Absorption of acetic, propionic and butyric acids is known to occur from the rumen of sheep but some confusion concerning relative rates of absorption of these acids has arisen, for although it is agreed that free acids are absorbed more rapidly than their sodium salts when aqueous solutions are in the rumen (Danielli, Hitchcock, Marshall & Phillipson, 1945;Gray, 1947) it has been claimed recently that no absorption at all occurs from the rumen when the pH is over 7 (Gray, 1948).Under normal feeding conditions the pH of the contents of the rumen rarely falls below 5'5; for this reason the quantity of free acid present is small compared to the quantity of anion. Absorption, therefore, is likely to be insignificant if the membrane lining the rumen is permeable only to free acid. Further investigations, which are reported in this paper, were undertaken to compare the mixture of acids present in the rumen with the mixture present in the blood leaving the rumen, for this should show whether the mixture of acids present in the rumen is indicative of the mixture passing to the liver from this organ.The majority of these comparisons were made while two of us (R. A. M. and A. T. P.) were working in the Unit of Animal Physiology, Cambridge, whilst confirmation of these results using a different method of analysis was obtained at the Rowett Research Institute. METHODSExperimental. Normal ewes were taken straight from pasture to the laboratory. They were anaesthetized by intravenous injection of nembutal, the abdomen was opened on the right side and the posterior vein of the rumen was located as previously described (Barcroft, McAnally & Phillipson, 1944). In addition, the right carotid artery in the neck was exposed. The experimental procedure was usually as follows.After exposing the posterior vein of the rumen 5 ml. of blood were withdrawn from the carotid
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