orbit (of the eye)
The orbit is the socket in the skull that contains the eyeball (see eye), protective pads of fat, and various blood vessels, muscles, and nerves. An opening in the back of the orbit allows the optic nerve to pass from the eyeball into the brain.
The orbit is a pyramid-shaped cavity with its base in front and its apex behind. The orbital margin is formed above by the frontal bone, which is notched or canalized for the passage of the supraorbital nerve and vessels. The lateral margin is formed by the processes of the frontal and zygomatic bones. The inferior margin is formed by the zygomatic bone and the maxilla. The medial margin is formed by the processes of the maxilla and the frontal bone.
The roof of the orbit is formed by the orbital plate of the frontal bone, which separates the orbital cavity from the anterior cranial fossa. The lateral wall is composed of the zygomatic bone and the greater wing of the sphenoid. The floor is formed by the orbital plate of the maxilla, which separates the orbital cavity from the maxillary sinus. The medial wall consists, from before backward, of the frontal process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid (which separates orbital cavity from the ethmoid processes), and the body of the sphenoid.
Openings into the orbital cavity
The main orbital opening lies anteriorly and is bounded by the orbital margin. The supraorbital notch, or canal, is situated on the superior orbital margin. The infraorbital groove and canal lie in the floor of the orbit. Anteriorly on the medial part of the floor lies the nasolacrimal canal.
Posteriorly is the inferior orbital fissure, which leads from the pterygopalatine fossa and the infratemporal fossa into the orbital cavity. The superior orbital fissure leads from the middle cranial fossa into the orbit. The optic canal also leads forward from the middle fossa into the orbit.
On the lateral wall are two small openings for the zygomaticotemporal and zygomaticofacial nerves.
On the medial wall along the upper margin of the ethmoid bone are the anterior and posterior ethmoidal foramina.
The orbital fascia is the periosteum of the bones that form the walls of the orbit. It is loosely attached to the bones and is continuous through the foramina and fissures with the periosteum covering, the outer surfaces of the bones. In the case of the superior orbital fissure, the optic canal, and the anterior ethmoidal canal, it becomes continuous with the endosteal layer of the dura mater. The muscle of Müller, or orbitalis muscle, is a thin layer of smooth muscle that bridges the inferior orbital fissure. It is supplied by sympathetic nerves, and its function is unknown.