The primate cranial base: ontogeny, function and - Harvard University
The primate cranial base: ontogeny, function and - Harvard University
The primate cranial base: ontogeny, function and - Harvard University
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138 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 43, 2000<br />
above interspecific analyses of adults with<br />
comparable analyses of infants at the period<br />
when the brain has ceased growing, but before<br />
much of the face has grown.<br />
Brain volume <strong>and</strong> posterior <strong>cranial</strong><br />
fossa shape<br />
Dean <strong>and</strong> Wood (1981, 1982) <strong>and</strong> Aiello<br />
<strong>and</strong> Dean (1990) hypothesized that increases<br />
in cerebellum size correlate with increases<br />
in the size of the posterior <strong>cranial</strong><br />
fossa. This correlation is purportedly a result<br />
of increases in basi<strong>cranial</strong> flexion; <strong>and</strong><br />
by lateral <strong>and</strong> anterior displacement of the<br />
lateral aspects of the petrous pyramids,<br />
which cause the petrous pyramids to be<br />
more coronally oriented in humans than in<br />
great apes. However, Ross <strong>and</strong> Ravosa<br />
(1993) found little support for a link between<br />
absolute cerebellum volume <strong>and</strong><br />
CBA4; in addition, Spoor (1997) did not find<br />
a correlation between cerebellum volume<br />
<strong>and</strong> petrous orientation. Rather, Spoor<br />
(1997) showed that more coronally oriented<br />
petrous pyramids in adult <strong>primate</strong>s correlate<br />
better with increases in brain volume<br />
relative to basi<strong>cranial</strong> length. In addition,<br />
the petrous pyramids, when viewed from<br />
the internal aspect of the <strong>cranial</strong> <strong>base</strong>, are<br />
not more coronally oriented in humans vs.<br />
other apes (Spoor, 1997).<br />
<strong>The</strong> probable explanation for these results<br />
may be that the posterior <strong>cranial</strong> <strong>base</strong><br />
(Ba-S) scales with isometry against both<br />
cerebellum volume <strong>and</strong> the volume of infratentorial<br />
neural structures (cerebellum, medulla<br />
oblongata, mesencephalon) (Table 4).<br />
If this scaling pattern characterizes those<br />
dimensions of the posterior <strong>cranial</strong> fossa not<br />
in the mid-sagittal plane, then increased<br />
infratentorial neural volumes would not necessitate<br />
changes in the proportions of the<br />
entire posterior <strong>cranial</strong> fossa. This would<br />
imply that variation in the orientation of<br />
the petrous pyramids may be linked to<br />
changes in other <strong>cranial</strong> systems <strong>and</strong> is not<br />
solely a structural response to changes in<br />
cerebellar volume.<br />
ASSOCIATIONS BETWEEN CRANIAL<br />
BASE AND FACE<br />
It has long been known that the <strong>cranial</strong><br />
<strong>base</strong> plays an important role in facial<br />
growth, but many details of how these regions<br />
interact remain poorly understood.<br />
While the face has some influence on <strong>cranial</strong><br />
<strong>base</strong> growth (see below), there are two major<br />
reasons to believe that the <strong>cranial</strong> <strong>base</strong><br />
exerts a greater influence on the face than<br />
vice versa during growth by setting up certain<br />
key spatial relationships. First, the majority<br />
of the <strong>cranial</strong> <strong>base</strong> (with the exception<br />
of the ethmoidal portions of the ethmomaxillary<br />
complex) attains adult size long before<br />
the face (Moore <strong>and</strong> Lavelle, 1974). Second,<br />
as noted above, most of the face grows anteriorly,<br />
laterally, inferiorly, <strong>and</strong> around the<br />
<strong>cranial</strong> <strong>base</strong>. In all mammals, the upper<br />
portion of the face (the orbital <strong>and</strong> upper<br />
nasal regions) grows antero inferiorly relative<br />
to the anterior <strong>cranial</strong> <strong>base</strong> <strong>and</strong> floor;<br />
<strong>and</strong> the middle face (mostly the nasal region)<br />
grows anteriorly relative to the middle<br />
<strong>cranial</strong> fossa. <strong>The</strong> lower portion of the face<br />
(the m<strong>and</strong>ibular <strong>and</strong> maxillary arches <strong>and</strong><br />
their supporting structures) interacts only<br />
indirectly with the <strong>cranial</strong> <strong>base</strong>, since the<br />
maxillary arch grows inferiorly from the<br />
middle face <strong>and</strong> anteriorly relative to the<br />
pterygoid processes of the sphenoid.<br />
<strong>The</strong>se spatial <strong>and</strong> developmental associations<br />
raise an important question: to what<br />
extent does the <strong>cranial</strong> <strong>base</strong> influence facial<br />
growth <strong>and</strong> form? In order to address this<br />
issue, we first discuss the relationships between<br />
two regions of the face <strong>and</strong> <strong>cranial</strong><br />
<strong>base</strong> that are contiguous across <strong>function</strong>al<br />
or developmental boundaries (the so-called<br />
growth counterparts of Enlow, 1990): 1) the<br />
anterior <strong>cranial</strong> fossa <strong>and</strong> the upper, orbital,<br />
<strong>and</strong> nasal portions of the face, <strong>and</strong> 2) the<br />
middle <strong>cranial</strong> fossa <strong>and</strong> the middle, ethmomaxillary<br />
portion of the face. We conclude<br />
with a brief discussion of the possible relationships<br />
between <strong>cranial</strong> <strong>base</strong> shape <strong>and</strong><br />
overall facial shape.<br />
Anterior <strong>cranial</strong> fossa shape <strong>and</strong> upper<br />
facial growth<br />
<strong>The</strong> upper face comprises the orbital cavities,<br />
the orbital superstructures, <strong>and</strong> the<br />
upper portion of the nasal cavity. <strong>The</strong> upper<br />
face therefore incorporates elements of the<br />
anterior <strong>cranial</strong> <strong>base</strong>, including the ethmoid,<br />
parts of the sphenoid, <strong>and</strong> significant<br />
portions of the frontal bone. <strong>The</strong> upper face