The orbit is the space in the skull in which the eye and extraocular
muscles are located. It is cone-shaped, widest anteriorly and tapering
to the orbital apex, through which the optic nerve passes. The orbits
are demarcated by the bones of the skull (Figure 26–1).
The roof consists of a portion of the frontal bone and the lesser
wing of the sphenoid bone. The frontal lobes of the brain lie above
this area. The medial orbital wall is composed of the lacrimal bone
and portions of the ethmoid, maxilla, and sphenoid bones. The ethmoid
sinuses are adjacent to this wall. The floor of the orbit is composed
of the maxilla and a portion of the zygomatic bone. It lies above
the maxillary sinus. The lateral wall is formed by the zygomatic
bone and the greater wing of the sphenoid.
The orbit is delineated by the cranial bones that surround
Openings in the orbit allow for passage of other structures (Figure
26–2). The optic nerve, ophthalmic artery, and sympathetic
fibers pass through the optic foramen posteriorly at the apex of the
orbit. Cranial nerves III, IV, and VI, and portions of cranial nerve
V travel through the superior orbital fissure. The infraorbital
nerve (a branch of cranial nerve V) is transmitted through the infraorbital
The posterior orbital openings, through which the optic
nerve, blood vessels, and nerves pass (see text).
All of the extraocular muscles except the inferior oblique muscle
arise at the annulus of Zinn at the apex of the orbit and extend
anteriorly to insert on the globe. A layer of connective tissue
joins these muscles to form a cone-shaped space. Orbital disorders
may be classified as intraconal (within
this group of muscles) or extraconal (outside
of the muscle cone but still within the orbit). The orbital septum
lies beneath the orbicularis muscle, and separates the orbit into
the preseptal and postseptal spaces.
The lacrimal gland is located in the lacrimal fossa in the superolateral
portion of the orbit.
Most of the orbital structures arise from the frontonasal and
maxillary processes of neural crest cells that surround the optic
cups. The various bones that comprise the orbital walls usually
fuse during the sixth month of gestation.
Measurements between various eyelid and ocular structures may
be used to describe some orbital disorders. Hypotelorism is
a smaller-than-normal distance between the medial orbital walls. Hypertelorism is a greater-than-normal
distance between the orbits. Telecanthus is
a greater-than-normal distance between the medial canthi. In telecanthus,
the orbits themselves are often normal (Figure 26–3).