Prop. VI.—That while in the blood, the medicine may undergo change, which in some cases may, in others may not, affect its influence. That these changes may be—
a. Of Combination.
b. Of Reconstruction.
c. Of Decomposition.
Before advancing to the consideration of the modes in which medicines operate in the cure of disease, it is of importance to mention that some of them are liable to undergo changes in the animal organism, because in particular cases these changes may materially affect their action, and in all cases they have some bearing upon it. With a view to this point I have divided these changes, somewhat arbitrarily, into three kinds,—of which the first does not hinder the effect of a medicine—the second may alter it slightly—and the third entirely changes or neutralizes it. Some of these change have to be considered more at length in the progress of the Essay, so that here I will only give an outline of them.
Changes of Combination.—The chief, and indeed almost the only way of detecting changes in medicines, is by taking notice of their effect upon the composition of the secretions.
Now in the blood we have a slight excess of alkali; in the urine, an excess of acid. Acids and alkalies are often given as medicines; and as it is not right that there should be much excess of either in the blood, both are generally soon neutralized in the system, and reduced to salts. This may sometimes occur before absorption, but perhaps more often after it. Now, it may seem strange for me to say that this neutralization does not destroy their influence. But it evidently does not; for in being neutralized they diminish in the blood, and in the system generally, the quantity of basic or of acid matter, and thus tend to alter the reaction of the secretions. Though an acid may combine in the blood with Soda, or with salts of Soda, yet by so doing it causes an excess of some other acid, probably an animal acid, which, being set free, acts on the secretion of urine much in the same way that the first acid would have done. It is easier to render the urine alkaline than to make it acid, for two reasons; first, as I shall show hereafter, an acid may pass out through other glands besides the kidneys; and secondly, an alkali is not so easily neutralized, either before or after absorption, the blood being already more or less alkaline.
A large quantity of acid would easily overcome the feeble reaction of the blood, and thus, by remaining free, Sulphuric and other acids are enabled to act as astringents on certain of the glands.
Other changes of combination have already been shown to take place during the process of absorption. Alkalies and their carbonates are more or less neutralized by the stomach acid. Substances soluble in alkalies are probably absorbed in such solution. Calomel, Chalk, Magnesia, and metallic oxides, as well as other insoluble medicines, are taken up in the soluble form, in which only they are capable of acting. But we are now concerned with medicines in the blood. And here I must protest against the idea that all chemical affinities have free play in that liquid. If it were so, many of our most valuable medicines would be decomposed and rendered insoluble there. All the mineral salts would be precipitated by the free Soda in that fluid. Acetate of lead would be immediately decomposed by sulphates, and Nitrate of Silver precipitated by chlorides. It is probable that the vital forces, as well as the viscidity of the plasma, exert a retarding or controlling power over such chemical tendencies.