Summary

Based on the analyses and discussion above, we conclude the following:

1. A stable position of the eye is required for normal oculomotor function. Minor low magnitude, low frequency displacements of eye position can be compensated for by several neural circuits. However, based on experimental and clinical data, moderate to high magnitude and frequency displacements cannot be compensated for in humans, and presumably other mammals.

2. Anterior temporalis and medial pterygoid muscle stimulation displaces the eye in anesthetized Otolemur and Felis. The magnitudes of these displacements exceed the magnitudes required to cause diplopia in awake animals. It is possible that diplopia may occur in awake masticating animals.

3. The presence of large postorbital processes in two Felis, and postorbital bars in one Otolemur and one Felis did not prevent eye displacement during isolated contractions of the anterior temporalis and medial pterygoid muscles. These data suggest that postorbital bars do not insulate the eye and orbital contents from the actions of the masticatory muscles in small-bodied primates and carnivorans.

4. Based on the anatomical and clinical literature, it is probable that the postorbital bar functions to maintain rigidity of the orbit required for the extraocular muscles to position and reposition the eye, and potentially compensate for movements due to masticatory muscle activity. The function of the postorbital bar is to maintain a stiff lateral orbit to prevent gross deformation of the orbital margin. This achieves two things. First, as suggested by Cartmill (1970, 1972) it ameliorates gross eye movements caused by deformation of the orbit. Second, it provides a stable substrate from which the extraocular muscle system can compensate for the remaining small scale eye movements identified by the present study as being caused by contraction of the anterior temporalis and medial pterygoid.

5. These ocular kinematic data collected during anterior temporalis and medial pterygoid muscle stimulation suggest that the haplorhine postorbital septum, the bony wall that largely insulates the orbit from the anterior temporal fossa, would prevent the masticatory muscles from disrupting eye position. This supports the hypotheses advanced for the function of the postorbital septum by Cartmill (1980) and Ross (1995, 1996), although these data do not allow us to distinguish between these hypotheses.

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