Anatomy of the Eye
ORBIT
The Bony orbit
§ The Orbit is shaped like a cone, with the apex pointing posteromedially.
§ The medial walls of the orbits (M) are parallel
§ lateral walls (L) are perpendicular to one another–important in understanding action of extra-ocular muscles.
Surface Anatomy
§ picture
The Eyelids: Layers
§ Skin
Thin and delicate
§ Muscular layer
Made up of orbicularis oculi which has an orbital portion encircling the orbit,
a palpebral portion in the lids
a lacrimal portion around the lacrimal sac.
§ Tarso-fascial layer
Orbital septum, continuous with periosteum
Tarsal plate with imbedded tarsal gland
Levator palpebrae
Superior tarsal muscle portion of levator palpebrae
§ Conjunctival layer
Palpebral conjunctiva
Orbital conjunctiva
Conjunctival fornices-the ducts of the lacrimal gland open into the lateral part of the superior fornix
Superior tarsal plate
Inferior tarsal plate
Medial palpebral ligament-also origin of orbicularis
Lateral palpebral ligament
Lacrimal gland – note how the aponeurosis of levator partitions the gland into two portions
Contents of the Orbit
§ Neural Elements
Abducens nerve
Ciliary ganglion
a small ganglion of the cranial parasympathetic nervous system.
Through it are relayed sensory and post ganglionic sympathetic fibers,
The synapse between pre and post ganglionic parasympathetic fibers occurs within the ganglion.
Pterygopalatine ganglion-
not actually within the orbit, but supplies the lacrimal gland with postganglionic parasympathetics through communications with V2 and then V1-lacrimal nerve.
§ Muscles
Orbital – orbicularis oculi, levator palpebrae, superior tarsus
Extraocular – the recti muscles and oblique muscles
Ocular – ciliary body, constrictor and dilator of the pupil
§ Vascular Elements
The eye is almost wholly supplied by the ophthalmic artery
the lids may be supplied by nearby surface arteries.
Veins accompanying the arteries drain into the cavernous sinus, other cranial sinuses or into veins of the face.
Actions of the extraocular muscles
§ Actions of the extraocular muscles are indicated in the diagram, which indicates the eye in the ‘position of rest,’ with the eye straight ahead.
§ Since the axis of the orbit is directed to the lateral or temporal aspect of the skull, this means all actions of the eye muscles are mixed, although each has a primary action.
§ The actions of a muscle are pure when the A-P axis of the eye coincides with the direction of a muscle’s fibers.
§ The primary action of the muscles are:
Lateral rectus-abduction
Medial rectus-adduction
Superior rectus and inferior oblique-elevation
Inferior rectus and superior oblique-depression.
The Eyeball: Coats of the Eyeball
§ Fibrous
Sclera
Cornea
§ Vascular or Pigmented – Also called Uveal tract, because the structure contains a deep purple pigment.
Choroid
Ciliary body
Iris
§ Nervous – Has three subdivisions;
Pars optica-light sensitive
The anterior limit is the Ora serrata
Ciliary and Iridial (not pictured)
Central artery of the retina
it supplies the retina and some of the pigmented layer,
but most of the vascular or pigmented layer receives its blood from other branches of the ophthalmic artery and the supplies of Uveal tract and retina do not mix.
The Ciliary Ganglion
§ The ciliary ganglion is an important relay for sensory and postganglionic sympathetic fibers and the site of synapses for the parasympathetic system.
§ Sympathetic fibers are dashed, parasympathetic fibers are solid, sensory fibers are not shown.
Autonomic Innervation of the Eye
§ Sympathetic innervation of the globe results in pupillary dilation.
§ Parasympathetic innervation of the globe results in pupillary constriction and accomodation.
Fascias of the Orbit
§ Fascias in the orbit play an important role in the support of the eyeball.
§ The fascias of the muscles and optic nerve combine to coat the eyeball and form a structure known as Tenon’s Capsule.
§ The capsule is so strong the globe may be ‘scooped’ out and the capsule remain.
§ Fascia around the inferior oblique becomes thickened and attaches laterally to the wall of the orbit and is called the Suspensory Ligament.
§ In addition, the fascias of the medial and lateral rectus muscles ‘fan out’ to form medial and lateral Check Ligaments.
Clinical Correlates:
§ Horner’s Syndrome is a group of disorders caused by loss of sympathetic innervation. This usually occurs due to neck injury or tumors. The characteristics include:
Constricted pupil-Miosis-due to loss of dilator function
Partial drooping of lid-ptosis-due to loss of superior tarsal muscle
“Sunken eye”-enophthalmos-poorly understood but probably due to loss of sympathetically innervated muscle such as the superior tarsal
Flushed skin-vasodilation-due to denervation of tunica muscularis of the arteries
Dry skin-anhidrosis-due to loss of innervation of the sweat glands
§ Fractures of the bones of the orbit are common. In a “blowout” fracture, increased pressure in the orbit due to a blow from the front fractures of the orbit–usually inferior–and the eyeball may sink if the defect is big enough. Muscles and nerves may be pinched in fragments of bone. Fracture of the roof of the orbit may result in leakage of CSF into the orbit.
§ The fascia of the globe and muscles–Tenon’s Capsule–is important in supporting the eye. A cup is formed when the eye is removed and this serves as a convenient receptacle for a prosthesis.
§ Veins in the orbit communicate with facial veins and with the cavernous sinus. Infections in the danger zone of the face can spread into the cavernous sinus and result in thrombosis. The wall of the carotoid artery may be eroded and an arterial-venous fistula result. A bruit–rasping sound–is heard by the patient and by the physician upon auscultation.
§ The sheath of the optic nerve is an extension of the the cranial dura. Increases in intracranial pressure are transmitted by the CSF around the nerve and compromise venous flow. The ophthalmic veins then have trouble draining and the optic disc becomes elevated–papilledema.
§ The retinal artery is an end artery–that is there is no anastomoses to supply the retina if the ophthalmic artery is occluded. Blindness will result.
§ Diplopia is double vision; conjunctivitis is an inflammation of the conjunctiva; a sty is an inflammation of a meibomian gland, a type of sebaceous gland, around the lashes of the eye; glaucoma is an increase in pressure in the aqueous humor and can result in blindness;cataract refers to the development of opacity in the lens; ptosis is a drooping of the lid; crying results in a runny nose because the nasolacrimal duct opens into the nose; eye color results from a combination of color of pigmentation and color of the iridial membrane.
§ An extraocular muscle is tested by positioning of the eye in such a manner as to minimize action of muscles other than the one being tested. Action of an extraocular muscle is pure when it is lined up in its field of action, e.g. medial rectus for nasal movement.
§ The sclera is fenestrated–lamina cribosa–where the optic nerve enters the globe. This is the weakest point of the globe and increased intraocular pressure causes a bulging outward of the area–’cupped disc.’
§ Fascias of the superior rectus muscle and levator palpebrae are interconnected such that there is a rule: “Lift eyes, lift lids.”
Tags: Abducens nerve, anhidrosis, Autonomic Innervation, bony orbit, Ciliary body, Conjunctival layer, enophthalmos, extraocular, eyelids, Horner's Syndrome, lacrimal gland, Lateral rectus, Miosis, ocular muscles, ophthalmic artery, orbicularis oculi, Pars optica, periosteum, Pterygopalatine ganglion, pupil, retina, Sclera, Sunken eye, Tarsal plate, Tenon's Capsule
