The aim of this study was to elucidate the detailed anatomic structure of arcuate expansion (AE) and its nearby structures. Eighteen orbits of 13 Korean adult male cadavers of a mean age of 76.5 years (range, 75-78 years) were used. Sixteen specimens were grossly dissected, and 2 were for histologic study.Arcuate expansion is a fibrous band expanding from the inferolateral orbital rim to the medial canthal ligament. A sector (fan shaped) of AE originated in the angle of 5 to 80 degrees at the circumference of the inferolateral orbital rim circle, falling within the range of 3 to 5.5 o'clock, and tapered and attached to the inferior border of the medial canthal ligament. Arcuate expansion was located deep to the orbital septum and superficial to the inferior oblique (IO) muscle. It overlapped the middle part of the IO muscle as it crossed and connected to the capsulopalpebral fascia. The tensile strength of the 5-mm width of AE at halfway was 6.7 +/- 3.8 N, and that at the orbital margin was 5.4 +/- 2.7 N.We think that AE prevents the protrusion of the eyeball and orbital fat and functions as a pulley holding overcontraction of the IO muscle.