Microbial activity in the omasum.

In a ruminant the omasum forms a relatively small ovoid chamber connecting the reticulum to the abomasum. The two orifices are fairly close together and connected by a channel in the omasal wall. The internal structure has been likened to a loosely-closed book with sheets of tissue forming the leaves and the spine opposite to the channel. ‘The omasal leaves are of various sizes; all leaves attach to the wall of the omasum at the “binding” and at the ends, but only part of them extend to the channel at the opposite edge’ (Hungate, 1966). Numerous observations have been made and discussed at length on the very complex processes governing digesta flow into, within and out of the omasum for a variety of conditions (Phillipson & Ash, 1965; Hungate, 1966; Sellers & Stevens, 1966; Bueno & Ruckebusch, 1974; Ehrlein, 1979). Detailed consideration of these is outside the scope of the present review but the net effect appears to be as follows. Digesta enters the omasum intermittently and, in the main, at times when the reticulum is contracting. Much of it is then forced between the omasal leaves where solid material is sequestered. There is some periodic backflow to the reticulum which carries with it any very coarse material trapped near the reticularomasal orifice. Flow to the abomasum is mainly of two kinds: (a) material of relatively-low dry matter (DM) which appears as a trickle or sometimes as a gush shortly after liquid enters the omasum with a reticular contraction, (b) material of higher DM which appears at irregular intervals as the omasal body contracts. One consequence of this very highly organized sequence of events is that in spite of the inlet and outlet of the omasum being so close together much material, and particdarly solid material, is retained for an appreciable period within the organ. Differential flow of solid and liquid is evidenced by findings in several ruminant species that the DM content of total omasal contents is about 1 ~ 2 9 % while omasal outflow has a DM content of only 5-770 (Hauffe & Engelhardt, 1975). This characteristic of the omasum will be considered in more detail later. It is difficult to avoid the conclusion that the omasum is an important component of the gastrointestinal tract of the ruminant but its actual role remains ill-defined. It has been pointed out (Engelhardt & Hauffe, 1975) that earlier conclusions on omasal absorptive function, based on the examination of material from slaughtered animals, greatly overestimated the amounts of water and some other digesta components, absorbed. This was mainly because differential turnover times of solid and liquid components were not recognized. It has now been shown that relatively little water is absorbed in the omasums of sheep or goats (on average approximately I 3% (Engelhardt & Hauffe, 1975)), although substantial