The thinner portion of the muscle of the palatopharyngeal arch, originating in the region of the midline where its fibers interdigitate with the contralateral partner, then passing posterior to the levator veli palatini muscle to join the longitudinal layer of pharyngeal musculature; acts as a sort of sphincter, reducing the caliber of the isthmus of fauces at the palatopharyngeal arch. [ http://medical-dictionary.thefreedictionary.com/velopharyngeal+sphincter ]