Wall C E, Larson S G, Stern J T
Doctoral Program in Anthropological Sciences, State University of New York, Stony Brook 11794.
Am J Phys Anthropol. 1994 Aug;94(4):549-67. doi: 10.1002/ajpa.1330940408.
Unlike all other primates, the digastric muscle of the orangutan lacks an anterior belly; the posterior belly, while present, inserts directly onto the mandible. To understand the functional consequences of this morphologic novelty, the EMG activity patterns of the digastric muscle and other potential mandibular depressors were studied in a gibbon and an orangutan. The results suggest a significant degree of functional differentiation between the two digastric bellies. In the gibbon, the recruitment pattern of the posterior digastric during mastication is typically biphasic. It is an important mandibular depressor, active in this role during mastication and wide opening. It also acts with the anterior suprahyoid muscles to move the hyoid prior to jaw opening during mastication. The recruitment patterns of the anterior digastric suggest that it is functionally allied to the geniohyoid and mylohyoid. For example, although it transmits the force of the posterior digastric during mandibular depression, it functions independent of the posterior digastric during swallowing. Of the muscles studied, the posterior digastric was the only muscle to exhibit major differences in recruitment pattern between the two species. The posterior digastric retains its function as a mandibular depressor in orangutans, but is never recruited biphasically, and is not active prior to opening. The unique anatomy of the digastric muscle in orangutans results in decoupling of the mechanisms for hyoid movement and mandibular depression, and during unilateral activity it potentially contributes to substantial transverse movements of the mandible. Hypotheses to explain the loss of the anterior digastric should incorporate these functional conclusions.
与所有其他灵长类动物不同，红毛猩猩的二腹肌没有前腹；后腹虽然存在，但直接附着在下颌骨上。为了了解这种形态新奇性的功能后果，研究了长臂猿和红毛猩猩二腹肌及其他潜在下颌降肌的肌电图活动模式。结果表明，两个二腹在功能上有显著程度的分化。在长臂猿中，咀嚼时后二腹肌的募集模式通常是双相的。它是重要的下颌降肌，在咀嚼和大张口时发挥这一作用。在咀嚼时，它还与舌骨上前方肌肉协同作用，在张口前移动舌骨。前二腹肌的募集模式表明，它在功能上与颏舌骨肌和下颌舌骨肌相关联。例如，虽然在下颌下降时它传递后二腹肌的力量，但在吞咽时它独立于后二腹肌发挥作用。在所研究的肌肉中，后二腹肌是唯一在两个物种之间募集模式存在重大差异的肌肉。红毛猩猩的后二腹肌保留了作为下颌降肌的功能，但从不双相募集，且在张口前不活跃。红毛猩猩二腹肌的独特解剖结构导致舌骨运动和下颌下降机制解耦，在单侧活动时，它可能导致下颌的大量横向运动。解释前二腹肌缺失的假说应纳入这些功能结论。
