The Morphology of the Anterior Segment and Periorbita of the Great Horned Owl (Bubo virginianus).

The unique visual system of the Great Horned Owl (Bubo virginianus) is a culmination of their interesting and variable ecology. Prior research has largely been centered on the posterior segment; however, a strong understanding of the anterior segment is critical for a complete understanding of their visual ecology and successful management of ocular diseases. The aim of this study was to, for the first time, describe in detail the normal anatomy of the periocular structures and anterior segment of the Great Horned Owl by gross dissection, bright field microscopy, scanning electron microscopy, and transmission electron microscopy. The extraocular muscles were well developed and consisted of four rectus muscles, two oblique muscles, and the pyramidalis-quadratus complex, which move the nictitans. A lacrimal gland was absent. The anterior corneal muscle was well developed and inserted primarily on the external wall of the scleral venous sinus. It is proposed that contraction of this muscle dilates the sinus and its associated drainage channels. The scleral venous sinus encircled the entire globe and was divided into anterior and posterior compartments by an annular artery. A small posterior corneal muscle was present. The annular pad of the lens consisted of radially arranged polygonal fibers. The lens cortex was composed of hexagonal fibers that interdigitated by means of ball-and-socket junctions.