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Comparative Study of Basicranial Morphology among Catarrhines

Takeshi NISHIMURA
Section of Morphology, Primate Research Institute, Kyoto University

     The uniqueness of the human basicranial flexion was showed by many studies. Laitman and Crelin (1978) suggested the flexion relate to the descent of the human larynx through comparison between human and apes. The anatomical relationships between these morphological changes during growth have not been studied. This report aims to explain the anatomical relationships between the basicranial morphology and the surrounding tissues with comparison between human, apes, and cercopithecoids. The basicranial shapes were measured three-dimensionally. The musculature and tendons inserting into the basicranium was scanned with Magnetic Resonance Imaging (MRI).
     The basicranial inclination against palate line of human is more similar to those of cercopithecoids than to those of apes. The basicranial flexion in cercopithecoids, however, is convex ventrally, that is inverse to that formed in human. The basicranial area is divided into two parts, basisphenoidal and basioccipital parts. The basioccipital inclination shows more variable than the basisphenoidal one, according to the analysis of the growth in Japanese macaques. The basioccipital inclination of human is obviously different from those of catarrhine monkeys that are similar to each other. The basisphenoidal inclination of human is similar to those of apes, but different from those of cercopithecoids. The observations of surrounding tissues showed that the longus capitis muscle (LCM) of human developed less than those of apes and cercopithecoids do.
     These results suggested that the function of the LCM of catarrhine monkeys is different from that of human, that is the LCM in catarrhine monkeys put head in position and rotate head, but that in human only let them bow. In human, those movements are functioned mainly by the sternocleidomastoideus muscle. The functional difference of the LCM would be related to that of basicranial flexion between human and nonhuman-primates.