age between war and poverty, and between conqueft and defolation.

With great refpect I have the honour to be

SIR,

Your most obedient humble Servant,

PHILADELPHIA,

Odber 26, 1786.

BENJAMIN RUSH.

A PHYSICAL INQUIRY into the POWERS and OPERATION of MEDICINES; by THOMAS PERCIVAL, M. D. F. R. S. and S. A. Lond. F. R, S. and R. M. S. Edinb. &c. &c.

CAUSA LATET, VIS EST NOTISSIMA.

OVID.

TO THE LITERARY AND PHILOSOPHICAL SOCIETY.

M

MANCHESTER, NOVEMBER 25, 1786.

EDICINES are the inftruments employed for the prefervation of health, or the cure of diseases: It muft, therefore, be an object of interefting fpeculation to the philofopher, and of practical importance to the phyfician to investigate the rationale of their action on the human body. But there is no branch of

the healing art which is in itself more intricate and obfcure; nor any one that has undergone fo many doctrinal viciffitudes. It would trespass too much on the time allotted for fuch difcuffions, in this Society, to enumerate the multifarious hypothefes which have been fupported by the fucceffive fectaries in phyfic, fince the days of Hippocrates. On many of thefe I have animadverted in a work, published near twenty years ago*. And I shall now only request your candid attention to a few obfervations, on the opinions which are beginning to prevail in our fchools; and your permiffion to offer fome hints towards the extenfion of our views, and the methodizing of our experience, relative to this curious and philofophical fubject.

Anatomy has now revealed the exquifite ftructure of our corporeal frame; and phyfiology has taught us that, in its animated ftate, the organs of which it is compofed are reciprocally connected with, and delicately adjusted to each other. The minuteft agent, therefore, may excite a movement capable of being propagated to any part of the fyftem, or even through the whole of it, by a fympathetic energy, independent and far beyond the power of the primary inftrument of motion. From these premises it is inferred, agreeably to the fimplicity which fub

募

Effays Medical and Experimental, vol. I.

fifts in all the operations of nature, that a medicine is only the cause of a cause, to adopt a phrase of the logicians; and that its proper action is confined to the nerves or fibres to which it is immediately applied. When received into the ftomach, after the first impreffion on the very fenfible coats of that organ, the nature of it is gradually changed, by the folvent powers of the gaftric juices: Or, if incapable of being digested into a mild and nutritious chyle, it is carried through the inteftinal canal, and ejected as useless and noxious to the body.

Error may be built on the bafis of acknowledged, if only partial, truth; and is then most fpecious in its form, and moft authoritative in its influence on the understanding. But the impofition ceases when we extend our views. And I fhall endeavour to fhew, that the operation of medicines is to be measured by a more enlarged fcale than the foregoing hypothefis applies to it, or any other which now occurs to my recollection.

I. Medicines may act on the human body by an immediate and peculiar impreffion on the ftomach and bowels, either in their proper form; in a ftate of decompofition; or by new powers acquired from combination, or a change in the arrangement of their parts. The fympathy of the ftomach with the whole animated fyftem is fo obvious to our daily experience, that it

cannot require much illuftration. After fafting and fatigue, we feel that a moderate quantity of wine inftantly exhilarates the fpirits, and gives energy to all the mufcular fibres of the body. It has been known even to produce a fudden and large augmentation of weight, after much depletion, by roufing the abforbent fyftem to vigorous action. Such power is peculiar to living mechanifm; and is properly denominated, by physicians, the Vis medicatrix naturæ. But apparent as is the fympathy of the ftomach, the laws by which it is governed are very infufficiently understood: And we have hitherto learned only from a loose induction of facts, that the nerves of this delicate organ seem to be endued with diverfified fenfibilities; that impreffions, made by the fame or different fubftances, have their appropriate influence on different and diftant parts; and that the ftomach itfelf undergoes frequent variations in its states of irritability. A few grains of blue vitriol, taken internally, excite inftantly the moft violent contractions in the abdominal, and other mufcles concerned in vomiting. A dofe of ipecacuanha, as foon as it produces naufea, abates both the force and velocity of the heart, in its vital motions; and affects the whole feries of blood veffels, from their origin to their minutest ramifications; as is evident by the palenefs of the skin, under fuch circumftances, and by the efficacy

efficacy of emetics in ftopping hæmorrhages. The head, when difordered with vertigo, fometimes derives fudden relief from a tea-fpoonful or two of æther, administered in a glafs of water. And I have known an inceffant cough to attack the lungs, in confequence of the ftimulus of a pin, which had been unwarily fwallowed. Of the action of medicines on the ftomach, under decompofition or recompofition, we have an example, familiar to every one, in magnefia. For this abforbent earth by neutralizing the acid in the prima via, acquires a purgative quality, and at the fame time yields a gas of great falubrity, as an anti-emetic, tonic, and antiseptic.

II. Medicines may pafs into the course of circulation in one or other of the ftates above defcribed; and, being conveyed to different and diftant parts, may exert certain appropriate energies. Chemistry furnishes numberless cafes wherein fubftances undergo changes, and put on new forms more remarkable than can be effected by the digeftion of the ftomach, retaining ftill the materia prima, and being capable of refuming the original arrangement of their particles, and confequently their original qualities. Now, a body altered in its texture, by the digestive organs, and carried into the fyftem with the aliment, may by fuch alteration acquire specific powers on particular found or dif

eafed