Circumstances affecting Absorption. Various circumstances more or less affect the facility or rapidity of absorption. Reference has already been made to the influence, in this respect, of the nature of the surface to which the medicine is applied. The following are other modifying influences.

* Isomorphism in chemical compounds has probably some influence over the absorption as well as the assimilation and elimination of those substances; those which have a similar constitution in this respect, obeying the same physiological laws. This may be inferred from the experiments of M. Roussin on fowls, as recorded in the Journal de Pharmacie, Se ser., xliii. 126. {Note to the third edition).

1. In relation to the blood-vessels, it is highly probable that the varying condition of their coats, under varying nervous influence, or different degrees of vital force, may considerably modify the process; but too little is precisely known on this subject to justify any definite statements.

2. The condition of the blood as to density may not be without effect, upon the principles of endosmose, in favouring or opposing the entrance of fluids from without.

3. The fact has been abundantly proved, that fulness of the bloodvessels is opposed to absorption; and hence, in a plethoric state of the circulation, medicines sometimes act with difficulty, because not readily taken into the system. It is well known that, in a state of high febrile excitement, the abstraction of blood very much favours the action of diaphoretics and diuretics, probably in part, at least, by removing an impediment to their absorption. Substances, too, which act powerfully as local irritants, by causing congestion of the blood-vessels in the part, impede their own absorption, and thus fail to act on the system; while, if applied in a dilute state, so as not to irritate, they may find a ready entrance. Diminution of atmospheric pressure upon the surface impedes absorption, by causing distension of the vessels. Consequently cupping-glasses, placed over a poisoned wound, delay or diminish the action of poison on the system. Compression, on the contrary, is said to favour the process. Whenever the blood-vessels are relatively empty, absorption is promoted. Hence, medicines act more powerfully after fasting, and in reduced states of the system generally, than in its ordinary condition.*

4. Age and sex have some influence on absorption. M. Briquet has found the process more active in the young than in the old, and in man than in woman. In the former case, the greater activity might be ascribed to greater rapidity of the circulation; but this explanation will not hold in the latter. (Archives Generates, 5e ser., x. 611).

5. Fodera has proved that galvanism or electricity promotes absorption.

* Some experiments recently made by Kohler, at Marburg, might seem to invalidate this conclusion. He found that, in starving animals, the symptoms of poisoning from strychnia, hydrocyanic acid, etc., to whatever surface applied, are retarded, instead of being accelerated. But this, if true, may be ascribed to other causes than to diminished absorption, as, for example, to the slower circulation of the blood, and possibly to modified susceptibilities in this condition of the system. (B. and F. Med.-chirurg. Rev., July, 1859, Am. ed., p. 168.) The nerve-centres, occupied by the agonies of starvation, would probably resist much more strongly than in their normal state impressions from the poison, just as violent neuralgic pains impede the action of anodyne remedies. (Note to the second and third editions).

6. The character and state of the medicine itself have also great effect. Allusion has been already made to the influence of density in opposing the absorption of medicines. Volatility appears in general to have an opposite effect; the most volatile medicines usually operating with greatest rapidity. Hence in part probably the very speedy effect of ether, chloroform, and hydrocyanic acid. Gases are readily absorbed.

7. Greater or less affinity between the medicine and the blood appears also to affect the facility of absorption; and substances which unite with that fluid with difficulty, are scarcely admitted into the system. Castor oil may be cited as an example.

8. The fluid form is usually considered essential to absorption, and consequently to medicinal activity, so far at least as the general system is concerned. It is true that many solid substances, nearly or quite insoluble in water, are powerful medicines. But they must undergo changes which bring them into the liquid state, before they can gain access into the blood-vessels. Such changes are often effected by the liquids of the alimentary canal. Thus, metals, in themselves insoluble and inert, often act energetically in consequence of oxidation and union with an acid in the stomach and bowels. But some recent experiments would seem to show that this principle is less universal than has been imagined. Rabbits were fed on finely powdered wood charcoal; and fine particles of this substance were afterwards found abundantly in the blood, the liver, and the lungs. Starch mixed with charcoal was given to frogs; and the corpuscles of the former were observed in the blood of the mesenteric veins, by the aid of the microscope, having their character-istic form, and exhibiting their characteristic reaction with iodine. Similar results, though less decisive, were obtained with mercury and sulphur, taken into the stomach, and applied by friction to the surface of dogs and rabbits* It is conceivable that mercury, being liquid, though insoluble, should be able to penetrate the tissues; but it is impossible, with our present views of the capillaries, that solid visible particles should enter them without some solution of continuity in their coats. It is not improbable that such particles, passing mechanically between the epithelial or epidemic cells, and thus separated from the blood of the capillaries only by the extremely tenuous coats of these vessels, and the equally tenuous basement membrane, may, by their contact with this delicate tissue, cause minute openings in it, so as to admit their passage, by a sort of physiological action such as that which causes two cells in contact to communicate together; and these openings, after the entrance of the particles, may close under the influence of the same vital law.