The absorption of volatile fatty acids from the rumen.

The fermentation of carbohydrates in the rumen of sheep is known to lead to the formation of large amounts of the lower fatty acids. The important part that these acids assume in the metabolism of ruminants has been remarked upon (Marston, 1939, 1946). While it is believed that acetic, propionic and butyric acids are the main fatty acid products of the fermentation, there is no certain knowledge of the proportions and quantities in which they are formed. There is evidence that absorption may proceed directly from the rumen (Trautmann, 1933; Davey, 1936; Phillipson & McAnally, 1942), and more recent work (Barcroft, McAnally & Phillipson, 1944) has demonstrated that fatty acids can find their way through the rumen wall directly into the blood stream. This being so, a knowledge of the relative proportions of the fatty acids in the rumen contents may, of itself, provide no indication of the proportions in which they are formed, as it would not be improbable that the individual fatty acids would be absorbed at different rates. It is clear, therefore, that a determination of the relative rates of absorption of the acids is required for a better understanding of the proportions in which the acids are formed in the rumen, and absorbed from it. Barcroft et al. (1944) presented evidence which was interpreted as elucidating this point. The case depended essentially on an increase of volatile fatty acids observed in the efferent blood from part of the rumen wall of an anaesthetized animal, after one or other of the acids had been introduced into the empty rumen. The composition of the lymph draining this organ was not determined, and for this reason alone the interpretation of the findings might be complicated; the possibility of differential absorption into the blood and lymph respectively is by no means remote. In order to check this point the feasibility of collecting from the rumen lymphatics sufficient material for critical chemical analysis was considered. Owing to the technical difficulties involved, however, this means of .approach was abandoned in favour of the direct determination of relative rates of disappearance of the lower fatty acid homologues when introduced into the empty, washed rumen as mixtures in dilute aqueous solution approximating the composition of the contents of this organ when active digestion is in progress. It was thought that this might provide less equivocal evidence.