George Ross
1844 The Lancet  
Protein not the final Principle of the Diges. tion of Azotised Aliment. MY last paper contained an account of the effects of the various reagents present during the digestive process on azotised principles. I am anxious that it should not be conceived that I admit any one actual and final principle of nutrition produced by digestion. Neither protein, nor any other substance, is an uniform product of the digestion of the various azotised principles of our food, although, like protein, the basis
more » ... f the albuminoid substances, there is one character which they must necessarily possess, namely, solubility in acid. This is the central point of a circle of changes, the extreme segments of whose circumference may exhibit different phases, displaying on one side the properties of albumen, on another those of casein. I have further endeavoured to show that the result of the digestion of azotised principles is the production of a substance from albumen resembling casein, and another from casein that has acquired the properties of albumen ; yet these principles are not identical. When albumen performs the part of a base it resembles casein,-when casein acts as an acid its reactions are similar to those of albumen. Albumen is subject to the same law as casein, with regard to alkalies, but in a modified degree, since a very small portion of albumen is precipitated by a large proportion of liquor potassse ; but, under ordinary circumstances, albumen is soluble in caustic alkali. Caseum is more readily inffueuced in this mode by alkali, for when thus treated precipitation takes place by the influence of heat almost invariably, acquÎ1" ing thus the peculiar character of albumen. We can hence understand how the incoagulable product of digestion becomes coagulable in the lymphatic fluid and the blood, for these fluids betray alkaline properties, and must necessarily confer coagulability on the nutritive substances which are poured into them. The process of digestion develops a series of changes, commencing in the reduction, in the alimentary canal, of some primitive organised substance, and terminating in the reconversion of this substance to its original character, or to another similar to it in composition, and also, like it, final. There does not appear to be any distinct principle to which such substances are reduced prior to their reconversion, but each stage is one of transition. I conceive that these remarks apply equally to the albumen and fibrine of the lacteals, since they are found to vary in properties under different circumstances, and are essentially transitional. Lactic Acid the Proximate Pl'inciple of the Digestion of Faf·ina 4-c. I endeavoured to show, also, in my last paper, that lactic acid was one stage of the transitional progress of the reduction of farinaceous food ; that this acid was not merely a product of the decomposition of indigestible aliment, nor of morbid irritation of the alimentary canal, but that it was an ordinary product of the digestion of farinaceous substances in the small intestines. Dr. Prout imagined that the collection of lactic acid in the stomach and bowels caused vomiting and diarrlaoea; but I apprehend that this idea is hypothetical, for I cannot conceive that an acid whose reaction is so weak can be possessed of any qualities obnoxious to the alimentary canal. The presence of any acid in the stomach will not necessarily cause vomiting. This symptom is attributable,under the circumstances in which lactic acid is found in the ejected fluid, simply to an abnormal fermentation going on in the indigestible substance; and to the production of other substances foreign to the natural conditions, such as carbonic acid, and other gases which cause distention and irritation of the organs of digestion. Moreover, the mass itself, not being assimilated, overloads 626 the bowels, excites nausea, and hence the usual sequelae. The nutritive properties of farinaceous food will next engage attention, but, in the first place, it will be necessary to decide which set of vessels, portal or lacteal, are those employed in the absorption of aliment. Absorption of Aliment by the Veins. Magendie, it is well known, established the fact of the capability of absorption by the veins, yet this fact has not been so productive of results as might have been expected. It is not yet known to what extent these vessels are engaged in the absorption of matters from the surface of the alimentary canal ; but we find Liebig, with his usual acuteness, remarking, " it is possible that the non-azotised constituents of food, after undergoing some change, are carried from the intestinal canal directly to the liver, and that they are converted into bile in this organ, where they meet with the products of the metamorphosed tissues, and subsequently complete their course through the circulation." I became convinced of the truth this suggestion embodies, some time since, after a careful examination of the experiments on digestion performed by Professors Tiedemann and Gmelin. These experiments produced results in many and the most essential points altogether contradictory to expectation, and apparently inexplicable in the opinion even of so profound a reasoner as Miiller; but this apparent contradiction arises, I apprehend, rather from an unwillingness to abandon received doctrines than from any fault in the experiments or anomaly in the operations of nature. Now, this calculation will be found to represent almost exactly the amounts of the various ingredients of the chyle of digestion ; for after the fatty matter is subtracted the amounts stand thus :-The only difference is in the amount of the substances soluble in water, and this is too trifling to disqualify the value of the calculation, and may be correctly attributed to a defect in the analysis. We can, therefore, prove, by strict calculation, based on unexceptionable and unbiassed experiments, that in the horse, at least, the lacteals absorb only fatty matter. These are the relative proportions of the component principles. Analysis of the Experiments by Tiedemann and Gmelin. These admirable physiologists found that the solid contents of the serum of the chyle in a horse, while fastin" contained. of Thus, after digestion, there is a less proportional amount of albumen than while fasting, which is quite inexplicable on the supposition of the absorption of this principle by the lacteals. Let us examine the point more narrowly. There are in the chyle of digestion 20 parts of fatty matter which do not exist in the other; these 20 parts are just one-fifth of the whole amount. Very well, then, if we add 20 parts of fatty matter to the original hundred, each of the ingredients, if no chemical change ensues, must lose a proportional quantity ; in other words, a 100 parts now must contain proportionally less than a 100 parts at first. Subtract, then, one-fifth from each ingredient and it will stand thus :-Further, the relative proportions of the whole amount of solid matters in the serum of the different portions of the lymphatic system of a horse fed on oats are thus expressed :-
doi:10.1016/s0140-6736(02)74282-7 fatcat:pelexd6einfnzmblsx4wapucne