several other explanations of a similar kind, of which the following is an example:-"The frightful effects of Sulphuretted Hydrogen and Hydrocyanic Acid are explained by the wellknown action of these compounds on those of Iron, when Alkalies are present, and free Alkali is never absent in the blood." (Organic Chemistry, p. 274.) Now in the first place it is not proved that the complete abstraction of iron from the blood would occasion sudden death, though doubtless it is a necessary constituent of that fluid. Further, Prussic Acid acts on the superficial nerves as an Anodyne when applied externally, which it can hardly do by displacing iron. Besides, by parity of reasoning, Ammonia, or Benzoic or Cinnamic Acid, should precipitate iron, if present in the blood in the soluble state, and Sulphuric or Nitric Acid should dissolve it, if in the state of peroxide; and yet none of these agents are frightful poisons. It is not to be imagined that chemical solutions and decompositions of every kind are allowed to take effect in the human system in the same way as in the laboratory of the chemist, for there are in the former many disturbing and controlling causes which suffice to hold them in check.

Another theory by Liebig, that Alcohol and similar materials operate as Narcotics, by appropriating to themselves the Oxygen which should maintain the vitality of the tissues by entering into combination with them,-and a somewhat similar explanation of the action of Narcotic remedies, given by Dr. Snow and others, may be mentioned here among chemical theories of the action of medicines, but must be treated more at length hereafter. (See Narcotics.)

We may for the present conclude, that though the actions of many remedies may be partly elucidated by chemical considerations, it is impossible to account for the influence of all alike in this way. For at least, as it seems to me, the actions of most nerve-medicines, and of gland-medicines, cannot be reasonably explained on any such hypothesis.

main there, for they are noxious and foreign to it. They must sooner or later be excreted from it by the glands. They are of use when disease depends on the presence and working in the blood of some morbid material or agency, which material or action they tend to counteract or destroy. They may be called vital antidotes; not strictly specifics, for they are not always efficacious, on account of variations in the animal poisons, or from the casual operation of disturbing causes. They are applicable in those many disorders which depend, not on the absence of a natural substance, but on the presence of an unnatural agent in the blood. These medicines are called Catalytics, from a Greek word which signifies to break up or to destroy. Having performed this their function, they then pass out of the blood.

All this requires to be proved.

In the Eighth Proposition it is stated of Neurotics, or nervemedicines, that they act by passing out of the blood to the nerves, which they influence. This is only to insist on the rule of local access, already laid down in Prop. V. It is further affirmed that they are transitory in action. They appear to effect molecular changes in nerve-fibre, similar to those by which the phenomena of the senses are produced, and which are by nature transitory in their results. And yet they may be very powerful, even so as to extinguish vital force. Thus, short and unenduring as is the operation of these agents, it may last long enough to cause death, and so a temporary influence produce a permanent result. There are three divisions of Neurotics. The first set are of use when there is a dangerous deficiency of vital action. These are Stimulants. They exalt nervous force, either of the whole nervous system, or only of a part of it. They vary very much in power. A second set, called Narcotics, first exalt nervous force, and then depress it. They have thus a double action; but they have also a peculiar influence over the functions of the brain, which is different from any possessed by other nerve-medicines.

They control the intellectual part of the brain, as distinguished from its organic function; the powers of mind more than those of life. Some Narcotics tend to produce inebriation; others, sleep; others again, delirium. In the third place some Neurotics tend simply and primarily to depress nervous force. They may act on the whole nervous system, or on a part of it only. They are often very powerful; and they are of use when, from any cause, some part of the nervous system is over-excited. They are called Sedatives. Like other Neurotics, they are used in medicine as temporary agents in temporary emergencies. If a permanent action be required, the remedy must be constantly administered, so that the effect may be kept up by continual repetition.

In the Ninth Proposition it is affirmed of Astringent medicines that they act by passing out of the blood to muscular fibre, which by their contact they excite to contraction. They do not so much influence the voluntary fibre of the muscles, which is under the direct control of the nervous system; but they chiefly manifest their action on the involuntary or unstriped muscular fibre, which is not directly controlled by the brain and nerve-centres, and for this reason more under the operation of external or irritating agents. Meeting this in the coats of the capillary vessels and of the ducts of glands, they are enabled to act as styptics, and as checkers of secretion. The action of Astringents appears to depend on a chemical cause; for we find that all of them possess the power of coagulating albumen.

The Tenth Proposition treats of Eliminatives. It is not said simply that these increase the secretions of a gland; or that they stimulate the glands while passing by them in the .blood. But it is laid down as a rule that they act by themselves passing out of the blood through the glands, and that while so doing they excite them to the performance of their natural function. They are substances which are unnatural to the blood, and must therefore pass out of it. In so doing

they tend to pass by some glands rather than by others: in these secretions they may be detected chemically; and it is on these glands that they have an especial influence. Their uses in treatment are various and manifold.

In these classes are included all medicines that act after

entry into the blood. On referring to the classification which precedes this chapter, it will be seen at a glance what groups of medicines are arranged as orders under each class or division.* In the third chapter I shall attempt at some length to prove the propositions which treat of these four classes; and I shall also attempt to explain the nature and mode of action of the orders, or small groups of remedies.

In the fourth chapter some of the more important medicines will be considered separately, either as individually interesting, or as illustrative of general modes of operation previously described.

I may point to some parts of the Essay as being more original than others, although not perhaps for that reason more valuable. For this purpose may be mentioned the treatment of the second and third propositions; the distinction which I have drawn between the two divisions of Blood-medicines; the account given of Tonics in one of these divisions, and of Anti-arthritics in the other; the theory of the action of Eliminative medicines; the experiments made on the action of some remedies in particular (Chap. IV.) ; &c.

*It may be of some use if I adduce here a characteristic example of each of the great groups of medicines to which I have alluded above: CLASS I. Hæmatics.

Div. 1. Restoratives. Iron, in Anæmia.

CLASS IV. Eliminatives. Cantharides, and Croton Oil.

17

CHAPTER II.

ON SOME OF THE MORE IMPORTANT CLASSIFICATIONS OF MEDICINES, AND OPINIONS OF AUTHORS RESPECTING THEIR ACTIONS.

I HAVE thought it necessary, before stating at length my own conclusions, to refer to some of the more important statements of authors concerning the subject of which I have to treat; because by so doing I may to some extent indicate what points are to be regarded as determined and proved, and what as still unsettled, and point out where I can agree with other writers, and where I am disposed to differ from them.

But should it seem to any reader that a brief and general review of the literature of the subject, comprising an enumeration of many various and often clashing opinions, would be likely to confuse rather than instruct him, I would advise him to pass on to the next chapter, with which the present one is not essentially connected.

The opinions of authors on the general action of medicines are in most cases best ascertained by observing the manner in which they have arranged and classified them, grouping together those which they consider to be alike in their mode of operation.

Differences of opinion respecting individual medicines will be best considered afterwards, when we come to discuss those

C