Abstract

The gizzard of the herbivorous domestic goose, Anser anser, was investigated in order to understand how it can break down food such as grass. The gizzard consists of two thick opposing muscles that are joined on its dorsal and ventral surface by large tendons. The long axis of the fibre bundles of these muscles are aligned with the rigid lumen of the gizzard. Thus, the distance between the two tendons cannot decrease during contraction. Instead, contractions reduce the curvature of the muscle fibre bundles. This reduction in curvature and associated swelling of the muscle bundles generates forces in the lumen of the gizzard. However, as the thick muscles are asymmetrical, these forces generate translational rather than simple compressive movements. Such movement is also suggested by the wear on the lining of the gizzard lumen. A band of thin muscle runs perpendicular to the thick muscles and positions the food in the lumen in readiness to allow for food breakdown by the thick muscles.