ヒト口蓋咽頭筋の形態的・機能的意義を考える

The human palatopharyngeus is composed of two divisions: longitudinal and transverse. The longitudinal division originates from the velum, and descends in and along the palatopharyngeal arch to reach the thyroid cartilage and submucosa of the pharynx. It is the portion generally accepted as the palatopharyngeus in standard anatomy textbooks. The transverse division originates from the velum and runs transversely on the lateral to posterior wall of the fauces to reach the pharyngeal raphe. In velopharyngeal closure during swallowing, the lateral and posterior walls of the fauces, opposite to the elevated velum, swell inwards to form Passavant’s ridge, and the pharyngeal isthmus is therefore closed as if strangled. The transverse division occupies the position in which its contraction should produce a ridge and may accordingly be termed the palatopharyngeal sphincter. The transverse division corresponds to the transition muscle bundle (Tr) between the longitudinal division and the superior constrictor of the pharynx (SCP). It remains controversial as to whether the Tr should be regarded as an independent sphincter or as the rostral-most portion of the SCP. In contrast, our previous study clarified that the transverse division was distinct from the SCP, and its contraction may have produced Passavant’s ridge to increase the efficiency of velopharyngeal closure by pressing the salpingopharyngeal fold and musculus uvulae ridge against the elevated velum. In mammals with an intranarial larynx, three muscles radiate from the velum to the tongue, the larynx, and the pharyngeal wall: the palatoglossus, palatothyreoideus (pt), and palatopharyngeus (pp). The latter two collectively correspond to the muscle designated here as the palatopharyngeus and are considered responsible for maintaining the intranarial larynx as follows: the pt draws the larynx near the velum as a retractor while the pp holds the larynx in that position as a sphincter. In humans, however, the larynx is positioned low and is largely separate from the velum, and therefore the anatomical states of the two muscles change as follows: in the pt and lower portion of the pp, the site of insertion descends together with the larynx and the muscles become vertical to form the longitudinal division, whereas the upper portion of the pp, which is not as markedly affected by the descending larynx, retains its primitive position and original sphincter function to form the transverse division.