The craniofacial haft resists forces generated in the face during feeding,
but the importance of these forces for the form of the craniofacial haft re
mains to be determined. In vivo bone strain data were recorded from the med
ial orbital wall in an owl monkey (Aotus), rhesus macaques (Macaca mulatta)
, and a galago (Otolemur) during feeding. These data were used to determine
whether: the interorbital region can be modeled as a simple beam under ben
ding or shear; the face is twisting on the brain case during unilateral bit
ing or mastication; the interorbital "pillar" is being axially compressed d
uring incisor loading and both axially compressed and laterally bent during
mastication; and the interorbital "pillar" transmits axial compressive for
ces from the toothrow to the braincase.
The strain data reveal that the interorbital region cannot be modeled as a
anteroposteriorly oriented beam bent superiorly in the sagittal plane durin
g incision or mastication. The strain orientations recorded in the majority
of experiments axe concordant with those predicted for a short beam under
shear, although the anthropoids displayed evidence of multiple loading regi
mes in the medial orbital wall. Strain orientation data corroborate the hyp
othesis that the strepsirrhine face is twisted during mastication. The hypo
thesis that the interorbital region is a member in a rigid frame subjected
to axial compression during mastication receives some support. The hypothes
is that the interorbital region is a member in a rigid frame subjected to l
ateral bending during mastication is supported by the epsilon (1)/ \ epsilo
n (2)\ ratio data but not by the strain orientation data. The timing of pea
k shear strains in the medial orbital wall of anthropoids does not bear a c
onsistent relationship to the timing of peak shear strain in the mandibular
corpus, suggesting that bite force is not the only external force influenc
ing the medial orbital wall. Strain orientation data suggest the existence
of two distinct loading regimes, possibly associated with masseter or media
l pterygoid contraction. Regardless of the loading regime, all taxa showed
low strain magnitudes in the medial orbital wall relative to the anterior r
oot of the zygoma and the mandibular corpus. The strain gradients documente
d here and elsewhere suggest that, in anthropoids at least, local effects o
f external forces are more important than a single global loading regime. T
he low strain magnitudes in the medial orbital wall and in other thin bony
plates around the orbit suggest that these structures are not optimally des
igned for resisting feeding forces. It is hypothesized that their function
is to provide rigid support and protection for soft-tissue structures such
as the nasal epithelium, the brain, meninges, and the eye and its adnexa. I
n contrast with the face of Otolemur, which appears to be subjected to a si
ngle predominant loading regime, anthropoids may experience different loadi
ng regimes in different parts of the face. This implies that the anthropoid
and strepsirrhine facial skulls might be optimized for different functions
. (C) 2001 Wiley-Liss, Inc.