We have now to treat of the actions of medicines in the blood. These are recited in the four remaining Propositions. The seventh treats of Hæmatics, acting primarily in the blood. The eighth treats of Neurotics, passing from the blood to the nerves. The ninth of Astringents, passing to muscular fibre. The tenth and last treats of Eliminatives, which pass out of the body through the glands. PROP. VII.--_That a first class of medicines, called_ HÆMATICS, _act while in the blood, which they influence. That their action is permanent._ 1. _That of these, some, called_ RESTORATIVES, _act by supplying, or causing to be supplied, a material wanting, and may remain in the blood._ 2. _That others, called_ CATALYTICS, _act so as to counteract a morbid material or process, and must pass out of the body._ Supposing that a medicine has fairly passed into the blood, and circulates round with it, there are now two ways in which it may behave itself. In the first place, it may have a tendency towards some tissues or parts of the body, on which to exert its powers, as the nerves, or the glands, or muscular fibre, and may use the blood only as a vehicle by which most readily and easily to attain to these. Such are Neurotics, Astringents, and Eliminatives. They may not affect the blood, but they must pass through it. But there is another and still more important class of medicines, whose action is particularly directed towards the blood itself. The blood, after their action, is different from what it was before. It may be a change for the better or for the worse; but there certainly is a change. Medical authors, with few exceptions, have been very backward to acknowledge the existence of medicines of this description. But even those who would fain have classed all medicines as stimulants or sedatives, differing only in the kind or degree of their action on the nervous system, have in many cases been obliged to confess that there is a set of remedies which they call "Alteratives," whose action, though slower, is more certain and more durable than that of the former. It is allowed that they alter the condition of the blood. To suppose that they do so by first influencing the nerves, is to adopt a circuitous and uncalled-for explanation. It is proved that they pass into the blood. It is known that when actually applied to nerves, they do not affect them. From these considerations merely, without further evidence, it would seem tolerably clear that they act by influencing the blood itself, simply and solely. But this it will be my business to prove more at length directly. Such medicines, then, I have designated Hæmatics, a simple and expressive term which has been used by others before me. It is obviously necessary that a medicine of this class should be absorbed.