The orbit lies below the
anterior cranial fossa, above the maxillary air sinus,
lateral to the nasal cavity and anterior to the middle cranial fossa (medially)
and the temporal fossa (laterally). It is a pyramidal cavity with a volume of
approximately 1.5 ounces (that of a standard shot glass). From its base, the
orbital margin, it tapers posteriorly to an apex, the entrance of the optic
canal. Two large discontinuities, the superior and
inferior orbital fissures,
converge upon one another in the back of the orbit just lateral to the apex.
The lateral wall of the orbit is offset approximately 45є from the medial wall, which is oriented anteroposteriorly, in a parasagittal plane. Thus the orbital axis is directed anterolaterally at a 22-23є angle.
The orbital plate of the frontal bone forms the superior margin and most of the roof of the orbit.
The maxilla forms the inferomedial margin and the floor of the orbit, which is also the roof of the maxillary sinus.
The zygomatic bone forms the inferolateral margin and the anterior part of the lateral wall of the orbit.
The lateral mass of the ethmoid bone forms much of the medial wall of the orbit.
The sphenoid bone contributes to the structure of the orbit in several ways.
The greater wing constitutes the lateral wall posterior to the zygomatic bone.
The lesser wing forms the posterior margin of the roof.
The roots of the lesser wing, connecting it to the body of the sphenoid, frame the optic canal in the apex of the orbit.
The small, curved lacrimal bone completes the medial wall of the orbit between
the frontal process of the maxilla and the lateral mass of the ethmoid.
The orbital process of the palatine bone, insinuated behind the maxilla and below the ethmoid, completes the posterior angle of the orbital floor.
The lateral margin and wall of the orbit is formed by the zygomatic bone, the wall is completed by the greater wing of the sphenoid.
The greater wing fails to articulate with either the lesser wing of the sphenoid or the maxilla. This leaves two gaps posteriorly- the superior orbital fissure between the lateral wall and the roof, and the inferior orbital fissure between the lateral wall and the floor. The fissures converge in the apex of the orbit, just lateral to the optic canal.
Roughly in line with the inferior orbital fissure, the zygomatico-orbital foramen opens into the orbital surface of the zygomatic bone. This foramen may be doubled or multiple, or it may be so unobtrusive as to be virtually invisible.
The inferior margin of the orbit is formed by the maxilla (medially) and the zygomatic bone (laterally). These two bones constitute virtually the entire orbital floor as well. All but the extreme lateral angle is formed by the orbital surface of the maxilla. The bone of the floor is also the roof of the maxillary air sinus, and it is very thin. The small orbital process of the palatine bone completes the floor immediately in front of the apex.
A prominent infraorbital groove leaves the inferior orbital fissure to run directly forward on the surface of the maxilla. Roughly half-way across the floor of the orbit, the groove becomes continuous with the infraorbital canal, which ends in the Infraorbital foramen on the anterior surface of the maxilla below the orbital margin.
The medial margin of the orbit is formed largely by the frontal process of the maxilla, which rises to meet the shorter maxillary process of the frontal bone. The maxilla itself contributes little more than the rim to the medial orbital wall.
Immediately within the orbit, the fragile, irregularly-shaped lacrimal bone articulates vertically with the frontal process of the maxilla. Together, the two bones frame the fossa for the lacrimal sac. The fossa is continuous with the bony nasolacrimal canal, which enters the inferior nasal meatus below (the space covered by the inferior nasal concha). The fossa is bounded posteriorly by a vertical ridge, the lacrimal crest, from which the orbital surface of the lacrimal bone
extends posteriorly to meet the lateral mass of the ethmoid.
The lateral mass of the ethmoid separates the orbit from the upper part of the nasal cavity. It is pneumatized extensively by the ethmoid air cells, which in many prepared skulls, give the medial wall of the orbit the appearance of an unviolated honeycomb. The orbital surface of the bone is so fragile that it has been called the “lamina papyracea” (paper-thin layer).
The medial wall and roof of the orbit meet in the fairly obvious frontoethmoidal suture. Across this boundary, the ethmoid air cells of the lateral mass extend upward into the overlying frontal bone. The anterior and posterior ethmoidal foramina open out of the orbit along the suture line into canals that pass horizontally between adjacent air cells. The first leads onto the upper surface of the cribriform plate, the second into the posterior ethmoidal and sphenoid air sinuses.
The frontal bone forms the superior margin of the orbit and all but the posterior extremity of the roof, which is provided by the lesser wing of the sphenoid. In closing the narrow interval between the greater wing of the sphenoid and the tip of the lesser wing, it terminates the superior orbital fissure. Laterally, the roof rises above the level of the rim of the orbit, creating a fossa for the lacrimal gland.
The contour of the superior orbital margin is sharp laterally and rounded medially. At the point of transition, the supraorbital notch (frequently a supraorbital foramen), leads from the orbit onto the face.
On the maxillary process of the frontal bone, in the anteromedial angle of the orbit, a small trochlear fossa (occasionally a minute spicule of bone) gives attachment to the trochlea, a cartilaginous loop, or pulley, through which the tendon of the superior oblique muscle changes direction on its way from the superior medial angle of the orbit to the globe of the eye.
The optic canal opens directly into the apex of the orbit from the middle cranial fossa. It transmits the Optic Nerve (Cranial Nerve II), covered by all three meningeal layers, and the ophthalmic branch of the internal carotid artery.
The superior orbital fissure is the main gateway between the orbit and the middle cranial fossa. It is subdivided in life by an annular tendon (annulus tendineus) that gives origin to four of the six extraocular muscles – the superior, inferior, lateral and medial recti, which pass directly forward to reach the globe of the eye. The tendon encompasses the expanded medial end of the superior orbital fissure as well as the orbital entrance of the optic canal.
[If we can draw in the annulus, can we add the nerves and vessels as they are mentioned?]
As a rule, structures leaving the middle cranial fossa to supply the rectus muscles and/or the eye itself, will pass within the embrace of the annular tendon. All others will enter the orbit through the lateral part of the superior orbital fissure. Thus, Cranial Nerves III (Oculomotor), VI (Abducent), and the nasociliary branch of the ophthalmic division of Cranial Nerve V (Trigeminal) pass within the annulus. Cranial Nerve IV (Trochlear), and the frontal and lacrimal branches of the ophthalmic division of the Trigeminal pass through the superior orbital fissure lateral to the annulus. So does the superior ophthalmic vein on its way back from the orbit to the cavernous dural venous sinus.
The anterior and posterior ethmoidal foramina transmit the anterior and posterior ethmoidal branches of the nasociliary nerve. These are accompanied by corresponding branches of the ophthalmic artery and by veins draining to the superior ophthalmic vein.
The anterior ethmoidal nerve and vessels pass onto the upper surface of the cribriform plate, through which they descend to reach the upper, anterior part of the nasal cavity.
The posterior ethmoidal nerve and vessels reach the sphenoid sinus and posterior ethmoid air cells.
[The nasociliary nerve continues as the infratrochlear nerve, passing forward below the trochlea for the superior oblique muscle to emerge onto the face.]
The supraorbital notch (or foramen) transmits the supraorbital nerve and vessels onto the forehead. The nerve is the larger branch of the frontal nerve. The artery is from the ophthalmic artery, and the vein drains to the superior ophthalmic vein.
[The smaller branch of the frontal, the supratrochlear nerve, and its companion vessels pass forward above the trochlea to emerge on the face.]
The inferior orbital fissure transmits the infraorbital and zygomatic nerves and vessels. The nerves arise in the pterygopalatine fossa as branches of the maxillary division of Cranial Nerve V. The arteries are branches of part three of the maxillary artery, and the veins drain to the pterygoid venous plexus in the infratemporal fossa.
The infraorbital groove and canal carry the infraorbital nerve and vessels from the inferior orbital fissure to the infraorbital foramen, through which they emerge onto the face.
Beyond the lateral end of the inferior orbital fissure, the zygomatico-orbital foramen transmits the zygomatic nerve and vessels into the zygomatic bone. The nerve is a branch of the maxillary division of Cranial Nerve V. The artery arises from part three of the maxillary artery in the pterygopalatine fossa, and the vein drains to the pterygoid venous plexus. Within the bone, the zygomaticofacial and zygomaticotemporal branches of the nerve diverge with their companion vessels.
The zygomaticofacial nerve and vessels emerge onto the face through the zygomaticofacial foramen. Occasionally it is not unusual for there to be two foramen rather than one. The zygomaticotemporal nerve and vessels emerge into the temporal fossa through the zygomaticotemporal foramen.