The experiments of MM. Magendie and Flandrin, the results of which were so opposite to those of Hunter, do not indeed lie open to the same objection ; but even here there is perhaps some ground for inquiry, before we implicitly adopt the conclusion that has been deduced from them. The experiment, as origi nally performed by Hunter, necessarily implies a degree of mechanical violence, which must produce a considerable derangement of the actions of the parts concerned. Acute inflamma tion of a peculiarly irritable and sensitive organ must have ensued, the vessels of all descriptions must have become much distended ; rupture and extravasation may have been not an impro bable consequence of this inflammation and distention, and, in short, a general derangement both of structure and functions may have oc curred, which must prevent us from drawing any positive inference respecting their natural condition.
These observations will apply with much greater force to a subsequent variation of the experiment, which consisted in entirely detach ing a portion of the intestine from the remainder of the tube, and filling this divided portion with the fluid, which, as in the former case, was detected in the vein of the mesentery. This arrangement was supposed to afford a still more decisive proof of venous absorption than the ex periment in its original state, and if we con sider the mechanical disposition of the organ only, we may admit that this would be the case. But it is obvious, on the other hand, that the vital actions of all the parts concerned must have been much more deranged, and that, on this account, we ought to be proportionally cau tious in the application of such experiments to our physiological theories.
We would venture to suggest, that the re markable discrepancy which exists between the experiments of Hunter and of the French phy siologists may perhaps be reconciled, by having recourse to the supposition, that in the former. case there was less violence used to the parts, and that they were left more in their natural condition ; whereas M. Magendie, as we pre sume, from a desire to render the effect more certain or more decisive, either produced a greater degree of distention of the intestines, or, in some other way, caused a greater derange ment of the parts, so as to produce a difference in the results. But this idea is offered merely as a conjecture, from which we do not venture to deduce any of our conclusions.
Upon the whole we feel disposed to regard the experiments of MM . Tiedemann andGmelin, and those of an analogous kind, in which extra neous substances were found in the venous blood, and in some of the secretions, when they could not be detected in the chyle, as more directly favouring the doctrine of venous absorption, because they are free from the objection which must always attach to those operations, where any considerable degree of mechanical violence has been employed. It may indeed be ob jected, that in these cases, the examination of the body did not take place at the proper point of time ; that, in some instances, it was made at too early a period, before the extraneous body had time to enter the lacteals, and, in other cases, not until it had left them, and had been discharged from the thoracic duct into the veins. But this contingency must be regarded
as rather a possible than a probable occurrence, and it is obvious that if any considerable num ber of experiments were performed, we can scarcely suppose it to exist.
The conclusion that we are disposed to draw from all the facts and arguments that have been brought forwards on the subject is in favour of the possibility of venous absorption, at least under peculiar circumstances ; at the same time that there are strong anatomical considerations, which would induce us to suppose, that in the ordinary actions of the system, the function of absorption is confined to the lacteals and the lymphatics §. 3. Inquiry into the mode in which the ab sorbents act.—In entering upon this inquiry there are two distinct subjects which present themselves for our consideration ; we must first ascertain by what means the substances that are absorbed enter the mouths of the vessels, and, in the second place, after they have entered the mouths, how they are conveyed along the ves sels themselves.
With regard to the first of these points we may remark, that while there is so much uncer tainty respecting the anatomical and physio logical structure of the mouths of the lacteals, and still more, while we are completely igno rant of that of the lymphatics, we cannot ex pect to arrive at any definite conclusion con cerning the mode of their action. We may, however, venture to say, that there is strong reason to believe, that the absorbents terminate in very minute or capillary vessels, that have open mouths, and that these mouths are brought into contact or close approximation with the substances to be absorbed. Hence, by an ana logy, which it must be acknowledged is some what vague, the action of these minute vessels has been referred to capillary attraction. But it may be doubted whether in this inference, as in so many other cases of physiology, we have not been misled by a mere nominal resem blance, and have applied the term capillary to the action of the lacteals, because it had been used to denote their dimensions. Perhaps, strictly speaking, there is scarcely a single cir cumstance, in which the action of the lacteals can be assimilated to that by which fluids are taken up by capillary tubes. The structure and consistence of the tube itself, the nature of the substance on which it is supposed to act, and their relative situation, are all of them more or less different from what occurs in the ordinary cases of capillary attraction. And if there is a difficulty with respect to the lacteals, where we have at least some indistinct evidence of the mechanical disposition of the parts, which may seem favourable to this hypothesis, in a much greater degree will it exist with respect to the lymphatics, where we have nothing to direct our opinion, except the analogy which may be presumed to exist between the two spe cies of absorbent vessels.