Anatomical study of the scleral ring and eyeball of the long-eared owl (Asio otus) with anatomical methods and diagnostic imaging techniques

Background: The scleral ring in birds consists of ossicles that are fixed as small plates by cartilage joints and have no articulation to other parts of the skeleton. Objective: Due to inadequate examination of the scleral ring anatomy and its specific form in owls, this study aimed to investigate the exact structure of the scleral ring and some morphometric characteristics of the eyeball in a long-eared owl (Asio otus). Methods: The eyes of 20 alive and 10 dead male and female owls were examined. In addition to common anatomical methods, computed tomography scans and radiographic and ultrasonographic imaging techniques were used in this study. Results: The structure consisted of 15 ossicles. In the ventral part of the ring, these tubercles were observed in the scleral rings of all owls; in each ring, there were four bones with these tubercles. Additionally, there was no significant difference between the left and right eye parameters. Most ocular parameters in female owls were larger than those in males, but in the case of some parameters, such as optic nerve length and optic nerve sheath diameter, this difference was not observed. Conclusions: According to this study, the scleral ring in the Asio otus has anterior and posterior parts, and the lens is in the immediate vicinity of the anterior part. The right and left scleral rings and eyeballs are bilaterally symmetrical in terms of the shape, size, and number of ossicles that form the ring.