An additional bone in the sclera of the eyes of owls and the common potoo (Nictibius griseus) and its role in the contraction of the nictitating membrane.

Morphometric, anatomical and histological examinations were made in 10 species of owls of the families Tytonidae and Strigidae and compared with the eyes of other species of nocturnal birds including common potoo (Nictibiidae) and three species of nightjars (Caprimulgidae) and two diurnal species: the roadside hawk (Accipitridae) and the domestic duck (Anatidae). In owls and the common potoo the nictitating membrane is situated on the dorsal edge of the eye. In these birds, the scleral ring bears an additional, previously undescribed bone of various forms and dimensions (1.4-6.8 mm of length and 0.8-3.3 mm of width), located on the trajectory of the tendon of the pyramidal muscle which is attached to the nictitating membrane. This bone has a groove that encloses the tendon of the pyramidal muscle, preventing it from separating from the sclera during contraction, as well as diverting the trajectory of the tendon to impede it from projecting itself over the cornea. In the ferruginous pygmy owl, Nacunda nighthawk, Pauraque, scissor-tailed nightjar, roadside hawk and domestic duck the additional bone was not seen. Based on the morphofunctional characteristics of the bone, we suggest that this bone should be named the scleral sesamoid bone.