This is a small sheath-like plate of bone that projects anteromedial from the base of the medial lamina of the pterygoid process. This process articulates anteriorly with the palatine and medially with the vomer.

Vagina is Latin for sheath. In osteologic nomenclature, this term describes thin plates of bone forming sheath-like coverings around other bones.

Processus vaginalis ossis sphenoidalis