Distribution of the muscle coat at the omasoabomasal junction and its vicinity in cattle.

Detailed studies on the distribution of the muscle coat at the omasoabomasal junction in cattle, especially in the pila omasi, were carried out in order to clarify the mechanism of closing of the ostium omasoabomasicum. Anatomical and histological observations revealed that the muscle coat forming the circumference of the ostium omasoabomasicum is composed of inner circular and outer longitudinal layers. The former was particularly thickened at the end of the sulcus omasi (pila omasi). Joined to the pila omasi was a thick muscle bundle which extended from the labium sinistrum (left lip) of the sulcus reticuli and ran obliquely along the floor of the sulcus omasi. Moreover, on the abomasal side of the ostium omasoabomasicum, vela abomasi were formed in such a manner as to surround the ostium omasoabomasicum. These were continuous with the edges of the sulcus omasi. Judging from its location and muscular structure, the pila omasi may contract in accordance with the contraction of the reticulum. As a result, the ostium omasoabomasicum may be narrowed and the vela abomasi pulled toward the omasum, perhaps obstructing the ostium omasoabomasicum. Accordingly, it is presumed that the retention of contents in the omasum may effectively prevent abomasal contents from moving backward into the omasum.