Bone · Head

Sphenoid

Os sphenoidale

A single bone shaped like a bat with its wings spread, set across the floor of the skull behind the eyes. It is the keystone of the head: it wedges into every other bone of the braincase, and almost all of it is buried where no examination can reach it. Its body is hollow — the sphenoid sinus — and its upper surface carries a small saddle of bone, the sella turcica, that cradles the pituitary gland. A pair of processes hangs down from it behind the upper jaw, each splitting into two plates, and the muscles that slide the jaw forward and swing it side to side arise from them — which is how a bone nobody can touch is nevertheless part of the machinery of chewing.