1. Mechanical theories of the action of medicines were greatly in vogue during the seventeenth and eighteenth centuries. There is a tendency in the human mind to explain every thing; and it was only natural for men who knew little of chemistry or of physiology to resort to the science of physics, which they could comprehend, in attempting the explanation of observed phenomena.
John Locke, in his essay concerning the Human Understanding, published in 1689, gave it as his opinion, that the shapes of the minute particles of medicines were sufficient to account for their several operations.
"Did we know," said he, "the mechanical affections of the particles of rhubarb, hemlock, opium, and a man, as a watchmaker does those of a watch, whereby it performs its operations, and of a file, which, by rubbing on them, will alter the figure of any of the wheels, we should be able to tell beforehand that rhubarb will purge, hemlock kill, and opium make a man sleep." This idea did not originate with the great metaphysician. The first rudiments are to be found in the doctrines of the Methodic Sect among the Romans, a medical branch of the Epicurean school. They held that diseases depended either on constriction or relaxation of the tissues, and that medicines operated by mechanically affecting these conditions.
The simple and philosophical statement of Locke was not improved by the various applications which were subsequently made of it. At the early part of the eighteenth century these ideas derived great support from the principles inculcated by Dr. Herman Boerhaave, the learned physician of Leyden. He likewise supposed that many diseases of the solid parts were to be attributed to a weakness or laxity of the animal fibres, and were to be cured by external or internal agents, which should act mechanically on those fibres so as to increase their tenacity. Also, that disorders of the fluids often depended on their being too viscid, and that this condition might be improved by agents which should attenuate this viscidity. Dr. Archibald Pitcairn, a Scotchman, the immediate predecessor and contemporary of Boerhaave, was elected to the Chair of Physic in Leyden in 1691, and was also an able exponent of the mathematical theories. But he applied to physiology those ideas which were employed by the other to throw light upon physic; if that may be called light which was at least an improvement on the ignorance which preceded it.[8] He explained the digestive process by the mechanical trituration to which the food was subjected in the stomach; and accounted for secretion by supposing the existence in glands of vascular pores of different sizes, which intercepted certain particles of the blood; actually giving for the process a mathematical formula. He was a vehement opponent of those who based their theories on the then youthful science of chemistry, who, having scarce yet shaken off from them the dust of alchemy, only substituting Acids, Alkalies, and Fermentations, for Salt, Sulphur, and Mercury, fell easy victims to his satire.
Dr. Charles Perry, in 1741, propounded a mechanical view of the action of Mercury and Arsenic. He thought that the particles of the former, being round and heavy, were able, when shaken about in the vessels, to break up and to annihilate those crude acrid humours which were the causes of disease; and that Arsenic acted as an irritant by the sharp and pointed nature of its atoms. He attributed the occasional poisonous effects of Mercury to the presence of Arsenic or some such substance as an impurity.[9]
Dr. Mead, in 1751, states that the administration of Mercury is dangerous in cases where there is carious bone, as there is a fear that its ponderous particles may break the weak lamellæ.[10] He was the Court Physician in the reign of George II. He accounted for the poisonous nature of the venom of serpents by asserting that it consisted of pointed particles, which pierced and destroyed the globules of the blood.
Dr. Perry conceived that some medicines, such as Steel and Antimony, did not act by their mere bodily presence, but by certain subtle vapours which emanated from them, and affected the vital spirits. This was a very misty notion. He stated that he borrowed this idea from a great philosopher of the German nation. This was probably Boerhaave, who lectured at Leyden in 1707.
Among those who regarded with favour the mechanical hypothesis, were Fourcroy and Hecquet in France, Van Swieten and Huygens in Holland, and Bellini in Italy. Excepting perhaps the case of external irritants, these explanations of the action of medicines have been universally condemned by scientific men at the present day. Doubtless these old authors were in the wrong, both in applying one hypothesis to the action of all remedial agents alike, and still more, in carrying their theories into such minute details, where it is impossible that they should be verified. And yet we may go too far in our condemnation of all such ideas. It does not seem to me to be so impossible, or even improbable, that the operation of some medicinal agents, particularly those which act on the nerves, may depend in some way on the shapes of the atoms of these substances, as related to those of the tissues which they influence. At least, there is no other possible explanation of the power of such substances. We know that the nerves are very much under the influence of mechanical impressions, upon which depend the phenomena of two at least out of the five senses, those of hearing and touch, as probably also of the other three, if we understood them better. We know also that if we accept the Atomic theory, by which so many chemical phenomena are cleared up and explained, we must admit a certain definite and peculiar arrangement and shape to the ultimate particles of every compound body. These considerations render it possible that the ultimate particles of a stimulant medicine may be of such a nature as to irritate, or to refuse to coincide with, the ultimate molecules of the sensitive nerve with which they come in contact; and those of a sedative may, on the other hand, be so shaped and arranged as to dove-tail with those particles, and by extinguishing, as it were, their salient points, to cloak their vital sensibility. This is obviously a mere conjecture, and the only value which can attach to it is, that it appears in some sort to explain a thing which without it is inexplicable.
Some modern writers have attempted to clear up the actions of certain medicines by their supposed influence on the physical process of endosmosis, as carried on through the coats of the stomach and intestines. Poissenille and Matteucci have attempted to prove that the action of saline liquids in causing purging, and that of a solution of morphia in preventing the same, may be explained by the endosmotic properties of these liquids, as ascertained by experiment. It does not seem to me that these ideas can be successfully maintained. (Vide Prop. II.)
2. Several attempts have been made to explain the general action of medicines on chemical principles.