A comparative morphological and morphometric study of the Musculi bulbi oculi and Apparatus lacrimalis in the ostrich (Struthio camelus) and emu (Dromaius novaehallandiae)

Abstract

The unique adaptation of the avian eye is reflected in its ocular anatomy. The ostrich and emu are commercially important species and a comparative study of the M. bulbi, lacrimal apparatus and their innervation would provide important ophthalmological data. Both species have large eye globes to which four recti, two oblique and two nictitating membrane muscles insert. Greater similarities in the origins of these muscles are evident between the two species, compared to their respective insertions. Branches of cranial nerves II to VII course within the orbit of both species, with cranial nerves II to VI innervating the eye and M. bulbi. The route of CN VI differs from that of other avian species whereas that of CN V and VII differs between the ostrich and emu. The M. bulbi in both species differ in mass, volume, isometric force and power generation, indicating possible variations in the dynamics of ocular motility. Each eye has an associated Harderian and lacrimal gland which empty at the inner margin of the nictitating membrane and lower eye lid, respectively. Morphological variation is evident, with the ostrich lacrimal apparatus being more robust, distinctly lobulated and pigmented. In both species, the lacrimal apparatus is compound in nature. A single large secretory duct extends into the body of both glands in the ostrich and emu, before branching into two to three smaller ducts into which the simple branched tubular units constituting the glands open. The secretory epithelium is simple columnar in nature. Concentrations of lymphocytes are observed in both glands confirming the general observation that the Harderian gland in particular plays an important role in local ocular immunity. The morphology and innervation of the ostrich and emu M. bulbi and Apparatus lacrimalis follow the general avian pattern. However, the small interspecies variations noted should be considered during diagnostic or surgical procedures on the eye or associated structures.