Split insertions of the oblique extraocular muscles in Carnivora

This study examines the presence or absence of split insertions (i.e., bifurcated/bifid tendons) in the oblique extraocular muscles (O-EOMs) in various carnivoran and non-carnivoran mammals. The O-EOMs were dissected in 44 mammalian orbital specimens (20 families, 36 species). Split insertions were present in Carnivora species exclusively. Split insertions were found in both superior oblique (SO) and inferior oblique (IO) muscles in all 6 Felidae specimens examined. The SO tendons split after the trochlea and coursed anterior and posterior to the global layer of the superior rectus (SR) before inserting onto the sclera at the temporal edge of the SR. The IO bifurcated after running between global and orbital layers of the inferior rectus and inserted onto the sclera anterior and posterior to the lateral rectus global insertion. All 4 Canidae, 1 Hyaenidae, and 1 Viverridae specimens had bifid insertions only in the IO, except the red fox, which had both SO and IO split insertions as in Felidae. Of the 7 Mustelidae species, 3 in subfamily Mustelinae had bifid oblique insertions, while 3 species in Lutrinae and Gulolinae lacked them. Bifid insertions were absent in other carnivoran and all non-carnivoran species examined. The presence of split oblique insertions is thus most common in terrestrial carnivorans with highly developed forward vision (high degrees of orbital convergence and frontation, retinal area centralis, etc.). The split oblique insertions in Carnivora may contribute to both torsional (anterior insertions) and vertical (posterior insertions) eye movements. This would parallel oblique functions in humans, but with different anatomical means.