The experiments, made on dogs, led him to the following results. The specific gravity of the serous portion of the chyle is from 1.012 to 1.021, whether it be formed from animal or vegetable diet. Vegetable chyle, when subjected to analysis, furnishes three times more carbon than animal chyle. The latter is highly disposed to become putrid; and this change generally commences in three or four days; whilst vegetable chyle may be kept for several weeks, and even for months, without becoming putrid. Putrefaction attacks rather the coagulum of the chyle than its serous portion.

The chyle from animal food is always milky; and, if kept at rest, an unctuous matter separates from it, similar to cream, which swims on the surface. The coagulum is opaque, and has a rosy tint. On the other hand, the chyle from vegetable food is almost always transparent, or nearly so, like ordinary serum. Its coagulum is almost colourless, and resembles an oyster; and its surface is not covered with the substance analogous to cream.

MAGENDIE, too, remarks, that the proportion of the three substances, into which the chyle separates, when left at rest;—namely, the fatty substance on the surface, the clot and the serum, varies greatly, according to the nature of the food; that the chyle, proceeding from sugar, for example, has very little fibrine; whilst that from flesh has more; and that the fatty matter is extremely abundant when the food contains fat or oil; whilst scarcely any is found if the food contains no oleaginous matter.

Lastly, the attention of PROUT has been directed to the same comparison. He found, on the whole, less difference between the two kinds of chyle than had been noticed by MARCET. In his experiments, the serum of chyle was rendered turbid by heat, and a few flakes of albumen were deposited; but, when boiled, after admixture with acetic acid, a copious precipitation ensued. To this substance, which thus differs slightly from albumen, Dr. PROUT gave the inexpressive name of incipient albumen. The following is a comparative analysis, by him, of the chyle of two dogs, one of which was fed on animal, and the other on vegetable, substances. Vegetable Food. Animal Food.

*THENARD has properly remarked, that the difference, in the time of putrefaction of these two substances, appears very extraordinary. It is, indeed, inexplicable.

The difference, between the chyle from food of such opposite character, as indicated by these experiments, is strikingly insignificant, and indicative of the great uniformity in the action of the agents of this absorption.

With regard to the precise quantity of chyle, that may be formed after a meal, we know nothing definite. When digestion is not going on, there can of course be none formed except from the digestion of the secretions from the digestive tube itself; and, after an abstinence of twenty-four hours, the contents of the thoracic duct will be chiefly lymph. During digestion, the quantity of chyle formed will bear some relation to the quantity of food taken; the nutritive qualities of such food, and the digestive powers of the individual. MAGENDIE, from an experiment made on a dog, estimated, that at least half an ounce of chyle was conveyed into the mass of blood, in that animal, in five minutes; and the flow was kept up, but much more slowly, as long as the formation of chyle continued.

Physiology of Chylosis.

The facts just referred to,-regarding the anatomical arrangement of the chyliferous radicles and mesenteric glands,-will sufficiently account for the obscurity of our views on many points of chylosis.

The impracticability of detecting the mouths or extremities of the chyliferous radicles has been the source of different hypotheses; and, according as the view of open mouths or of the spongy gelatinous tissue has been embraced, the chyle has been supposed to enter immediately into the vessels, or to be received through the medium of this tissue; or, again, to pass through the parietes of the vessels by imbibition.

Let it be borne in mind, however, that not only the action of absorption, but the vessels themselves, are seen only by the "mind's eye;" and that the chyle does not seem to exist any where but in the chyliferous vessels. In the small intestine, we see a chymous mass, possessing all the properties we have described, but containing nothing resembling true chyle; whilst, in the smallest lacteal we can detect, it always possesses the same essential properties. Between this imperceptible portion of the vessel, then, and its commencement,-including the latter,-the elaboration must have been effected. LEURET and LASSAIGNE, indeed, affirm, that they have detected chyle in the chymous mass within the intestine, by the aid of the microscope. They state that globules appeared in it similar to those that are contained in the chyle, and that their dissemination amongst so many foreign matters alone prevents their union in perceptible fibrils. These globules they regard as true chyle, for the reason, that they observed similar globules in the artificial digestions they attempted; and, on the other hand, never

detected them in the digestive secretions. In their view, consequently, chyliferous absorption would be confined to the separation of the chyle, ready formed in the intestine, from the excrementitious matters united with it. We have already more than once referred to the caution, which it is necessary to adopt, regarding minute microscopic researches; and to the difference, presented to the observer by glasses of different magnifying powers. We must have stronger evidence than this to set aside the overwhelming testimony in favour of an action of selection and elaboration by the absorbents of all organized bodies-vegetable as well as animal. The nutriment of the vegetable may exist in the soil and the air around it; but it is subjected to a vital agency the moment it is laid hold of, and is decomposed to be again united, so as to form the sap. How else can we understand the conversion of the animal matters in the manure into the substance of the vegetable? A like action is doubtless exerted by the chyliferous radicles; and hence all the modes of explaining this part of the function, under the supposition of their being passive, mechanical tubes, are inadequate and unphilosophical. BOERHAAVE affirmed, that the peristaltic motion of the intestines has a considerable influence in forcing the chyle into the mouths of the vessels; whilst Dr. YOUNG is disposed to ascribe the whole effect to capillary attraction; and he cites the lachrymal duct as an analogous case, the contents of which, he conceives, and we think with propriety,are entirely propelled by capillary attraction.

The objections to these views, as regards the chyliferous vessels, are sufficiently obvious. The chyle must, according to them, exist in the intestines; and if the view of BOERHAAVE were correct, we ought to be able to obtain it from the chyme by pressure. As the chyle is not present, ready formed, in the intestine, the explanations by imbibition and by capillary attraction are equally inadmissible. There is no analogy between the cases of the lachrymal duct and the chyliferous vessels. In another part of this work, we have affirmed, that the passage of the tears, through the puncta lachrymalia and along the lachrymal ducts, is one of the few cases, in which capillary attraction can, with propriety, be invoked, for the explanation of functions executed by the human frame. In that case, there is no conversion of the fluid. It is the same on the conjunctiva as in the lachrymal duct, but, in the case of the chyliferous vessel, a new fluid is formed; there must, therefore, have been an action of selection exerted; and this very action would be the means of the entrance of the new fluid into the mouths of the lacteals. If, therefore, we admit, in any manner, the doctrine of capillary tubes; it can only be, when taken in conjunction with that of the elaborating agency.

"As far as we are able to judge," says BOSTOCK, "when particles, possessed of the same physical properties, are presented to their mouths (the lacteals,) some are taken up, while others are re

jected; and if this be the case, we must conceive, in the first place, that a specific attraction exists between the vessel and the particles, and that a certain vital action must, at the same time, be exercised by the vessel, connected with, or depending upon, its contractile power, which may enable the particles to be received within the vessel, after they have been directed towards it. This contractile power may be presumed to consist in an alternation of contraction and relaxation, such as is supposed to belong to all vessels, that are intended for the propulsion of fluids, and which the absorbents would seem to possess in an eminent degree." This is all specious; but it is not the less hypothetical.

By other physiologists, absorption is presumed to be effected, by virtue of the peculiar sensibility or insensible organic contractility or irritability of the mouths of the absorbents; but these terms are, as MAGENDIE has remarked, the mere expression of our ignorance, regarding the nature of the phenomenon. The separation of the chyle is, doubtless, a chemical process; seeing that there must be both an action of decomposition and of recomposition; but it is not regulated, apparently, by the same laws, as those that govern inorganic chymistry.

It has already been remarked, that the chyle always possesses the same essential properties; that it may vary slightly according to the food, and the digestive powers, of the individual, but that it rarely if ever contains any adventitious substance;-the function of the chyliferous vessels being restricted to the formation of chyle. The facts and arguments, in favour of this view of the subject, will be given hereafter.

The course of the chyle is, as we have described, along the chyliferous vessels, through the mesenteric glands into the receptaculum chyli or commencement of the thoracic duct; along which it passes into the subclavian vein.

The chief causes of its progression, are,-first of all, the inappreciable action, by which the chyliferous vessels form and receive the chyle into them. This formation being continuous, the fresh portions must propel those already in the vessels towards the mesenteric glands. It is probable, too, but we have no direct evidence of the fact, that the vessels themselves are contractile; although, their fibrous coat is seemingly not muscular,-notwithstanding the opinions of SHELDON, SCHNEIDER and CRUIKSHANK, to the contrary, and when observed on a living animal, does not exhibit any contraction. They have by some physiologists, however, been looked upon as possessing this property, for the following reasons. First. They are small, and tonic contractions are generally admitted in all capillary vessels. Secondly. The ganglions or glands, which cut them at intervals, would destroy the impulse given by the first action of the radicles; and hence require some contraction in the vessels to transport the chyle from one row of these ganglions to another. Thirdly. If a chyliferous vessel be opened in

a living animal, the chyle spirts out, which could not be effected simply by the absorbent action of the chyliferous radicles; and, fourthly: in a state of abstinence, these vessels are found empty; proving, that notwithstanding there has been an interruption to the action of chylous absorption, the whole of the chyle has been propelled into the receptaculum chyli. It is obvious, however, that most of these reasons would apply equally to the elasticity as to the muscularity of the outer coat of these vessels.

A more forcible argument is derived from an experiment by LAUTH. He killed a dog, towards the termination of digestion; and immediately opened its abdomen, when he found the intestines marbled, and the chyliferous vessels filled with chyle. Under the stimulation of the air, these vessels began to contract, and, in a few minutes, were no longer perceptible. The result he found to be the same, whenever the dissection was made within twenty-four hours after death; but, at the end of this time, the irritability of the chyliferous vessels was extinct; and they remained distended with chyle, notwithstanding the admission of air. These experiments lead to what seems, in the absence of less direct proof, scarcely doubtful:-that the chyliferous vessels possess a contractile action, by the aid of which the chyle is propelled along the vessels.

In addition to these propelling causes, the pulsation of the arteries in the neighbourhood of the chyliferous vessels; and the pressure of the abdominal muscles in respiration have been invoked. The former has probably less effect than the latter. It is not, indeed, easy to see how the former can be possessed of any. Of the agency of the latter we have experimental evidence. If the thoracic duct be exposed in the neck of a living animal and the course of the chyle be observed, it will be found accelerated at the time of inspiration, when the depressed diaphragm forces down the viscera; or when the abdomen of the animal is compressed by the hands. We shall find, too, hereafter, that the mode in which the thoracic duct opens into the subclavian, exerts considerable effect on the progress of the chyle in its vessels.

We have reason to believe, that the course of the chyle is slow. It has been already stated, that in an experiment on a dog, which had eaten animal food at discretion, MAGENDIE found half an ounce of chyle discharged from an opening in the thoracic duct in five minutes. Still, as he judiciously remarks, the velocity will be partly dependent upon the quantity of chyle formed. If much enters the thoracic duct, it will probably proceed faster than under opposite circumstances.

In the commencement of the thoracic duct the chyle becomes mixed with lymph. Under the head of lymphatic absorption we shall show how they proceed together into the subclavian, and the effect produced by the circumstances under which the thoracic duct opens into that venous trunk.