schweigger-seidel sheath schweigger-seidel sheath schweigger-seidel sheath is a phagocytic sleeve that is part of a sheathed arteriole of the spleen, and is sometimes referred to as a splenic ellipsoid. it is a spindle-shaped thickening in the walls of the second part of the arterial branches forming the penicilli in the spleen. it is named after german physiologist franz schweigger-seidel (1834-1871).