This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
The preparations of lead are here considered, because one of their most prominent properties, and that for which probably they are most employed, is their astringency; though, in other respects, they are quite peculiar, and different in their mode of action from all other medicines. In the metallic state, lead is believed to have no effect on the system. It is true that its introduction into the stomach has been followed by symptoms ascribable to its presence; but there can be little doubt that it underwent chemical change, under the influence of the gastric liquids, before these symptoms were experienced, at least any other than such as could be referred to a mere mechanical agency. The vapour arising from melted lead is capable of affecting the system through the lungs; but it is probably an oxide, and not the metal itself, which acts in this case. According to Mialhe, all the salts of lead, however insoluble in pure water, are in a greater or less degree soluble in the liquids of the alimentary canal, through the agency of the chloride of sodium or potassium there present. All of them, therefore, are capable of being absorbed, and of operating on the system. He does not except even the sulphate, which has usually been considered inert in consequence of its great insolubility. It is now generally believed that none of the known combinations of lead are without some effect, excepting only the sulphuret.*
* Sulphate of lead is soluble, to a certain extent, in solution of hyposulphite of soda, and of course might act on the system if it should encounter this salt. (Journ. de Pharm. et de Chim., Janv. 1859, p. 77.) Even the sulphuret of lead is said, when applied as a dressing to an ulcerated surface, to have caused lead-poisoning; so that the metal can hardly be considered safe in any state. (Dr. N. J. Pittman, Trans, of Med. Soc. of N. C, 1856, p. 39).
The preparations of lead may act in two ways; first, by simply irritating the part with which they may come into contact; and secondly, by exercising their peculiar influence, through absorption, either on the part itself, or the system. To a certain extent, these influences are inconsistent with each other; for irritation in a part is well known to impede absorption; and, in proportion as the irritating substance is taken up, and removed from the point of application, is its power of irritating, in any given quantity, diminished. This fact has an important practical bearing. If the peculiar effect of lead is wanted, care must be taken that the preparation be as little irritant as possible. Thus, a quantity of acetate of lead, large enough to irritate the stomach, will be much more likely to operate on the system, when administered in small doses frequently repeated, than when the whole is taken at once. In the latter case, not only is absorption impeded by the fulness of the irritated vessels, but the agent is apt to be removed by vomiting or purgation.
In relation to the irritant effect of the preparations of lead, there is nothing peculiar. They are apt to operate in this way when applied largely, and to delicate surfaces; and the important point for the physician is to be able to regulate the dose of each preparation, according to the susceptibility of the several parts with which it is brought into contact. The skin will bear more than the gastric mucous membrane, and the latter more than the delicate conjunctiva. Some of the preparations act as irritant poisons when swallowed in over-doses. Too highly concentrated, they may indeed occasion the death of a part, either by excess of irritation beyond its vital capacity, or by chemical combination with one or more of its organic constituents.
This, so far as it can be generalized, is the influence conjointly of an astringent and sedative. The sedative influence, though felt in some degree in the circulation, is more especially directed to the nervous system, and appears to affect the nerve tissue directly, rather than through the nervous centres, though these may also be involved. When the preparations of lead are given in such doses as gradually to bring the system under their influence, no observable effects may be experienced for some time, in a state of health; but, after a shorter or longer period, which varies much in different cases, the secretions are generally somewhat diminished, and the pulse often lessened in frequency and fulness. These may be considered as their legitimate effects, when they are used therapeutically; and they will sometimes prove useful in restraining morbid discharges, even before any change is noticed in the healthy functions. If longer continued, whether accidentally, or for medical purposes, they are capable of inducing a poisonous condition, which presents very peculiar phenomena, and not unfrequently ends in death if neglected. Practically, this poisonous condition results much more frequently from exposure to the influence of the metal accidentally, or in the pursuit of business, than from its use as a remedy; no doubt because, in the latter case, its operation is carefully watched, and its employment suspended upon the occurrence of unpleasant symptoms. The toxicological influence of lead was investigated, with peculiar care. by M. Tanquerel des Planches; and, though much was known before the publication of his treatise, and many observations have since been made, yet his account of the symptoms has served as the basis of most subsequent descriptions.