Like other hollow organs, the muscles of ventricles consists of an external layer of longitudinal fibers, a middle layer of circular, and an internal layer of longitudinal fibers. The outermost muscle fibers (muscle tracts) of the longitudinal layer together surround the two ventricles (B7). The longitudinal fibers arise from the cardiac skeleton, mainly from the fibrous trigones. They run anti-clockwise in spirals toward the apex of the ventricle where individual fiber bundles enter the circular layer. Many muscle tracts go to the apex of the heart, where the form a whorl, the vortex of the heart (C). From this, as well as from the circular layer of fibers, some longitudinal fibers segragate and return clockwise in steep spirals to the heart skeleton. Among them are the trabeculae carneae and papillary muscles. The left ventricle and the outflow tracts of both chambers are surrounded by strong circular fibers. The inflow and outlow tracts contract one after another.