The mechanism of absorption from the rumen as exemplified by the behaviour of acetic, propionic and butyric acids.

Absorption of the lower fatty acids occurs directly from the rumen of sheep, in which organ they are produced as a result of bacterial fermentation. Previous work on this subject, in which the pH of the rumen was not controlled (Barcroft, McAnally & Phillipson, 1944), showed that: (a) The quantity of acid absorbed is substantial. (b) The rate of absorption from the normally filled rumen bears no simple relation to the concentration of volatile acid in the ingesta. (c) When equimolar solutions of the sodium salts of acetic, propionic and butyric acids are placed in the rumen, they are absorbed at different rates. The smaller molecules are absorbed more rapidly than the larger ones. The present research deals with the relative rates at which these three substances leave the rumen by passage through its stratified squamous epithelial lining. Normally the pH. of the ingesta of the rumen remains comparatively stable in spite of the fermentations that proceed within it, and the reaction of the ingesta rarely exceeds the limits oipH 5'8-7-5, the average value being in the vicinity of pH. 6-4 (Phillipson, 1942). This is an indication that the quantity of free acid present is never excessive. The proportions of the individual acids also remain fairly stable and are in the regions of acetic 65 %, propionic 20 % and butyric 15 % (Elsden, 1945). These physiological facts have been taken as the basis for the design of the present experiments. At pH 6-4 both fatty acid anion (A~) and free acid (HA) are present in significant quantities, so that either or both of these may be passing through the rumen wall. By keeping the rumen contents at pH 7-5 the proportion of free acid is reduced to negligible proportions, and the permeability of the rumen wall to fatty acid anion may be studied. Then by changing the rumen pH to say 5-8, where a substantial amount of free acid is present, the permeability to both anion and free acid can be studied, and the loss of material as free acid readily calculated with knowledge of the permeability to anions provided by the first half of the experiment. The permeabilities to free acid and to anion can then be examined by the methods given by Danielli (1941, 1943) to elucidate the nature of the process of passage through the rumen epithelium.