PROP. II.--_That the great majority of medicines are capable 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._
It has already been shown of medicines in general, that it is necessary for them to pass away from the mucous surface into the circulation. Mention has been made of their discovery in the blood by chemical means; and as we proceed it will be shown more particularly of different sets of medicines that each of them has been found in that fluid. Now, to gain entrance there, the medicine must first pass through the soft mucous membrane lining the stomach and intestine. This passage is performed by a process to which the names of _Absorption_ and _Endosmosis_ have been applied. Immediately outside this membrane, and between the tubes and cells which are formed by its involution, is a close net-work of very small veins, having thin and delicate walls. Now the same forces, whatever they be, which conduct the medicinal solution through the mucous membrane, cause it to pass on through the fine walls of these vessels. The two membranes, lying in such juxtaposition, are to all intents the same as one. Thus the medicine passes into the blood, and this is the only direct way by which it can gain entrance into it. These small veins, or capillaries, lead at length to the Mesenteric veins, which pour into the Portal vein, by which the blood proceeding from them is conducted into the Liver. The process of Absorption is the only mode by which remedies can thus enter the blood; we have now to consider what it is, and to show in what manner and by what agents different medicines are fitted to undergo it.
The subject of this Proposition divides itself naturally into three parts. We shall have to consider--1. What is the nature, and what the function, of the gastric and intestinal secretions. 2. The laws of this process of Endosmotic absorption, and how they are fulfilled in this case; and, 3. The mode in which the great majority of medicines are reduced to a state of solution, which is necessary before they can be absorbed.