Comparative anatomical study of the hyoid apparatus in selected primates.

This paper embodies a comparative study of the hyoid apparatus in six species of primates. There is a direct correlation between a predominantly herbivorous diet and an increase in the relative size of the basihyal (the body) of the hyoid bone. The basihyal has a deep "cup-shaped" concavity in the howler but is very flat in the marmoset. The baboon, rhesus and capuchin fall in between these two extremes. The configuration and size of the basihyal are directly proportional to the presence or absence and the size of the median air-sacs. Of the species studied, the posterior cornu (thyrohyal) of the hyoid bone is present in all the species, but the hypohyal is present only in the baboon, rhesus and capuchin. A true styloid process is found only in the baboon, although the rhesus has a bony spicule. All except the howler have a stylomandibular ligament from which arise the stylohyoid, stylopharyngeus and styloglossus muscles. The anterior bellies of the two sides of the digastric muscle are truly united only in the marmoset, but are approximated in the baboon and rhesus. The stylohyoid muscle at its insertion into the basihyal is pierced by the intermediate tendon of the digastric muscle in the rhesus and capuchin, but not in the howler and baboon. In the marmoset the presence of a stylohoid muscle is variable.