Superior oblique muscle layers in monkeys and humans.

PURPOSE Rectus and the inferior oblique extraocular muscles (EOMs) consist of orbital layers (OLs), inserting on connective tissues, and global layers (GLs), inserting on the sclera. This study was performed to clarify the anatomic relationships of the corresponding layers of the superior oblique (SO) muscle. METHODS Two whole human and two monkey orbits were serially sectioned en bloc at 10-mum thickness in the coronal plane and stained for collagen with Masson's trichrome and for elastin with van Gieson's stain. The SO muscles of one human and one monkey were sectioned longitudinally. The structure of the SO muscle was examined by light microscopy, and muscle fibers in the OL and GL of selected sections were counted. RESULTS The deep SO muscle consisted of a central GL contiguous with the tendon, surrounded coaxially by a peripheral OL inserting on the SO sheath posterior to the trochlea. The maximum number of SO fibers was 14,400 to 19,200 in the human and 7,000 to 7,400 in the monkey. In the monkey, approximately 60% of total fibers were in the GL, and 40% in the OL. The SO sheath was in mechanical continuity with the superior rectus pulley. CONCLUSIONS The primate SO has a substantial OL configured to contribute to positioning the superior rectus pulley in the coronal plane. Whereas the direction of application of the SO's GL force is determined by the rigid trochlea, the SO's OL influences the direction of application of rectus EOM forces. This insight extends the concept of active control of pulley positions to include a contribution from the SO muscle.