The cyclic motility of the omasum and its control in sheep

1. The mechanical activity of different parts of the omasum was recorded by means of small balloons and/or strain-gauges in conscious sheep. These events were related to the electrical activity of the omasal body, the omasal groove and the leaves and to the changes in diameter of the reticulo-omasal orifice. Humoral, behavioural and nervous influences were examined.2. Two patterns of contraction of the omasum were found. One, confined to the oral and middle thirds of the organ, originated at the omasal groove and proceeded to the right and then the left surfaces of the body and ceased at the onset of reticular contraction. The other was limited to the aboral third of the organ near the abomasum; in this part the contractions were prolonged regardless of the reticular contraction and some gave rise to a reversed propagation over the organ. The characteristic cyclic motility of the omasum was slightly reduced in sheep fasted for 48 hr and in those fed on pelleted food, and it persisted after vagotomy.3. The reticulo-omasal orifice always dilated strongly during the second phase of reticular contraction. It also dilated noticeably during the extra-contraction associated with regurgitation. In each case alternating opening and closing movements at a frequency of 5-7/min followed. Contractions of the omasal leaves, which were independent of those of the omasal body, occurred at a rate of approximately 2-3/min. They passed from the free border to the base and travelled in an aboral direction. After local application of fatty acids the frequency of their contractions increased to approximately the same rate as that of the orifice.4. It is concluded that the motility of the omasum in sheep involves a wave of contraction starting at a point on the omasal groove just posterior to the reticulo-omasal orifice and passing slowly over the omasal body. With the exception of the occasional reversed contractions this pattern is closely associated with rumino-reticular cycles even though the omasal contractions and the movements of the leaves are partially independent of vagal innervation.