Vetnews | Julie 2024 40 « BACK TO CONTENTS The unique ecology of birds (flight, foraging- and sexual behaviour) requires them to have high acuity vision, which is a function of a complex, multidimensional organization of their visual system. Bird ecology is obviously diverse and therefore their visual systems are adapted to allow each species to successfully occupy their specific ecological niche. Examples of the adaptation of their visual systems, related to their ecology, are best highlighted when considering the differing visual needs of a Peregrine falcon, which hunts other bids on the wing, to a Pel’s fishing owl, which hunts fish at night perched on a branch and, diving species like kingfishers, where accurate location of prey in another medium (water) is required while on the wing and rapid, considerable lenticular and corneal accommodation is needed when the birds enter water to allow clear vision when they catch prey. A basic understanding of the anatomy and physiology of the avian eye, and how it differs from that of other species, is not just interesting but also valuable, especially when we are required to examine these animals. The basic anatomy (cornea, sclera, iris, lens, retina) is similar to mammals but needless to say there are some significant differences. In birds, the nictitating membrane [or third eyelid] is pulled over the cornea during blinking by the pyramidalis muscle, which is a striated muscle. It originates on the posterior sclera and loops around the optic nerve through a sling formed by the bursalis muscle, also known as the quadratus muscle. The orbit is occupied predominantly by the globe and the extraocular muscles are poorly developed in most species, the result of these adaptations is minimal to no ocular movement, depending on the species. This decreased ocular movement is compensated for by wide cervical mobility. The shape of the globe varies considerably between species with 3 shapes described, these are flattened, globose or tubular (Fig1). The avian eye is large relative to skull size and body mass, and sensibly so as it is the most important sensory organ in the majority of species. In some owl species, the posterior segment is tubular allowing a considerably magnified image to be projected onto the retina. Another striking difference from the mammalian eye is the presence of a ring of scleral ossicles immediately behind the limbus. These bony structures provide a firm origin for the muscles which allows for an exceptional range of accommodation. The scleral ossicles should be taken into consideration when enucleating a bird’s eye. The iris colour varies dramatically between species, with agerelated differences and sexual dimorphism also described. Varying visibility of iridal vascularization may be present and this can also affect the iris colour. Pupil size is under both voluntary and involuntary control as striated and smooth muscle is present in the iris, although striated muscle predominates. This must be kept in mind when mydriasis is required for ophthalmic examination and surgery. Parasympatholytic drugs like tropicamide or atropine, commonly used to produce mydriasis in mammals, do not work in birds and non-depolarising muscle relaxants are required. Avian accommodation is a function of both corneal (change in curvature) and lenticular (anterior movement or deformation) mechanisms. Crampton’s muscle is responsible for corneal accommodation through flattening of the peripheral cornea. The lens is generally soft which allows for rapid accommodation. Lenticular accommodation is achieved through the contraction of Brucke’s muscle, which exerts pressure on the annular pad, resulting in axial movement of the ciliary body and ciliary process which then compresses the lens. Extreme lenticular accommodation in some diving birds is through direct lenticular compression of the lens by the pupil, resulting in rapid accommodative ranges over 50 dioptre! For context, a 3D lens will bring parallel rays of light to focus at 1/3 meter. The avian retina is avascular and atapetal and the fundus may appear bluish to gray to red. Avian retinas are thick compared to mammalian retina due to the increased density of neuronal cells. The pecten oculi is a highly pigmented vascular structure that protrudes into the vitreous body. Studies suggest that the pecten has a high metabolic activity and supports the hypothesis of a nutritional function and/or intraocular pressure regulation. Regulars I Ophthalmology Column Wild Bird Vision Dr Antony Goodhead, Specialist Veterinary Ophthalmologist, Johannesburg and Cape Animal Eye Hospitals, www.animaleyehospital.co.za Figure 1
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