Orbit

• Bones

  The frontal bone forms the roof of the orbit, parts of the medial and parts of the lateral wall. It also forms small ridges superior to the orbit (supraorbital ridges) which are covered by the eyebrows

  The frontal bone forms the roof of the orbit, parts of the medial and parts of the lateral wall. It also forms small ridges superior to the orbit (supraorbital ridges) which are covered by the eyebrows

  
• The lacrimal bone is associated with the lacrimal apparatus and the nasolacrimal duct that carries excess lacrimal fluid (tears) into the nasal cavity. The ethmoid bone forms part of the medial wall but also contributes to the nasal cavities. The sphenoid bone is quite a complex bone shaped like a butterfly with a central body and wings spread out either side. It forms the optic canal (CNII) and the superior orbital fissure (CN III, CN IV, CN V, CN VI) it also contributes to the inferior orbital fissure (ophthalmic veins).

  
• Orbital contents

  •Eyeball

  •Fat and connective tissues

  •Extraocular muscles

  •Nerves

  •Blood vessels

  
• Tarsal Plates

  The orbital contents is protected by the orbital septum, which is composed of strong connective tissue. The corners of the eyelid are supported by the medial and lateral palpebral ligaments and the dense connective tissue of the tarsal plates (contributing to the eyelids) can also be seen. Each tarsal plate physically protects the eye but also contains glands that release oily and watery secretions to lubricate the surface of the eye.

  
• Ligaments

  There are also ligaments that restrict the movements of the extraocular muscles (the check ligaments) and the suspensory ligament that supports the position of the eyeball within the orbit.

  
• Eyeball

  The outer coat is relatively strong, and the sclera provides an attachment site for the recti muscles.

  
• Retina

  Rods and cones

  Blind spot = optic disc

  Macula lutea = fovea

  area where there are no rods and cones because the optic nerve itself leaves the retina (the blind spot). The macula is an area rich in both rods and cones (the retinal cells that responded to light).

  
• Aqueous humor

  Anterior to lens, helps to maintain intra-ocular pressure

  Made by ciliary body, drains into corneo-scleral junction (canal of Schlemm)

  The space between the lens and the cornea is called the anterior chamber of the eye and is filled with a thin fluid called aqueous humor. This is continuously made by the cells of the ciliary body and recycled back into the veins at the junction of the cornea and sclera.

  
• Vitreous humor (body)

  •Embryological

  •Transparent jelly

  •Posterior to lens

  •Supports retina

  The posterior chamber of the eye is filled with a gel. The function of the gel is to support the retina by gently pressing it to the choroid below. The choroid has the blood vessels that supply the retina so keeping the retina and choroid in close proximity helps to prevent damage to these vessels. The vitreous humor is transparent to allow the passage of light through to the cells of the retina.

  
• Intraocular (intrinsic) muscles

  Dilator pupillae increases the diameter of the pupil sympathetic, carried on blood vessels

  Sphincter pupillae decreases the diameter

  parasympathetic via oculomotor nerve CN III

  The iris contains small radial muscle fibres that pass from the edge of the pupil out across the iris. When these fibres contract they pull the medial edges of the iris outwards increasing the size of the pupil. The sympathetic nervous system controls this- ‘fight or flight’ giving as much light to the retina as possible to detect potential danger.

  
• Intraocular (intrinsic) muscles

  The iris also has circular fibres around the pupil called the sphincter pupillae. When these fibres contract they narrow the pupil, restricting the amount of light entering the eye and acting to protect the delicate retinal cells from high light intensities. These fibres are under parasympathetic control carried by the oculomotor nerve (CN III).

  
• Ciliary muscle

  intrinsic eye muscle = change the shape of the lens for accommodation    oculomotor nerve

  The other intrinsic muscles of the eye act to change the shape of the lens. This causes the light entering the eye to refract (bend) as it passes through the lens. It can then be focused on the retinal cells. This allows the lens to accommodate for near and far vision.

  
• Extraocular (extrinsic) muscles

  7 extraocular muscles -levator palpebrae superioris from orbital roof to upper eyelid

  Superior, inferior, medial and lateral recti from annular ring to sclera (anterior to equator)

  Superior oblique (lesser wing sphenoid) and inferior oblique (orbital floor) to sclera (posterior to equator)

  The other eye muscles are outside the eyeball and act to move the eyeball as a whole or raise the upper eyelid.

  
• Extraocular (extrinsic) muscles

  The name of levator palpebrae superioris tells you where it is and what it does – raises/eyelid/upper. It does not move the eyeball as it is not attached to the sclera

  The 4 recti at the cardinal points (north, south, east and west) attach in front of the equator and the oblique muscles behind the equator. Where they attach helps to determine which eye movements they can assist with.

  
• The muscles are very small

  
• Axes

  Axes of orbit (bony cavity) and optical axis (line of gaze) different so eye muscles can move eye in more than one direction

  The other structural factor that helps to determine which movements are made is where the centre of the eyes sit in relation to the walls of the orbit. This determines how much room for movement the eyeball has, as the medial wall extends farther out from the skull.

  
• Adduction is towards the nose, abduction is away from the nose. RADSIN is a mnemonic to help you remember the Recti ADDuct (except for lateral rectus) and the Superiors INtort. The chemical style formula helps with the nerve supply – all are oculomotor except Lateral Rectus (CN VI) and Superior Oblique (CN IV).

  
• Nerves - optic nerve CN II

  The main nerve for the eyeball is CN II the optic nerve which gathers impulses from the light sensitive cells of the retina and passes them to the brain to be interpreted as images. The optic nerve leaves the orbit via the optic canal. CN II forms a chiasma over the pituitary fossa, then sends optic tracts towards the brain. The optic radiations then take information to be processed in the primary visual cortex. These communicate with association areas to put the images into context

  
• Nerves

  cranial nerves pass through the superior orbital fissure to enter the orbit and supply the intrinsic and extrinsic muscles of the eye. The trigeminal nerve supplies sensory innervation to the orbit and carries fibres from the facial nerve to the lacrimal gland.

  
• Cavernous sinus

  Before the nerves reach the superior orbital fissure, they pass though the cavernous dural venous sinus. This lies around the body of the sphenoid bone and the pituitary fossa. CN III, CN IV and CN V pass along the walls close to the dura. CN VI passes through the middle of the sinus close to the internal carotid artery. So, the abducent nerve is the odd one out here.

  
• Annular ring

  As the nerves come into the posterior part of the orbit, they reach an annular ring of connective tissue that provides an attachment site for the recti muscles. Some nerves pass through the ring, but parts of the trigeminal nerve and CN IV do not. So, the trochlear nerve is the odd one out here.

  
• Nerves – trigeminal CN V

  CNV1 is the ophthalmic division and this passes into the superior orbital fissure. Frontal branches are easiest to see as they lie superior to the extraocular muscles. The frontal branch divides to give supraorbital and supratrochlear branches which pass onto the face via the supraorbital notch to supply sensory innervation to the skin of the forehead. The nasociliary branch will form ethmoidal branches which pass through the ethmoid bone. The lacrimal nerve acts as a carrier for parasympathetic secretomotor fibres from the facial nerve.

  
• Nerves - CNV1

  the frontal branch as it passes over the superior surface of levator palpebrae superioris.

  
• Nerves - oculomotor CN III

  The oculomotor nerve (CN III) is the main nerve supply to the extraocular muscles. The superior division of the nerve supplies levator palpebrae superioris and superior rectus with motor innervation. The inferior division supplies medial rectus, inferior rectus and inferior oblique with motor innervation. It also carries parasympathetic fibres to the intrinsic muscles of the eye. The ciliary ganglion is a small piece of nerve tissue posterior to the eye which communicates with CN III and sends short ciliary nerves into the eyeball.

  
• Nerves - CN III

  CN III lies deeper in the orbit than CNV1 and the ciliary ganglion also receives sympathetic fibres carried by the vessels of the orbit.

  
• Nerves – trochlear CN IV and abducent CN VI

  CN VI and CN IV are small as they only supply one muscle each. CN VI can be seen associated with the surface of lateral rectus and CN IV crosses the superior surface of superior oblique before piercing and innervating it.

  
• Arteries

  The blood supply to the orbit is from the ophthalmic artery – a branch of the internal carotid artery. The ophthalmic artery also gives branches to the forehead and scalp, to the nose. The ophthalmic artery enters the orbit through the optic canal with the optic nerve.

  
• Veins

  The ophthalmic veins of the orbit communicate with the cavernous venous sinus through the superior orbital fissure; with the pterygoid venous plexus through the inferior orbital fissure and with the facial vein.

  
• Lacrimal apparatus

  The lacrimal gland secretes a thin watery fluid to lubricate the surface of the eye and help remove debris. The gland lies at the lateral edge of the orbit and releases tears onto the eye via a number of tiny ducts. The tears wash across the surface of the eye and then are gathered through lacrimal punctum into the lacrimal canaliculi  and then into a membrane lined space called the lacrimal sac. This sac communicates with the inferior meatus of the nose via the nasolacrimal duct.

  
• Nerves – CN VII

  The facial nerve provides secretomotor fibres to the submandibular and sublingual salivary glands via the chorda tympani by joining with the lingual nerve (CNV3). The fibres to the lacrimal gland also travel with the trigeminal nerve but in this case CNV2 and V1. The facial nerve branches to form the greater petrosal nerve. It carries parasympathetic secretomotor fibres. It then joins with sympathetic fibres from a plexus around the internal carotid artery to form the nerve of the pterygoid canal.

  
• Nerves – CN VII

  This nerve passes to the pterygopalatine ganglion to join with CNV2 these fibres then join the lacrimal nerve (CNV1) to finally reach the lacrimal gland. So, lesser petrosal and greater petrosal are parasympathetic from CN IX and CN VII. Deep petrosal are sympathetic fibres, and the nerve of the pterygoid canal has both type of fibres present. Small nerves passing from the pterygopalatine ganglion also allow the facial nerve to supply tiny glands in the nose.