the greater wing (Figure 8-5): the foramen rotundum, through which the maxillary nerve passes; the foramen ovale, through which the mandibular nerve passes; and the foramen spinosum, through which the middle meningeal artery passes.

The single ethmoid bone resembles a rectangular box that contains a midline perpendicular plate. This plate bisects the top of the box, the horizontal cribriform plate, which is perforated for the passage of the olfactory nerves. The sides of the box parallel to the perpendicular plate are the orbital plates and are separated from the perpendicular plate by the ethmoid air cells. The ethmoid bone articulates with the sphenoid and frontal bones superiorly and with the vomer inferiorly; the orbital plates also articulate with the maxillary and lacrimal bones.

The two nasal bones form the bridge of the nose and articulate with each other, with the frontal bone, and with the frontal processes of the maxillary bones (see Figure 8-4). The vomer is a single bone that forms the posterior part of the nasal septum. It articulates with the palatine and maxillary bones inferiorly and with the ethmoid bone superiorly. The inferior conchae are separate bones located along the lateral walls of the nasal cavity.

The lacrimal bone (one in each orbit) is the smallest bone of the face and articulates with the maxillary bone, ethmoid bone, and frontal bone.

There are two palatine bones. Each is an L-shaped bone that extends from the hard palate at the back of the mouth to the orbit. The horizontal plate is found in the oral cavity; the vertical stem runs along the posterior aspect of the nasal cavity and articulates with the pterygoid process of the sphenoid bone. A small, flattened area at the top of the vertical stem is located in the orbital floor at the posterior edge of the orbital plate of the maxilla.

The paired zygomatic bones form the lateral part of the cheekbones and articulate with the zygomatic process of the temporal bones to form the zygomatic arches (see Figure 8-1). The zygomatic bones also articulate with the maxillary bones and with the greater wings of the sphenoid bone.

The mandible forms the movable lower jaw. It is a horseshoe-shaped bone consisting of a curved horizontal body and two perpendicular processes, the rami.

T H E O R B I T

The orbits are bony cavities on either side of the midsagittal plane of the skull below the cranium. They contain the globes, the extraocular muscles, and orbital nerves, blood vessels, and connective tissue.

The orbit is shaped like a four-sided pyramid, the base of which is at the anterior orbital margin and the apex at

the posterior margin within the skull. The orbital walls are referred to as the roof, floor, and medial and lateral walls. The medial walls run approximately parallel to each other, whereas the two lateral walls, if extended posteriorly, would form approximately a 90-degree angle with each other1,2 (Figure 8-6). The orbit has also been described as pear shaped, having its widest portion 1.5 cm inside the orbital margin.1 The orbital floor extends to approximately two-thirds the depth of the orbit; the other three sides extend to the apex.

Each orbit is composed of seven bones—the frontal, maxillary, zygomatic, sphenoid, ethmoid, palatine, and lacrimal bones (Figure 8-7). The frontal, sphenoid, and ethmoid are each a single bone and take part in the formation of both orbits.

ORBITAL WALLS

Roof

The roof is triangular and is composed primarily of the orbital plate of the frontal bone in front (Figure 8-8). The lesser wing of the sphenoid contributes a small posterior portion. The orbital plate of the frontal bone is thin in the area that separates the orbit from the anterior cranial fossa. In an elderly adult, bone in this area may resorb, leaving only the periosteal connective tissue in contact with the dural covering of the frontal lobe of the brain. The small area of the lesser wing of the sphenoid that is involved in this wall runs slightly downward, and an oval foramen, the optic canal, lies between it and the body of the sphenoid (see Figure 8-7). This optic foramen is located roughly at the apex of the orbit.

The frontal bone forms the ridge of the superior orbital margin. Behind the lateral aspect of this margin is an indentation in the frontal bone: the fossa for the lacrimal gland. A U-shaped piece of cartilage, the trochlea, is attached to the orbital plate of the frontal bone approximately 2 mm behind the medial aspect of the superior orbital margin. The tendon of the superior oblique muscle passes through this pulleylike structure.

Floor

The floor is also triangular and is composed of the orbital plate of the maxillary bone and the orbital plate of the zygomatic bone in front and the small orbital process of the palatine bone behind (­ Figure 8-9). The maxillary bone makes up the largest part of the floor, and most of the remainder is provided by the zygomatic bone. The orbital process of the palatine bone is a small, flattened area at the top of the vertical arm and is located at the most posterior edge of the orbital plate of the maxilla. Often in the adult skull, the

Medial orbital walls

Lateral orbital walls

FIGURE 8-6

Angular relationship of orbital walls. Medial walls are approximately parallel to each other; if lateral walls were extended, an approximate right angle would be formed.

suture between the orbital process of the palatine bone and the maxilla is indistinguishable.

The floor does not reach all the way to the apex and is separated from the lateral wall posteriorly by the inferior orbital fissure. The infraorbital groove runs across the floor from the inferior orbital fissure and anteriorly is bridged by a thin plate of bone, thus becoming a canal running within the maxillary bone. This canal opens on the facial surface of the maxilla below the inferior orbital margin as the infraorbital foramen (see Figure 8-7). The inferior orbital margin is composed of the maxilla and the maxillary process of the zygomatic bone.

Clinical Comment: Blow-Out

Fracture­ of the Orbit

The orbital rim is strong and can withstand considerable impact. However, a blow to the orbital rim can cause compression of the orbital contents, and such a sudden increase in intraorbital pressure might cause a fracture in one of the orbital walls. In the classic blow-out fracture, the orbital rim remains intact. The floor of the orbit is particularly susceptible to such a fracture, which usually occurs in the thin region along the infraorbital canal2-4 (Figure 8-10). Clinical signs and symptoms accompanying

this damage include orbital swelling, ecchymosis, anesthesia of the area innervated by the infraorbital nerve, and diplopia caused by restriction of ocular motility (particularly

Medial Wall

The medial wall is rectangular. From front to back, it is formed by the frontal process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid, and a part of the body of the sphenoid (Figure 8-11). A ridge on the frontal process of the maxilla that forms the anterior part of the medial orbital margin also forms the anterior lacrimal crest, which demarcates one border of the fossa for the lacrimal sac. The lacrimal bone, a small bone approximately the size of a thumbnail, together with the frontal process of the maxillary bone, forms the wall of this fossa. The lower portion of the fossa is a groove that is continuous inferiorly with the nasolacrimal canal, which continues into the nasal cavity. A ridge in the lacrimal bone forms the posterior lacrimal crest and is continuous superiorly with the prominence of the frontal bone, forming the posterior part of the medial margin of the orbit.

The ethmoid bone forms most of the medial wall. The orbital plate of the ethmoid sometimes is said to be “paper thin” (lamina papyracea); thus the medial wall is the thinnest of the orbital walls. The small part of the sphenoid bone present in this wall is part of the body and is located at the posterior end adjacent to the wall of the optic canal. The floor is joined to the medial wall at the sutures connecting the bones of the two walls, and the anterior and posterior ethmoidal canals are located within the frontoethmoidal suture at the junction of the roof and medial wall.

Lateral Wall

The lateral wall is roughly triangular and is composed of the zygomatic bone in front and of the greater wing of the sphenoid bone behind (Figure 8-12). The zygomatic bone separates the orbit from the temporal fossa. One or more foramina may be present in the zygomatic bone as a conduit for nerves and vessels between the orbit and facial areas. The lateral or marginal orbital tubercle (Whitnall’s tubercle) is a small, bony prominence located on the orbital surface of the zygomatic bone and is the attachment site for the aponeurosis of the superior palpebral levator muscle, the lateral palpebral ligament, and the lateral check ligament.7

The greater wing of the sphenoid separates the orbit from the middle cranial fossa. The roof is separated from the lateral wall in back by the superior orbital fissure and in front by the frontozygomatic and