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The following conclusions were reached by placing a solution of sodium acetate, propionate and buty-rate in the rumen of sheep and studying the rate at which these substances are absorbed: (1) When the rumen contents are at pH 7.5, fatty acid anion only is absorbed, i.e. no free fatty acid is absorbed, and the anion is accompanied by a roughly equivalent amount of sodium. The anion is absorbed by passage through water-filled pores, the diameter of which is large compared with retric acid. These pores probably lie in the intercellular cement of the rumen epithelium. (2) When the rumen contents are at pH 5.8, free acid is lost from the rumen in large amounts in addition to the loss of fatty acid anion. Part of this free acid is absorbed by passage through water-filled pores, but the greater part of it passes through the lipoid membranes of the epithelial cells. (3) In consequence of the difference in the two mechanisms, at alkaline pH the substances are lost in the order acetate > propionate > butyrate, whereas at acid pH the order is butyrate > propionate > acetate. (4) All the substances studied (fatty acid anions, sodium and water) move from regions of higher to regions of lower concentration. (5) The total fatty acid absorbed from the rumen of a sheep on pasture grass may readily be twice that absorbed from the rumen by the same sheep on hay. (6) The permeability of the rumen epithelium is such that the pH of the ingesta tends to move towards neutrality, independently of the neutralizing action of the saliva.

A mixture of volatile organic acids, consisting for the most part of the fatty acids, acetic, propionic and butyric, is a characteristic product offermentation within the alimentary tract. It was shown previously that volatile acids are absorbed directly from the organ in which they are produced (Barcroft, McAnally & Phillipson, 1944), and this fact, together with a consideration of the quantities present in the digesta—a subject with which the present paper is concerned—suggest that they form a valuable part of the calorific material utilized by the animal.

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