periorbits

The periosteum of the orbit.

Synonym(s): periorbit, periorbital membrane

[peri- + L. orbita, orbit]

The periosteum of the orbit.
Synonym(s): periorbit.

[peri- + L. orbita, orbit]

A rigid bony cavity in the skull which contains an eyeball, orbital fat, the extraocular muscles, the optic nerve, nerves and blood vessels, lacrimal system and fibrous tissue of various kinds. This packing serves to keep the eyeball reasonably well fixed in place as it rotates. The orbital cavity has the approximate form of a pyramid. The walls of the orbital cavity are formed by seven bones. The medial wall of the orbit consists of: (1) the frontal process of the maxilla (maxillary); (2) the lacrimal bone; (3) the lamina papyracea of the ethmoid; and (4) a small part of the body of the sphenoid. The floor of the orbit consists of: (1) the orbital plate of the maxilla; (2) the orbital surface of the zygomatic (malar) bone and (3) the orbital process of the palatine bone. The lateral wall of the orbit consists of (1) the orbital surface of the greater wing of the sphenoid, and (2) the orbital surface of the zygomatic. The roof of the orbit is made up mainly by the frontal bone and behind this by the lesser wing of the sphenoid. The orbit is lined with a membrane of tissue called the periorbita (or orbital periosteum) which extends to the orbital margin (anterior rim of the orbit) where it becomes continuous with the periosteum covering the facial bones. The periorbita is loosely attached to the bones except at sutures, foramina and the orbital margin where it is firmly attached. The bones are much thicker at the margin (rim) than they are along the walls of the orbital cavity. There are many apertures and gaps in the orbit through which blood vessels and nerves pass (see Table O4). See orbital axis; optic canal; inferior orbital fissure; superior orbital fissure; orbital fracture; cavernous haemangioma; lamina papyracea.

Table O3 Bones forming the walls of the orbit
roof	medial wall
1. frontal	1. maxilla
2. lesser wing of sphenoid	2. lacrimal
	3. ethmoid
	4. sphenoid
floor	lateral wall
1. maxilla	1. greater wing of sphenoid
2. zygomatic
3. palatine	2. zygomatic

Table O4 Orbital apertures
aperture	location	contents
optic canal	at the apex (in lesser sphenoid)	optic nerve ophthalmic artery sympathetic nerve fibres
superior orbital fissure	at the apex (gap between greater and lesser sphenoid)	III, IV, V, VI nerves sympathetic nerve fibres ophthalmic vein recurrent lacrimal artery
inferior orbital fissure	between lateral wall and posterior part of the floor	infraorbital nerve zygomatic nerve branch of inferior ophthalmic vein nerve fibres from the pterygopalatine (sphenopalatine) ganglion to orbital periosteum
ethmoidal foramina (anterior and post.)	medial wall (frontal/ethmoidal suture)	ethmoidal vessels ethmoidal nerve/external nasal nerve
zygomatic foramen	lateral wall	zygomatic nerve and vessels
nasolacrimal canal	medial wall (maxilla/lacrimal)	nasolacrimal duct