The orbital cavity, commonly known as the eye socket, is a paired, cone-shaped space situated on the anterior side of the skull. It is designed to house, protect, and facilitate the movement of the eyeball. The orbit is typically described as a four-sided pyramid, with its base opening forward onto the face and its narrow apex pointing toward the back of the skull. Its primary function is to serve as a secure physical container while also providing pathways for the nerves and blood vessels that supply the eye and surrounding tissues.
The Bony Framework
The sturdy architecture of the orbit is a composite structure formed by seven distinct bones of the skull. These bones meet at various sutures to create the four walls of the pyramidal cavity. The composition of these walls determines their relative strength and their anatomical relationship with adjacent structures.
The roof, or superior wall, is formed primarily by the orbital plate of the frontal bone and the lesser wing of the sphenoid bone. This separation of the orbit from the anterior cranial fossa is an important boundary. The floor, which is the inferior wall, is composed of the maxilla, the zygomatic bone, and the palatine bone. It is the thinnest wall, separating the orbital contents from the underlying maxillary sinus.
The lateral wall is the strongest, formed by the zygomatic bone and the greater wing of the sphenoid bone. This wall separates the orbit from the temporal fossa. The medial wall is the most delicate and is formed by four bones: the ethmoid, lacrimal, maxilla, and sphenoid bones. The largest component is the paper-thin ethmoid bone, which separates the orbit from the ethmoid sinuses.
Key Structures Housed Within the Orbit
The most prominent structure within the orbital cavity is the globe, or eyeball. The globe is suspended within the bony socket, surrounded by various soft tissues that provide support and function. Its position and movement are facilitated by the surrounding environment of the orbit.
A large portion of the non-bony space is filled with orbital fat. This fat acts as a resilient cushion, stabilizing the globe against shock and allowing the eye to rotate smoothly within the socket. It helps maintain the eye’s proper position and facilitates the actions of the extraocular muscles.
The lacrimal apparatus handles the production and drainage of tears. The lacrimal gland, situated in a fossa on the roof’s anterolateral side, produces the watery component of tears. Tears then drain through a system of ducts, ultimately passing through the nasolacrimal canal on the medial wall to reach the nasal cavity.
Extraocular Muscles: Controlling Eye Movement
Movement of the eye is controlled by six extraocular muscles. These muscles are divided into two main groups: the four recti muscles and the two oblique muscles. The recti muscles—superior, inferior, medial, and lateral—share a common origin at the back of the orbit, known as the common tendinous ring, or Annulus of Zinn.
The recti muscles extend forward in a straight path to insert onto the anterior half of the sclera. The medial rectus turns the eye inward toward the nose (adduction), while the lateral rectus turns it outward (abduction). The superior rectus primarily elevates the eye, and the inferior rectus primarily depresses it.
The two oblique muscles have a different orientation and action. The superior oblique muscle originates near the common tendinous ring but passes through a cartilaginous pulley called the trochlea before inserting on the posterior sclera. Its main actions are to rotate the eye inward (intorsion) and depress it. The inferior oblique muscle is the only one that originates from the front of the orbit near the medial floor. It rotates the eye outward (extorsion) and elevates it.
Neural and Vascular Connections
The orbit serves as a major conduit for neurovascular structures connecting the eye to the brain. Two principal openings in the bony framework allow passage for these connections: the optic canal and the superior orbital fissure. These openings are located at the orbital apex, the narrow posterior part of the cavity.
The optic canal transmits the optic nerve (Cranial Nerve II), which carries visual information from the retina to the brain. The ophthalmic artery, a branch of the internal carotid artery, also enters through this canal. This artery is the primary source of blood for the eye and its associated tissues.
The superior orbital fissure is a larger opening that transmits the major motor nerves controlling the extraocular muscles. These include the oculomotor nerve (Cranial Nerve III), the trochlear nerve (Cranial Nerve IV), and the abducens nerve (Cranial Nerve VI). The superior ophthalmic vein, which provides the main venous drainage for the orbit, also passes through this fissure as it drains into the cavernous sinus.