In consequence of these difficulties, and of the supposed inadequacy of the mechanical theory, many physiologists have had recourse to a certain specific action of the vessels, and have conceived that the chyle was taken up by a power, which has been supposed to be ana logous to an elective attraction between the vessel and the substance that is absorbed.*' There are indeed many circumstances which would appear to indicate, that a certain kind of selection is exercised by the mouths of the vessels, for, as far as we are capable of judging, when substances possessed of the same con sistence and physical properties are placed in contact with these mouths, some of them are received, while others are rejected. But we must remark, that the same objection may be urged against this as against the former expla nation, that the term elective, which is borrowed from the chemical relation of bodies to each other, is perhaps as little applicable to the case under consideration as that of capillary, which refers more to their mechanical action.
Discarding therefore all these analogical illustrations, which are at least of doubtful application, we may remark, that the lacteals ought to be regarded, like every other part of the animal frame, as vital organs, possessed of appropriate and specific powers ; that, in this instance, we are not able to refer to any general principle the train of events now under con sideration, and that we must therefore be satis fied with simply stating the fact, that the lac teals have the power of taking up by their extremities certain substances, with which they are in close approximation ; that, for the most part, the substances which they receive are the elements of the chyle, that they select these from the contents of the intestinal canal, and that, except under peculiar circumstances, they reject every other substance.* When the elements of the chyle have been received into the lacteals, it appears to undergo a certain degree of elaboration, by which it is farther assimilated and perfected, an operation, the intimate nature of which we are unable to explain, but which, as well as its entrance into the mouths of the vessels, we correctly refer to their vital action. After the chyle has entered the lacteals, there is less difficulty in conceiving the subsequent steps of the process. We are at least able to generalize the operation, by referring it to contractility, the same power which ori ginates motion in other parts of the system. It must, no doubt, be admitted, that the exis tence of the muscular fibres of the lacteals has not been satisfactorily demonstrated, and that, until this has been accomplished, our opinion can only be regarded as hypothetical : but we have here the advantage of being able to assign a probable and sufficient cause of the effect, and are aware of the point towards which we must direct our future investigations.- Before we conclude this branch of the subject,we may remark concerning the contents of the lacteals, that, under ordinary circumstances, we have no decided proof of these vessels containing any substance except the elements of the chyle, and that, although in some of the experiments referred to above, extraneous bodies have been occasionally found in them in minute quantity, these cases must be regarded as exceptions to the general fact.
With respect to the chyle itself, it has been a subject of examination by the chemists, whe ther its properties are always uniform in the same animal, or class of animals, under the various circumstances of age, constitution, and still more of diet, to which they are subject. But it may be necessary, before we enter upon this inquiry, to premise a few remarks upon the meaning of the terms chyme and chyle. By the older physiologists they were very gene rally employed as synonymous, and this is still the case with some of the modern writers, more especially on the continent.X A clear distinc tion between them has, however, been pointed out and recognized, and as there appears to be an essential difference between them, it is desi rable that it should be generally adopted. The first of these substances is the immediate pro duct of the action of the gastric juice on the aliment, as received into the proper digestive stomach, while the latter is the substance which is produced by a subsequent part of the pro cess of digestion. The conversion of chyme into chyle seems to commence shortly after it leaves the stomach, and while it still remains in the duodenum, is so far advanced as to be reduced into a condition proper for being re ceived into the lacteals. There is, however, reason to believe that the completion of the process takes place in the lacteals themselves, and even that it is not until the chyle arrives at the thoracic duct, or at least at the great trunks of the lacteals, that it is fully elaborated. The nature of the change which the chyme expe riences in the duodenum, and the agents by which this change is effected, what share the secretions of the part itself, the bile, or the pancreatic juice have in the operation, are questions that still remain in discussion, and which will be considered in the appropriate parts of this work.* For the analysis of the chyle we are prin cipally indebted to Vauquelin, Marcet, and to Dr. Prout. Vauquelin employed the chyle of the horse, as taken from the large trunks of the lacteals and from the thoracic duct.i. The experiments of Marcet were principally directed to the inquiry, how far the chyle of the same kind of animal was affected by dif ferences in the diet, according as it consisted principally of animal or vegetable substances.I Dr. Prout's experiments on the chyle extended both to Pits general properties, and to the dif ferences produced by different kinds of diet, while, in addition to these points, he entered into a very interesting examination of the suc cessive changes which it experiences, from its first entrance into the lacteals until its final deposition in the thoracic duct.§ The result of these experiments, as far as our present inquiry is concerned, tends to show that the vegetable chyle differs somewhat, in its physical and chemical properties, from that of animal origin, and that the chyle, when it first enters the lacteals, is in a less perfect state, while it be comes more assimilated to the blood in pro portion as it advances towards the thoracic duct.