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D. A. Samuelson, J. L. McGee, K. Maciejewski, M. M. Strobel, P. A. Lewis; A Uniquely Constructed Extraocular Myology in the Florida Manatee (Trichechus manatus latirostris). Invest. Ophthalmol. Vis. Sci. 2010;51(13):6334.
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In our studies of the Florida manatee, we have observed swift movement of its third eyelid, similar to that performed by birds. Concomitantly, the eyes do not appear to retract into the orbit. A complete description of the musculature within the orbit of the Florida manatee has yet to be made. The present study examines anatomically by gross and histological observations the organization of the extrinsic ocular muscles in this species and relates these findings to observed ocular movement and previously described visual behavior.
For this study, complete orbital specimens of six animals (one juvenile and five adults) that had died from natural causes were collected at the Marine Mammal Pathobiology Laboratory in St. Petersburg, FL., and placed in 10% formalin, further dissected and photographed. Selected portions were embedded in paraffin, sectioned, and stained using either H&E, Masson’s trichrome or elastic stain.
Orbital dissections of all specimens revealed consistently the absence of a true set of musculi bulbi. Instead, four muscles have been observed in the orbit, each associated primarily with the palpebrae, including a prominent muscle that ends at the base of the upper eyelid, and another attached to the base of the lower eyelid, both originating from the area of the optic foramen. The two other muscles are attached to the third eyelid, one inferiorly and the other medially. All musculature has been verified to be skeletal. The upper and lower eyelids have a confluent, uninterrupted set of orbicularis oculi muscles.
The upper and lower eyelids are designed for squinting rather than blinking, which when contracted brings the palpebral fissure to a small rounded point, much like that in miosis of rounded iridal pupils. The actual blinking and concomitant sweeping of tears over the surface of the cornea is performed by the third eyelid. The collective changes and differences of the extrinsic ocular muscles of the Florida manatee that we have observed most likely reflect adaptations needed for their survival in an immensely divergent aquatic environment.
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