Musculus sphincter ani externus in the rat

M. sphincter ani externus (MSAE) in the rat appears as a circular ring 1-1.25 mm in height and 250-350 microns thick, completed by a plate composed of longitudinal musculature in its posterior third. The main structures serving for the structural arrangement of MSAE are columns composed of longitudinal smooth musculature of the gut lining the posterior third of the anal circumference. Ventrally to the columns there are several structures in the MSAE showing fluent mutal transitions, namely the horshoe-like pars profunda, the ring-shaped pars superficialis and the pars subcutanea. The ends of the fibres of the horshoe shaped pars profunda join with the columns and pass along longitudinally to gradually replace the smooth musculature. The extension of the longitudinal muscle fibres of the columns upon the dorsal circumference leads to the development of the dorsal plate of MSAE. Some of the fibres of MSAE cross each other ventrally in the pars profunda and pars subcutanea, as well as dorsally in the pars subcutanea and in the area of longitudinally oriented fibres. Some fibres of MSAE are also fixed to the surrounding structures.