Chapter 26

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.

###### FIGURE 26–1

The orbit is delineated by the cranial bones that surround it.

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 fissure.

###### FIGURE 26–2

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.

### Embryology

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).

###### FIGURE 26–3

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