Functions of the Omasum in the Stomach of the Sheep

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Abstract
The processes which occur in the omasum of the ruminant have long been a subject of speculation and debate (Favilli, 1937). It was at first thought that the fluid portion of the digesta passed on from the reticulum was squeezed out in the omasum, and the larger food particles comminuted there, but no evidence has been offered which proves that either of these processes actually take place. As an alternative explanation of the characteristically high concentration of solids in the omasum it was later suggested that large amounts of water might be absorbed through the walls of the organ, and this suggestion has recently received strong support from the observation (Garton, 1951) that the concentrations of certain inorganic ions are often much higher in the omasal fluid than in the rumen fluid. Nevertheless, the possibility of permeation of the omasal wall by these ions makes them unreliable as accurate markers of the absorption of water; and the further possibility that some of the reticular contents may bypass the omasum and flow directly to the abomasum via the omasal groove has rendered equivocal the use of markers for the assessment of the extent to which water is absorbed. It has been suggested (Elsden, Hitchcock, Marshall & Phillipson, 1946) that the greater part of the fatty acids in solution in the liquid portion of the material received from the reticulum are absorbed from the omasum, but here again the evidence is incomplete because of lack of knowledge of how liquids and solids are passed through the omasum. Fatty acids are undoubtedly absorbed there (Barcroft, McAnally & Phillipson, 1944), and the ratio of acids to solids is much reduced (Elsden et al. 1946), yet if fluid were separated from the solids mechanically, rather than absorbed through the walls of the omasum, the absorption of fatty acid need not be as extensive as has been implied.