The sphenoidal sinuses are two large, irregular cavities hollowed out of the interior of the body of the bone, and separated from one another by a bony septum, which is commonly bent to one or the other side. They vary considerably in form and size, are seldom symmetrical, and are often partially subdivided by irregular bony laminæ. Occasionally, they extend into the basilar part of the occipital nearly as far as the foramen magnum. They begin to be developed before birth, and are of a considerable size by the age of six. They are partially closed, in front and below, by two thin, curved plates of bone, the sphenoidal conchæ, leaving in the articulated skull a round opening at the upper part of each sinus by which it communicates with the upper and back part of the nasal cavity and occasionally with the posterior ethmoidal air cells. The lateral margin of the anterior surface is serrated, and articulates with the lamina papyracea of the ethmoid, completing the posterior ethmoidal cells; the lower margin articulates with the orbital process of the palatine bone, and the upper with the orbital plate of the frontal bone.