Gross morphology of Musculi bulbi of the ostrich (Struthio camelus) and emu (Dromaius novaehollandiae)

Abstract

The avian eye is uniquely adapted and reflects some structural peculiarities in comparison to mammals. M. bulbi consists of four rectus, two oblique and two nictitating membrane muscles. These have been described in several avian species, including the ostrich. The ostrich and emu are commercially important and a description of M. bulbi would aid in diagnostics and surgery. Three sub-adult ostrich and three sub-adult emu heads were collected after slaughter in a commercial abattoir and immersion fixed in 10% neutral-buffered formalin. Enucleation involved removing extra-ocular tissues and incising the origins of M. bulbi in half the specimens and the insertions in the remainder. M. bulbi was described and compared. In both species M. bulbi was composed of eight thin, sheetlike muscles; with the two nictitating membrane muscles being more robust. M. rectus dorsalis / ventralis / lateralis / medialis originated peripherally to the For. optici. M. obliquus dorsalis / ventralis originated dorsally and rostrally to those above. The straight and oblique muscles inserted near the equator. Minor differences in the attachments of these muscles were noted between the species. M. quadratus / pyramidalis membranae nictitantis originated dorsally and ventro-medially on the equator, respectively. The pyramidal muscle inserted on the nictitating membrane via a long tendon to which the quadrate muscle also attached. M. bulbi was similar in both species and the minor differences need to be morphometrically quantified. Functions of the muscles were comparable in both species. A similar surgical procedure for enucleation is recommended in the ostrich and emu.