And it may be further remarked concerning these experiments, without impugning the accu racy or the dexterity of the operator, that they imply a degree of minuteness in the execution, and of attention to a variety of concurrent cir cumstances, and are altogether of so extremely delicate a nature, as to render it undesirable that any physiological conclusion should be founded on them. If a single bloodvessel be divided, however minute, and its extremity be exposed, or even if a single cell of the membranous texture be laid open, so as to admit of the introduction of the fluid, the essence of the experiment is destroyed, and its results must become equivocal.
Another hypothesis respecting the nature of absorption has been lately brought forward by Sir D. Barry, according to which it immediately depends on atmospheric pressure, either ex ercised directly on the surface of the body, or acting indirectly on the absorbents through the medium of the great internal cavities. The experiments on which the hypothesis rests con sisted in introducing a portion of some poison ous substance into a wound, and forming a vacuum over it by means of a cupping-glass; when, by contrasting the effect of the poison in this case with that which ensues from the same application where the cupping-glass was not employed, he concludes that the process of ab sorption was suspended by removing the at mospheric pressure, and he hence infers that this pressure is the cause of absorption.* The results of these experiments, in a prac tical point of view, are of great interest, but with respect to the physiological conclusion that has been drawn from them, there are various circumstances to be taken into account, which appear not to have been duly attended to.
In the first place, a similar kind of objection occurs in this case as in the experiments of MM. Magendie and Delille related above, that the poison was introduced into a wounded part, and would therefore be immediately mixed with the blood and carried into the general circulation. The effect of a vacuum formed over the divided extremity of a vessel, must be to retard the progress of its contents, whatever be its description, or in whatever cause it ori ginates. This effect is therefore not specifically applicable to absorption, even in the natural state of the parts; and when we consider that in this case there was an artificial opening made into the vessel, we may venture to affirm that the conclusion which was drawn from it is in no respect the necessary inference from the facts.
And besides this general objection, it may be fairly questioned how far the removal of pres sure from the surface of the body could act in retarding the progress of a fluid along a vessel which has no external opening, and which is provided with valves, such as is strictly the case with the lacteals, and may be almost said to be so with the lymphatics. And with re spect to the lacteals, it appears a very obvious objection to the hypothesis, that they are alto gether defended from the effects of atmospheric pressure, either as applied directly, or as in directly acting on them through the medium of any of the internal cavities. Besides, we have sufficient proof of the spontaneous and inde pendent action of these vessels, whatever may be our opinion respecting the existence of their muscular coat, and to whatever principle we may refer this action, and we have thus an actual cause for the propulsion of their con tents, although it is impossible to estimate its actual amount, it would appear unnecessary to search for any farther agent, unless we have good ground for concluding that the existing cause is inadequate to produce the effect re- quired.
Cutaneous absorption.—There is a branch of the subject to which we must now direct our inquiry, the existence and extent of what has been termed cutaneous absorption. When we trace the progress of the lymphatic vessels from their great central trunks, and follow them through all their minute ramifications, we find that many of them appear to have their origin from the surface of the body,* and hence we are led to suppose that the function of ab sorption is exercised, to a certain extent, by the cutis, or the parts immediately connected with it. That this is the case is proved by various pathological facts; we have daily opportunities of observing, that various medicinal substances, by mere application to the surface, and still more when aided by friction, produce the same effect upon the system as if they had been received, in the ordinary way, through the medium of the stomach. By this means mercury manifests its specific action on the salivary glands, the salts of lead destroy the contractility of the muscular fibre, while opium, tobacco, and other narcotics produce their pe culiar effects on the nervous system.