytics, as mercury in syphilis).

II. Neurotics, which act on the nervous system, either exalting its force (stimulants, as ammonia), exciting its powers, to be followed by their depression (narcotics, as opium), or depressing them directly (sedatives, as hydrocyanic acid).

III. Astringents, which act by causing contraction of the tisuses composed of albuminoids, especially muscular fibre, as tannic acid.

IV. Eliminatives, which act by exciting to increased secretion the glands whereby they pass away. Such are sialogogues, which increase saliva, as mercury, iodine, betel; expectorants, which increase bronchial mucus, as tartar emetic, hippo, squills; cathartics, which increase intestinal fluids, as jalap, elaterium, sulphate of magnesia; cholagogues, which increase bile, as mercury, manganese, taraxacum; diaphoretics, which increase perspiration, as antimony, nitre, guaiacum; and, lastly, diuretics, which increase urine, as water, salines, cantharides.

That many substances have their poisonous action destroyed by the action of the stomach is shown by many animals feeding on wourali, so fatal if inserted into a vein. The poison of the viper does not injure, it is said, snakes, tortoises, leeches, &c.

The peptone and sugar being removed from the food, the acidity of which promotes osmose towards the alkaline blood or chyle, there still remains to be absorbed