Seven bones make up the orbit. Click on the photograph to enlarge and test yourself by naming the numbered bones. The roof of the orbit is composed of 2 bones, the frontal bone and the sphenoid bone. The frontal bone (#1 in blue) comprises anterior part of the roof of the orbit and the lesser wing of the sphenoid (#2 in tan) surrounds the optic canal and forms the posterior part of the roof.
The medial wall of the orbit is composed of 4 bones: sphenoid, ethmoid, lacrimal and maxillary bone. The lesser wing of the sphenoid (#2 in tan) is most posterior and is joined to the ethmoid bone (#3 in dark green), moving anteriorly to the lacrimal bone (#4 in light red) and then to the maxillary bone (#5 in light green).
The floor of the orbit is composed of 3 bones: maxillary bone (#5 light green); zygoma (#6 in pink) and posteriorly the palantine bone (#8 in bright red). The palantine bone borders on the inferior orbital fissure. Notice that the inferior orbital fissure narrow posteriorly, a useful landmark in CT scans.
The lateral wall of the orbit is composed of the zygoma (#6 in pink) and the greater wing of the sphenoid (#7 in tan). There is an important landmark on the zygoma that every ophthalmologist and emergency physician must know. The lateral orbital tubercle (that dark area above #6 in the figure) is the place where the lateral canthal tendon joins the orbit. The lateral canthal tendon, which is a fibrous extension of the tarsi of both eyelids, constrains the orbital contents. In traumatic injury with orbital hemorrhage the intraorbital pressure and intraocular pressure may rise high enough to snuff out blood flow and vision. In this case the lateral canthal tendon can be lysed at the lateral orbital tubercle. The inferior crux of the lateral canthal tendon can be divided as well. This emergency procedure, lateral canthotomy and cantholysis may prevent permanent vision loss.
Some foramina or openings through the bones of the orbit and their associated contents are shown in this depiction of the boney posterior left orbit. It is important for students interested in ophthalmology to know the contents of these canals. With this knowledge, findings in such disorders as superior orbital fissue syndrome and carotid cavernous fistulas will make much more sense. See how many entering nerves and vessels you can name correctly.
Backlighting of the cranial cavity with the skull-cap removed, exposes some foramina (optic canal, superior orbital fissure and pterygoid canal); light is shining in from the cranial cavity. The inferior orbital fissure does not enter the cranial cavity and therefore is relatively dark in this illustration.
Superior orbital fissure:
1. lacrimal nerve
2. frontal nerve
3. trochlear nerve
4. oculomotor nerve (superior division)
5. nasociliary nerve
6. abducens nerve
7. oculomotor nerve (inferior division)
8. superior ophthalmic vein
blue arrow- sympathetic fibers of ciliary ganglion
Inferior Orbital Fissure:
9. maxillary nerve
10. nerve entering from the pterygoid canal
11. inferior ophthalmic vein
12. optic nerve
13. ophthalmic artery
red arrow- sympathetic fibers carotid plexus