Lamina papyracea The viscerocranium (facial skeleton) consists of 15 irregular bones: 3 single bones centered on or lying in the midline (mandible, ethmoid, and vomer) and 6 bones occurring as bilateral pairs (maxillae; inferior nasal conchae; and zygomatic, palatine, nasal, and lacrimal bones). Several bones of the cranium (frontal, temporal, sphenoid, and ethmoid bones) are pneumatized bones, which contain air spaces, presumably to decrease their weight. The ethmoid bone is exceedingly light and spongy, and cubical in shape; this bone is situated at the anterior part of the base of the cranium, between the two orbits, at the roof of the nose, and contributes to each of these cavities. The ethmoid bone consists of four parts: a horizontal or cribriform plate, forming part of the base of the cranium; a perpendicular plate, constituting part of the nasal septum; and two lateral masses or labyrinths.

• Cribriform plate: Contains many olfactory foramina. The olfactory nerves pass through these foramina. Note: Damage to this area typically results in the loss of sense of smell. • Perpendicular plate: The crista galli is a midline projection from the perpendicular plate that serves as an attachment for the falx cerebri.

• Lateral masses (right and left) project downward from the cribriform plate. They contain the ethmoid sinuses and the orbital plate of the ethmoid bone (lamina papyracea). The lamina papyracea forms the paper-thin medial wall of the orbit. The superior and middle nasal conchae are scroll-like projections that extend medially from the lateral masses into the nasal cavity.

1. Each ethmoidal sinus is divided into anterior, middle, and posterior ethmoidal air Notes cells. 2. The superior wall or roof of the orbit is formed almost completely by the orbital plate of the frontal bone. Posteriorly, the superior wall is formed by the lesser wing of the sphenoid bone.