Role of the levator palpebrae superioris (LPS) muscle in effecting nictitating membrane movement in the rabbit

Examination of the origin and course of the muscle fibers attached to the nictitating membrane (NM) of the rabbit demonstrated that these muscle fibers constitute part of the nasal branch of the levator palpebrae superioris (LPS) muscle. Other fibers of the nasal branch of the LPS muscle were observed to terminate in the nasal half of the upper eyelid. No further anatomical division of the nasal LPS into distinct eyelid and NM branches was observable by dissection. Contraction of the fibers inserting in the NM, produced by direct electrical stimulation, resulted in movement of the extended membrane towards its resting position in the corner of the eye. The observed anatomical relationships demonstrate in part that functional control of NM movement is structurally linked to control of upper eyelid elevation in this species. The origin and distribution of fibers inserting in the NM, and the effects of contraction of the muscle fibers on movement of the NM, indicate that contraction of these fibers could both oppose extension of the NM and/or initiate or contribute to retraction of the membrane following extension. Because of their position and the direction of NM movement caused by their contraction, these muscle fibers cannot actively cause or contribute to extension of the NM. Therefore, it is suggested that the LPS has a role in effecting movement of the rabbit NM by actively contributing to its retraction or reopening.