The principal affirmations which I have to make on this subject are divisible into Ten Propositions, as seen in the Table of Contents.
The first four of these concern the general conduct of medicines after their introduction into the stomach, and before their passage into the blood. Some broad rules are laid down by which the course which they take must be determined. The action of some few on the mucous membrane is also defined.
The remaining six propositions treat of the subsequent behaviour of those medicines which pass into the blood and fluids of the body. Of these, the fifth specifies their general course. The sixth states that they may undergo certain changes in the system. And the concluding four treat of the various modes in which these agents may operate in the cure of disease.
The first proposition lays down the great fundamental rule of the action of medicines through the medium of the blood and fluids.
Prop. I.—That the great majority of medicines must obtain entry into the blood, or internal fluids of the body, before their action can be manifested.
This is to say, that the mere contact of a medicine with the stomach is not in general sufficient for the production of its peculiar action. It will be seen that the only apparent exception to this rule consists of agents having a mere local action on the mucous membrane, for which simple contact is all that is required.
Even when acting on any part of the system removed from this mucous surface, as when applied to the skin, it is necessary that the medicine pass away from it to enter the blood or internal fluids. In the great majority of instances it enters the blood directly. But we know that it would be sufficient for its operation if it were to enter through the chyle, or into the serous fluid which exists in the interstices of the tissues throughout the body. For by these it might at length be conducted to distant parts. This is what is meant by internal fluids.
We are not just now concerned with the way in which this passage into the blood and fluids is obtained. It is by the process of absorption. The question of passage by absorption is treated in the second and third propositions. What we have now to decide is, whether a medicine acts by mere contact with the stomach, its influence being propagated to distant parts by means of the nerves; or by passage thence into the system, acting through the medium of the blood and fluids. I affirm that it operates in this latter way; and this is to be proved by four considerations, which we will consider in succession.
A medicine introduced into the system elsewhere acts in the same way as when introduced into the stomach.