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This study tests several hypotheses of integration between the cranial base and face in primates. After reviewing the definition and anatomical basis for the posterior maxillary (PM) plane, which demarcates the back of the midface at its junction with the sphenoid, we demonstrate how the PM plane can be identified accurately on radiographs, and confirm that it maintains a 90°angle relative to the Neutral Horizontal Axis of the orbits in all primates. In addition, we use the PM plane to test Dabelow's (1929) hypothesis that the orbits and anterior cranial base are more highly integrated in anthropoids than in strepsirrhines, and we test the hypothesis that the midline anterior cranial base (planum sphenoideum) and anterior cranial floor (planum sphenoideum plus cribriform plate) in primates are highly correlated with each other relative to the PM plane. The mean angle between the anterior cranial base and the PM plane does not differ significantly from 90°in anthropoids, but differs significantly in strepsirrhines. The anterior cranial base and anterior cranial floor, however, correlate well with each other relative to the PM plane in both suborders of primates, independent of orbital orientation and configuration. The PM plane, anterior cranial base, and anterior cranial floor, therefore, form an integrated structural complex, a "facial block," whose orientation relative to the posterior cranial base influences craniofacial shape among anthropoids in which orbital orientation influences the orientation of the anterior cranial base. One such effect is that increases in cranial base flexion shorten the antero-posterior length of the nasopharynx. Anat Key words: posterior maxillary (PM) plane; neutral horizontal axis; integration; "facial block"The posterior maxillary (PM) plane 1 is defined as the line connecting two radiographically determined termini projected onto the midsagittal plane: (1) the average most posteroinferior point on the maxillary tuberosities, and (2) the average point on the anterior-most extent of the greater wings of the sphenoid (Fig. 1). Although the PM plane was developed for, and has principally been used in, radiographic studies of cranial growth in humans Enlow and Moyers, 1971;Enlow and McNamara, 1973;Enlow and Azuma, 1975;Enlow, 1990), it may have a wider utility for comparative studies of primate and hominin craniofacial growth and architecture 1 Technically, the PM plane is not a plane (which is defined by three points), but is really a line defined by the average of two points inferiorly and two points superiorly. We use the term "plane" because it is established in the literature. Bromage, 1992;Lieberman, 1998). According to Enlow (1990:171), the PM plane is "one of the most basic and important planes in the whole head" for several architectural and developmental reasons. Spatially, the PM plane is an attempt to characterize the posterior margin of the ethmomaxillary complex (the ethmoid, maxilla, and palatine) where it articulates with the boundary between the anterior...

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Despite centuries of investigation, the function of the maxillary sinus (MS) and underlying patterns governing its form remain elusive. In this study, we articulate a methodology for collecting volumetric data for the MS and nasal cavity (NC) from computed tomography (CT) scans and report details for a small sample of 39 dried human crania of known ecogeographic provenience useful for assessing variation in MS size and shape. We use scaling analyses to preliminarily test the hypothesis that volumes of the nasal cavity (NCV) and maxillary sinus (MSV) are inversely correlated such that the NC covaries with size of the face, whereas the MS "fills in" the leftover space [proposed by Shea: Am J Phys Anthropol 47 (1977):289-300]. Against expectation, MSV is not significantly correlated with NCV or any cranial size variable. NCV, on the other hand, scales isometrically with facial size. The results of this pilot study suggest that NCV covaries with facial size, but that the MS does not simply fill in the leftover space in the face. The role, if any, of the MSs in midfacial function and architecture remains unclear. Larger sample sizes, additional environmental variables, and assessment of MS and NC shape are necessary to resolve this issue.