Oil, although possessed of the fluid form, does not appear to observe the law which governs the disposal of these bodies; it is not absorbed, but is entirely transformed into chyme in the stomach. To effect this, however, it seems essential that the stomach should be in a state of high energy, or it undergoes chemical decomposition, and becomes rancid: nor will the stomach, unless it be educated to it, like those of some northern nations, digest any considerable quantity of it; and since it cannot be absorbed, it must find its exit through the alimentary canal, and consequently prove laxative.

99. I have endeavoured to show in the Pharmaco-logia, that certain salts are absorbed with the water which holds them in solution, unless they so far increase the peristaltic motion as to produce catharsis, in which case, they pass at once through the alimentary passage; but should this not occur, they enter the circulation, and will sometimes exhibit their effects on distant organs. It is on this account that sulphate of magnesia proves diuretic in horses, for the bowels of these animals are not stimulated by that salt; and the same theory will explain why persons frequently experience an increase in their urine on drinking the weaker waters of Cheltenham.

1 See Pharmacologia, edit. 8. art. Vinum.

100. It has been stated that the chyle, together with the lymph, is poured into the sub-clavian vein. We know very little of the use of this latter fluid: it is conveyed by vessels, termed lymphatics, which appear to spring from extremely small roots in the substance of the membranes and of the cellular tissue, and in the parenchyma of the different organs. In man, these vessels uniformly traverse glands before they arrive at the venous system. The general opinion entertained upon the subject of lymphatic absorption, assigns to this order of vessels the office of returning the broken-down and useless parts of the various structures to the blood, in order that they may be finally ejected from the body. In examining the chemical composition of lymph, we find it to have a considerable analogy with the blood; which has induced M. Majendie to conclude that it is a part of that fluid, which, instead of returning to the heart by the veins, follows the course of the lymphatic vessels; for what object, it is impossible even to offer a conjecture. He therefore doubts whether these vessels have any absorbing power, but endeavours to prove by experiments, which certainly have the merit of great plausibility, that the faculty of absorption is possessed by veins.

Be this, however, as it may, it is certain, that the chyle, as well as those fluids which are absorbed from the stomach, are transmitted through the lungs for the purpose of undergoing such changes as may perfectly assimilate them with the blood. Every meal, therefore, must impose a certain labour upon these organs; and it is probable, that the extent of this labour will vary with the nature of the food. It becomes, then, an object of the greatest pathological interest, to inquire into the relations which subsist between the functions of chylification and respiration, and to ascertain what species of food will require the greatest, what the least effort of the lungs, to complete its sanguification. In cases of pulmonary disease, such a discovery would be of the highest value, in order that we may not impose upon an enfeebled organ more duty than is necessary for the preservation of life. Unfortunately, the facts which have been collected upon this subject are few, and even discordant. The faint light which science afforded us in the investigation of the preceding stages of digestion is here extinguished, and experience, embarrassed with a thousand sources of fallacy, is all that remains for us.

Notwithstanding the numerous experiments which have been instituted, and the various theories which have been formed, how extremely vague and doubtful is our knowledge respecting the physiology of respiration! It is true, that the chemist, by a refined examination of the air, before and after it has been respired, has ascertained that a quantity of oxygen is absorbed, and replaced by an equivalent proportion of carbonic acid; but can any person accustomed to reflection believe, that respiration serves no other purpose than that of the removal of a certain portion of carbon from the blood? A function which cannot be suspended for a minute without the certain destruction of life, must surely have some relation with the vital energy more intimate and far more important1. The quantity of carbonic acid does not appear to have that striking connexion with the quality or quantity of our ingesta which theory would have led us to suppose. Whoever will read an account of the results obtained by Dr. Prout, in his examination of the "Quantity of carbonic acid gas emitted from the lungs during respiration, at different times, and under different circumstances 2," will retire from the perusal of the essay with the mortifying conviction, that little or nothing is known upon the subject: such, at least, was my conviction, upon that occasion.

It must, however, teach us the folly of hasty generalization, for nothing tends more to the perpetuation of error.