When solutions of sodium acetate, propionate and butyrate are placed separately in the empty rumen of sheep anaesthetized with nembutal, analysis of the blood leaving the rumen shows that absorption of the acids occurs and that the rate of absorption appears to decrease as the molecular size increases (Barcroft, McAnally & Phillipson, 1944).. Recently this was reinvestigated by Gray (1948), who concluded that no absorption at all occurred from the rumen of sheep when the pH was slightly alkaline as would be expected when solutions of the sodium salts are introduced. Gray's method consisted of introducing solutions of sodium acetate and propionate into the empty washed rumen of an unanaesthetized sheep through a rumen cannula. Pectin was included in the solution as a marker. The solution was allowed to remain in the rumen for 5 hr. or more and was then removed and the interior of the organ was washed with water. The loss of pectin from the rumen was taken as indicative of the quantity of solution that had passed on to the abomasum; on this basis the quantity of acetate and propionate absorbed from the rumen was calculated and found to vary between -10 and + 22 m.equiv. during 5 or 6 hr. In addition, Gray (1948) introduced solutions of pectin or sodium acetate into the empty rumen of anaesthetized sheep from which the exits were blocked by ligature, and found a loss of less than 5 % of either substance over an experimental period of 4 hr.Acetic, propionic and butyric acids are absorbed more rapidly than the corresponding anions, and the rate at which free acid disappears from the rumen increases with the length of the hydrocarbon chain (Danielli, Hitchcock, Marshall & Phillipson, 1945;Gray, 1947). If the epithelium of the rumen is impermeable to the anion, as Gray (1948) contends, then the ratio propionic/ acetic and butyric/acetic should be greater in the blood leaving the rumen than in the rumen contents, but this is not so (Kiddle, Marshall & Phillipson, 1951); in fact, the mixture of acids in the blood leaving the rumen is much the same as that in the rumen except that it contains less butyrate. 190 M. J. MASSON AND A. T. PHILLIPSON On account of these contradictions a further series of experiments on the absorption of acetate, propionate and butyrate from the rumen has been conducted, and the results are recorded in this paper. METHODSChemical. Acetic, propionic and butyric acids were determined by steam distillation, after acidifying with sulphuric acid, using a 'Markham' still. The distillates were titrated with carbonfree 0-02 or 0-05N-potassium hydroxide under carbon-dioxide-free conditions using phenol red as indicator. In one experiment in which rumen liquor was used total volatile acid was determined in the same way after precipitation of protein by McAnally's reagent .Volatile acid in blood was determined by a modification of the method of McClendon (1944), the details and accuracy of which are described elsewhere (Kiddle et al. 1951).Chloride was determined in earlier experiments by the Volhard meth...

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It is frequently stated that the passage of food through the stomach of the ruminant is a continuous process and this conception is based on the fact that normally food is always found in all parts of the stomach after slaughter, even after a 24 hr. fast. Studies on the rate of disappearance of stained particles from the rumen and their appearance in the faeces can be explained on this assumption while the well-known continuous cycle of contraction ofthe stomach pouches suggests continual mixing and propulsion of food. There is no information, however, on the quantities of food passing from the stomach-whether it stops completely at intervals through the day and night-or on the manner in which the rate of passage is influenced by feeding or rumination. The experiments to be described were undertaken in an attempt to assess the quantity of food leaving the stomach of sheep maintained on constant rations and to study the flow of food residues to the intestines. METHODThe flow of food from the abomasum was measured directly by diverting the flow outside the body through a cannula.Permanent exteriorization of the flow was achieved in one sheep. This method was based on a technique described by Markowitz (1949) for the exteriorization of the flow of digesta along the small intestine of the dog. The duodenum was sectioned approximately 1J in. below the pylorus and the two cut ends were closed by continuous sutures involving the mucosa and submucosal tissues. The closed ends were then inverted and buried by a further continuous suture. An ebonite cannula was inserted into the pyloric part of the abomasum and another into the duodenum. These were exteriorized through stab wounds in the abdominal wall, on either side of the operation wound, so that each cannula was an equal distance from the vertebral coluimn of the sheep. The operation wound was then closed and the two cannulae joined by a glass tube bent at each end to fit the shafts of the two cannulae to which it was joined by rubber tubing.One sheep in which the flow of food from the abomasum to the duodenum was permanently exteriorized was maintained in good condition for 3 weeks after the operation. After this time its * This research was started in the

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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