Orbicularis Oculi Muscle Fibers Are Relatively Short and Heterogeneous in Length. Timothy Lander, Jonathan D. Wirtschafter, Linda Kirschen McLoon. http://iovs.arvojournals.org/
Dear Arwa, You might find these two publications useful - both describe ultrastructure of M. orbicularis oris in humans (in health and with cleft palate), check the literature quoted as references.
1. de Chalain, T., et al. (2001). "Histologic, histochemical, and ultrastructural analysis of soft tissues from cleft and normal lips." Plastic and Reconstructive Surgery 108(3): 605-611.(you can get pdf from Web of Science)
There is a also Cleft Palate Craniofacial Journal and old issues of Virchows Archives (ultrastructure) where you might find some useful data for both human and rabbit M orbicularis oris.. Good luck with your study.