Theorbital orhorizontal part of thefrontal bone (pars orbitalis) consists of two thin triangular plates, the orbital plates, which form the vaults of the orbits, and are separated from one another by a median gap, theethmoidal notch.
Theinferior surface of each orbital plate is smooth and concave, and presents, laterally, under cover of thezygomatic process, a shallow depression, thelacrimal fossa, for thelacrimal gland; near the nasal part is a depression, thefovea trochlearis, or occasionally a small trochlear spine, for the attachment of the cartilaginous pulley of theobliquus oculi superior.
Thesuperior surface is convex, and marked by depressions for the convolutions of thefrontal lobes of thebrain, and faint grooves for the meningeal branches of theethmoidal vessels.
Theethmoidal notch separates the two orbital plates; it is quadrilateral, and filled, in the articulated skull, by thecribriform plate of the ethmoid.
The margins of the notch present several half-cells which, when united with corresponding half-cells on the upper surface of the ethmoid, complete theethmoidal air cells.
Two grooves cross these edges transversely; they are converted into the anterior and posterior ethmoidal canals by theethmoid, and open on the medial wall of theorbit.
In front of the ethmoidal notch, on either side of the frontal spine, are the openings of thefrontal air sinuses.
These are two irregular cavities, which extend backward, upward, and lateralward for a variable distance between the two tables of the skull; they are separated from one another by a thin bony septum, which often deviates to one or other side, with the result that the sinuses are rarely symmetrical.
Absent at birth, they are usually fairly well-developed between the seventh and eighth years, but only reach their full size afterpuberty.
They vary in size in different persons, and are larger in men than in women.
This article incorporates text in thepublic domain frompage 137 of the 20th edition ofGray's Anatomy(1918)
