tonsillar fossa tonsillar fossa the tonsillar fossa (or tonsillar sinus) is a space delineated by the triangular fold (plica triangularis) of the palatoglossal and palatopharyngeal arches within the lateral wall of the oral cavity. tonsillar fossathe mouth cavity. (tonsillar fossa, which is a part of the oropharynx, visible but not labeled.)detailsidentifierslatinfossa tonsillarista98a05.2.01.009ta22851fma55041anatomical terminology in many cases, however, this sinus is obliterated by its walls becoming adherent to the palatine tonsils.