Hypoglossal Nerve Size
The hypoglossal nerve (the 12th cranial nerve) supplies motor innervation to all of the intrinsic and all but one of the extrinsic muscles of the tongue. The hypoglossal nerve passes through the hypoglossal canal in the occipital as it exits the cranial vault. Humans have larger hypoglossal canals (both absolutely and relative to oral cavity volume) than other great apes, suggesting a larger hypoglossal nerve in humans. Given the role of the hypoglossal nerve in tongue movement, enlargement of the nerve in humans has been argued to reflect finer motor control of the tongue than possible in other apes, which is important to the rapid production of speech sounds in language. While the size of the hypoglossal canal is absolutely and relatively larger than that of other great apes (although in absolute terms the difference from gorillas is not statistically significant), relative to oral cavity size the human hypoglossal canal is smaller than that of gibbons. Recent studies also suggest that the size of the hypoglossal nerve itself may not differ between humans and chimpanzees.
Kay et al., 1998. The hypoglossal canal and the origin of human vocal behavior. PNAS 95: 5417-5419.
DeGusta et al., 1999. Hypoglossal canal size and hominid speech. PNAS 96: 1800-1804.
Jungers et al., 2003. Hypoglossal canal size in living hominoids and the evolution of human speech. Hum Biol 75: 473-484.
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