The roof (superior wall) of the bony orbit is made up of the orbital part of the frontal bone with a small contribution from the sphenoid bone (Fig. 8.76). This thin plate of bone separates the contents of the orbit from the brain in the anterior cranial fossa.
Unique features of the superior wall include: anteromedially, the trochlear fovea, for the attachment of a pulley through which the superior oblique muscle passes, and the possible intrusion of part of the frontal sinus; anterolaterally, a depression (the lacrimal fossa) for the orbital part of the lacrimal gland.
Posteriorly, the lesser wing of the sphenoid bone completes the roof.
The medial walls of the paired bony orbits are parallel to each other and each consists of four bones—the maxilla, lacrimal, ethmoid, and sphenoid bones (Fig. 8.76).
The largest contributor to the medial wall is the orbital plate of the ethmoid bone. This part of the ethmoid bone contains collections of ethmoidal cells, which are clearly visible in a dried skull.
Also visible, at the junction between the roof and the medial wall, usually associated with the frontoethmoidal suture, are the anterior and posterior ethmoidal foramina. The anterior and posterior ethmoidal nerves and vessels leave the orbit through these openings.
Anterior to the ethmoid bone is the small lacrimal bone, and completing the anterior part of the medial wall is the frontal process of the maxilla. These two bones participate in the formation of the lacrimal groove, which contains the lacrimal sac and is bound by the posterior lacrimal crest (part of the lacrimal bone) and the anterior lacrimal crest (part of the maxilla).
Posterior to the ethmoid bone the medial wall is completed by a small part of the sphenoid bone, which forms a part of the medial wall of the optic canal.
The floor (inferior wall) of the bony orbit, which is also the roof of the maxillary sinus, consists primarily of the orbital surface of the maxilla (Fig. 8.76), with small contributions from the zygomatic and palatine bones.
Beginning posteriorly and continuing along the lateral boundary of the floor of the bony orbit is the inferior orbital fissure. Beyond the anterior end of the fissure the zygomatic bone completes the floor of the bony orbit.
Posteriorly, the orbital process of the palatine bone makes a small contribution to the floor of the bony orbit near the junction of the maxilla, ethmoid, and sphenoid bones.
The lateral wall of the bony orbit consists of contributions from two bones—anteriorly, the zygomatic bone and posteriorly, the greater wing of the sphenoid bone (Fig. 8.76). The superior orbital fissure is between the greater wing of the sphenoid and the lesser wing of the sphenoid that forms part of the roof.
The upper and lower eyelids are anterior structures that, when closed, protect the surface of the eyeball.
The space between the eyelids, when they are open, is the palpebral fissure.
The layers of the eyelids, from anterior to posterior, consist of skin, subcutaneous tissue, voluntary muscle, the orbital septum, the tarsus, and conjunctiva (Fig. 8.77).
The upper and lower eyelids are basically similar in structure except for the addition of two muscles in the upper eyelid.
The skin of the eyelids is not particularly substantial, and only a thin layer of connective tissue separates the skin from the underlying voluntary muscle layer (Fig. 8.77). The thin layer of connective tissue and its loose arrangement account for the accumulation of fluid (blood) when an injury occurs.