Intraorbital anatomy of the koala (Phascolarctos cinereus)
Kempster, RC, Bancroft, BJ & Hirst, LW 2002, The Anatomical Record, vol. 267, pp. 277-287.
Detailed descriptions of the koala anatomy generate baseline information for understanding and treating diseases in the koala. This study provides baseline anatomical details of the normal koala orbit, excluding the bulbus oculi.
Following immediate autopsy of koalas that were already deceased or had been euthanised, the nasolacrimal system, which serves as a conduit for tear flow from the external eye to nasal cavity, was described with the use of casts made using polymethyl methacrylate (PMMA). The lymphatic drainage of the conjunctival tissues was analysed following the injection of dye into the palpebral conjunctiva in addition to Microfil casts of efferent lymphatics. In doing so, the authors found that the extraocular muscles in the koala eye are consistent with most of mammalian species, excluding a retractor bulbi muscle which is not present in humans. In koalas, this muscle supports the eye to help prevent injury by retracting the globe back into its socket. The study describes in detail these extraocular muscles as well as the nerve supply, arterial system and venous system of the orbital. The superficial lymphatic system of specimens varied, most likely due to lack of circulation these koalas originally had in their poor condition. The authors also described in detail the koala eyelids, nasolacrimal apparatus, lacrimal gland, nictitating membrane and nictitans gland. The study described how the koala derives its blood supply from the internal carotid artery and not the external carotid, which is unlike other placental mammals. The PMMA cast results showed an unusual central nervous system, much like the opossum and kangaroo, in which terminal vascular units supply the brain, retina, and optic nerve. Most other components of the koala intraorbital anatomy were similar to other mammals; however, more research will be needed to accurately compare the koala to other species and the implications of these differences.
The detailed descriptions of the intraorbital anatomy of the koala provided by this study improve our understanding of the ocular adnexa and structure of the koala eye. This information combined with further research on a broader range of koala subjects will assist in the understanding transmission pathways and implications of chlamydial pathogens, as the nasolacrimal system may serve as a pathway for the bacteria.
Summarised by Robyn Boldy
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