The Orbit

Question 1

Describe the border of the orbit

Answer 1

The apex of the orbit is the optic canal.
Superior border - Frontal and spenoid bone
Lateral border - zygomatic and sphenoid
Medial border - ethmoid, maxillary, lacrimal and the sphenoid
Inferior border - maxillary and zygomatic

Question 2

Which bones are involved in the orbit

Answer 2

Frontal, sphenoid, ethmoid, lacrimal, maxilla and zygomatic

Question 3

How is the eye kept moist? And where does this drain into in the nasal cavity?

Answer 3

The eye is kept moist by the lacrimal gland, this liquid drains through the nasolacrimal sac and the duct into the inferior meatus of the nasal cavity. A layer of this liquid is place over the cornea every time we blink requiring the orbicularis oculi muscle.

Question 4

What holes are there into the skull at the back of the orbit and which bone are they in?

Answer 4

Holes in the back of the eye optic canal and the superior and inferior orbital fissure (all of which are in the sphenoid bone).

Question 5

What structure pass through the 3 holes in the back of the orbit?

Answer 5

The superior orbital fissure contains the ophthalmic nerve, trochlear nerve, superior ophthalmic vein, abducent nerve, and the oculomotor nerve. The inferior orbital fissure contains the inferior ophthalmic vein and maxillary nerve.
Optic canal - optic nerve and ophthalmic artery

Question 6

What is the iris, ciliary body, macula/fovea and the optic disc?

Answer 6

Iris – contracts and relaxes to change the amount of light entering the lens
Ciliary body – contracts and relaxes to change the power of the lens
Macula/Fovea – point of highest concentration of cells allowing for must in focus vision.
Optic disc – blind spot where the cells aren’t present due to optic nerve

Question 7

What are the three layers that form the eyeball?

Answer 7

3 layers to the eyeball
Outer fibrous layer – tough white sclera and the cornea at the front
Middle Vascular layers – choroid, iris and ciliary body providing nutrients to the eye
Inner layer – retina i.e. rod and cone cells.

Question 8

What side of the eye is the optic disc on in relation to the fovea and the nose?

Answer 8

Optic disc is always on the nasal side of the fundus

Question 9

What happens to the optic disc if there is a raised intracranial or ocular pressure?

Answer 9

If raised intracranial pressure, then the optic disc will appear to bulge outwards – clinically termed papilledema. In raised intra ocular pressure the optic disc will appear cupped.

Question 10

What is retinal detachment

Answer 10

Retinal Detachment – retina comes parted from the choroid. During development, the retina develops separately and so can come away easily. It requires a blood supply so if it’s not caught early enough that part of the retina may die leaving you partially blinded.

Question 11

Why is the optic nerve special and what clinical importance does this cause?

Answer 11

Optic nerve is essentially an extension of the forebrain because they still have the meninges i.e. Pia Mata, Arachnoid Mata and Dura Mata, because of this meningitis will cause photophobia – pain upon looking at light.

Question 12

What are the chambers of the eyeball?

Answer 12

Posterior segment – vitreous body/humour – a liquid substance that helps to keep the eye round
Anterior segment – split into the anterior and posterior chamber

Question 13

How does the lens get its nutrients?

Answer 13

Lens is an avascular structure and so get its nutrient supply from the surrounding fluid in the posterior chamber which is produced by the ciliary body. This fluid travel around the iris into the anterior compartment and drains through the trabecular meshwork and into the venous drainage.

Question 14

What is open angle glaucoma?

Answer 14

Open angle Glaucoma – if there is a blockage in the trabecular meshwork resulting in a build-up of fluid in the anterior segment resulting in the angle between the cornea and the iris to enlarge (hence open angle). As the pressure increases more in presses back onto the vitreous s humour and eventually onto the optic nerve causing a distortion in vision.

Treated with eye drops that assist in unblocking the trabecular meshwork.

Question 15

What is closed angle glaucoma?

Answer 15

Closed angle glaucoma – in this access to the trabecular meshwork is lost acutely causing a rapid rise in intra orbital pressure. This is an ophthalmic emergency.

Question 16

What are cataracts?

Answer 16

Lens becomes opaque causing very blurry vision. Treated with small easy operation that replaces or break up the original lens.

Question 17

What is accommodation?

Answer 17

Accommodation – the changes in iris dilation and lens size to allow us to move between focusing from afar to close or vice versa – can be tested in cranial nerve test. This can becomes harder to do as we age due to the lens becoming inflexible and dense this is known as presbyopia.

Question 18

Describe the 7 muscles involved in eye movement and their innervation. Hint LR6 SO4 R3

Answer 18

Lateral rectus muscle – supplied by the abducens nerve VI
Superior oblique passing through the trochlear sling – supplied by the trochlear nerve IV
Everything else – supplied by the oculomotor nerve
Inferior oblique helps to move the eye inwards and upwards when looking medially
Superior oblique helps to move the eye inwards and downwards when looking medially Superior rectus – superior movement
Inferior rectus – inferior movement
Medial rectus – medial movement
Lateral Rectus – lateral movement

Question 19

How does the occulomotor nerve split and which parts innervate which muscles?

Answer 19

Note oculomotor nerve split into the superior and inferior division.
Superior innervates the superior rectus and levator palpebrae superioris
Inferior innervates the medial rectus, inferior rectus and inferior oblique.

Question 20

What is a orbital blow out fracture?

Answer 20

Squash ball to eye injury (blow out fracture)
Increase in orbital pressure causes tear drop sing through the maxillary floor (weakest point). This will damage the inferior oblique muscle and so they won’t be able to move the eye up properly.

Question 21

What is the blood supply to the eyeball and what is the clinical correlate for its venous drainage?

Answer 21

Ophthalmic artery from the internal carotid artery. This gives rise to the central retinal artery which passes within the dural sheath of the optic nerve to supply the retina. This artery is an end artery.

Venous drainage is very similar – central retinal vein into the superior ophthalmic vein and from here into the cavernous sinus. Note that again this is valve less and blood can flow both ways

Question 22

What is the conjunctiva?

Answer 22

This is a film over the top of the cornea which is what becomes infected in conjunctivitis. It has two parts – bulbar conjunctiva and palpebral conjunctiva. The conjunctival membrane reflects off the sclera to line the inner surfaces of the eyelids. In conjunctivitis, the cornea is not affected as the conjunctivae ends at the limbus i.e. conjunctiva does not overlie the cornea.

Question 23

What is the role of the eyelid and what glands are present here?

Answer 23

Protects against injury, excessive light and dryness. The eyelid is lifted by the levator palpabrae superioris innervated by the oculomotor nerve. The superior tarsal muscle is an accessory muscle innervated by the sympathetic nervous system – hence Horner’s syndrome causes a partial ptosis.

The tarsal gland (Meibomian gland) are modified sweat glands that secrete oily substance to prevent evaporation of the tear film. Ciliary glands are sebaceous glands that secrete liquid to maintain the eye lash.

Meibomian cyst is inflammation of the tarsal gland and are usually located above the ye lashes. A stye is inflammation of the ciliary gland, usually contains pus and is found within the eye lashes.

Question 24

Which nerve forms the afferent limb of the corneal reflex?

Answer 24

The cornea is innervated by the ophthalmic division of the trigeminal nerve and allows the corneal reflex to take place – efferent of the corneal reflex is the facial nerve.

Question 25

Describe the direct and consensual light reflex

Answer 25

Direct light reflex is when the light is received in one eye and the change occurs in the same eye whilst a consensual is where it is seen in one eye and occurs in the other. The pathway for this is: Sensory afferent from retina (CNII) to brainstem at the pretectal nucleus. There is then a connection with BOTH EDW nuclei (Edinger-westphal nucleus), then a signal is sent down parasympathetic fibres from here and leave the brain stem hitch hiking on the CN III passing the ciliary ganglion and affecting the sphincter pupillae.

Question 26

What are all the different muscles involved in opening and closing the eye.

Answer 26

Tarsal Muscle - secondary muscle keeping eyelid open innervated by sympathetic fibers on opthalmic artery and then occulomotor nerve

Levator Palpebrae Superioris - main muscle keeping eye lid open and innervated by the occulomotor nerve

Orbicularis Oculi - innervated by facial nerve shuts the eye
Orbitar part - scrunches up the eye
Palpebral part - shuts eyelid i.e. blinking