Applied Anatomy and Physiology of the Orbit and the Eye

Question 1

The fibrous coat (white, outermost) of the eye has two components - what are they?

Answer 1

• Sclera

- Cornea

Question 2

Describe the structure and function of the sclera.

Answer 2

• Component of the fibrous coat
• Opaque posterior 5/6
• Gives attachment to eye muscles

Question 3

Describe the structure and function of the cornea.

Answer 3

• Component of the fibrous coat
• Transparent anterior 1/6
• A window that allows light to enter eyeball

Question 4

The vascular coat (orange, middle, contains a rich supply of blood vessels) of the eye has three components - what are they?

Answer 4

• Choroid
• Ciliary body
• Iris

Question 5

Describe the structure and function of the choroid.

Answer 5

• Component of the vascular coat
• Posterior
• Supplies blood to outer layers of retina

Question 6

Describe the structure and function of the ciliary body.

Answer 6

• Component of the vascular coat
• Middle
• Suspends the lens
• Produces aqueous humor

Question 7

Describe the structure and function of the iris.

Answer 7

• Component of the vascular coat
• Anterior
• Hangs from ciliary body
• Incomplete due to anterior aperture i.e. the pupil
• Controls the diameter of the pupil and thereby controls the amount of light rays entering eyeball

Question 8

The fibrous coat (innermost) of the eye has one component - what is it?

Answer 8

Retina.

Question 9

Describe the structure and function of the retina.

Answer 9

• Contains light-sensitive rods and cones
• Has ten different layers
• Outer layers supplied by choroid

Question 10

Describe the structure and function of the lens.

Answer 10

• Transparents, crystalline, biconvex structure
• Suspended by zonules (suspensory ligaments) from ciliary body
• Can change shape i.e. become more/less convex, thus control focal distance of the eye

Question 11

Where are the ciliary body and suspensory ligaments present?

Answer 11

360° around the lens.

Question 12

Describe the structure and function of the anterior segment of the eye.

Answer 12

• In front of the lens
• Contains aqueous (liquid) humor, which helps to maintain ocular pressure (approx. 21mmHg)

Further divided into;

• Anterior chamber in front of iris
• Posterior chamber behind iris

Question 13

Describe the structure and function of the posterior segment of the eye.

Answer 13

• Behind the lens
• Filled with vitreous (gel) humor, which helps to cushion the retina (can become liquid in trauma, allowing more chance for retina to detach from choroid)

Question 14

Describe the movement of aqueous humor through anterior segment of the eye.

Answer 14

• Aqueous humor produced in ciliary body
• Flows into posterior chamber
• Flows through pupil to anterior chamber
• Drains out of eye through the angle of the anterior chamber; trabecular meshwork –> Schlemm’s canal –> episcleral veins

Question 15

Describe the skin of the eyelid.

Answer 15

• Thin with hair follicles and sebaceous glands

- Margins of eyelid have special hair follicles with their own sebaceous glands (blocked gland results in sty)

Question 16

Describe the structure and function of the levator palpabrae superioris.

Answer 16

• Starts at apex of orbit
• Runs just under roof of orbit to front where its tendons attach to conjunctiva, tarsal plate and skin of eyelid
• Elevates upper eyelid

Question 17

Describe the structure and function of the orbiculares oculi.

Answer 17

• Runs through eyelid and orbit

- Helps to close eyelid

Question 18

Describe the structure and function of the tarsal plate of the eyelid.

Answer 18

• Gives eyelid form
• Meibomian glands are inside it, which produce meibum (oily substance that prevents the evaporation of the eye’s tear film)

Question 19

Describe the structure and function of the conjunctiva.

Answer 19

• Mucus membrane, thin, vascular
• Secretes fluid
• Covers inner surface of eyelids and loops back over sclera
• Does not cover cornea (as it is highly vascularised so leakage over the cornea would affect vision)

Question 20

Describe the structure and function of the lacrimal apparatus.

Answer 20

Situated in the orbit laterally.

• Ducts open into conjunctival sac
• During each blink, eyelids spread tears evenly on surface of cornea
• Tears drain through punctae on medial side of each eyelid
• Drains into lacrimal sac, which sits over lacrimal bone
• Then drains through nasolacrimal duct into inferior meatus of nasal cavity

When blinking, the sac is pulled open, creating a negative pressure that pulls the tears through the canaliculi and into the sac.

Question 21

Describe the innervation of the lacrimal apparatus.

Answer 21

Parasympathetic (secretomotor) fibres from facial nerve (CN VII).

Question 22

Describe the structure and function of the intrinsic muscles of the eye.

Answer 22

• Begin and end inside eyeball- Present in iris or ciliary body
• Control pupil diameter
• Help alter lens curvature to enable us to see nearby objects

Question 23

Name the intrinsic muscles of the eye.

Answer 23

• Ciliaris muscle
• Constrictor pupillae
• Dilator pupillae

Question 24

Describe the function and innervation of the ciliaris muscle.

Answer 24

• Contracts and changes shape of lens, allowing ability to see nearby objects (accommodation)
• Innervated by parasympathetic fibres (oculomotor nerve, CN III)

Question 25

Describe the structure, function and innervation of the constrictor pupillae.

Answer 25

- Concentric muscle - Constricts pupil - Innervated by parasympathetic fibres (oculomotor nerve, CN III)

Question 26

Describe the structure, function and innervation of the dilator pupillae.

Answer 26

- Radial muscle - Dilates pupil - Innervated by sympathetic plexus around blood vessels

Question 27

Describe the structure and function of the extrinsic muscles of the eye.

Answer 27

- Originate from the orbit and attach to the sclera | - Help to move the eye when contracted

Question 28

Name the extrinsic muscles of the eye.

Answer 28

4 straight muscles; - Medial rectus - Lateral rectus - Inferior rectus - Superior rectus 2 oblique muscles; - Superior oblique - Inferior oblique

Question 29

Describe the structure of the superior oblique.

Answer 29

- Starts at apex of orbit - Runs close to medial wall - Comes anterior, attaches to roof of orbit via trochlea before changing direction to attach to sclera posteriorly

Question 30

Describe the structure of the inferior oblique.

Answer 30

- Starts at front of orbit (all other muscles start at apex) | - Slings under eyeball and attaches laterally and posteriorly to sclera

Question 31

Describe the innervation of the extrinsic muscles of the eye.

Answer 31

``` Oculomotor nerve (CN III) supplies; - Medial rectus - Inferior rectus - Lateral rectus - Inferior oblique (- LPS) ``` ``` Trochlear nerve (CN IV) supplies; - Superior oblique ``` ``` Abducens nerve (CN VI) supplies; - Lateral rectus ```

Question 32

CNs III, IV, VI all enter the orbit through which opening?

Answer 32

Superior orbital fissure.

Question 33

Name the three branches of the ophthalmic division of the trigeminal nerve (CN V).

Answer 33

- Nasociliary - Lacrimal - Frontal

Question 34

The ophthalmic division of the trigeminal nerve (CN V) enters the orbit through which opening?

Answer 34

Superior orbital fissure.

Question 35

The maxillary division of the trigeminal nerve (CN V) enters the orbit through which opening?

Answer 35

Inferior orbital fissure.

Question 36

What artery supplies the orbit?

Answer 36

Ophthalmic artery (branch of ICA).

Question 37

Which veins drain the orbit?

Answer 37

Superior and inferior ophthalmic veins --> cavernous sinus --> IJVs.

Question 38

The internal jugular veins leave the skull through which openings?

Answer 38

The jugular foramen.

Question 39

Describe the structure of the cavernous sinuses.

Answer 39

- Sit on either side of body of sphenoid bone | - Situated between two layers of dura mater (lined by squamous endothelium)

Question 40

Embryology: describe the development of the eye from the neural tube.

Answer 40

- Develops as an extension of the brain - The brain develops from the neural tube - The notochord send signals to the ectodermal cells to enlarge and form a neural plate - With further development, the neural plate sinks down and the ends fold up to meet and form the neural tube at the dorsal end of the embryo

Question 41

Embryology: describe the formation of the optic vesicles.

Answer 41

- Optic vesicles grow outwards from diencephalic part of neural tube towards surface ectoderm - Once at surface, they send signals for epithelium to thicken, forming the lens placode - Lens placode invaginates inwards and pushes the optic vesicle inwards to form an optic cup - A stalk connects the optic cup to the neural tube

Question 42

Embryology: describe the formation of the eye's components from the optic cup.

Answer 42

- A 2 layered optic cup is formed in the retina - The mesenchyme sitting around it condense to form the choroid and sclera - The lens vesicle loses its connection with the surface ectoderm and forms the crystalline lens - The anterior epithelium form the cornea - The 10th layer of the retina forms from the outer layer of the optic cup while the other 9 form from the inner layer (potential space between these layers, important clinically as they can separate)

Question 43

Embryology: describe the formation of the choroid fissure.

Answer 43

As the optic cup is forming, blood vessels create a groove in the developing retina and optic nerve which is called the choroid fissure. This fissure eventually becomes closed off but the artery remains and goes on to become the central retinal artery, which supplies the surface of the retina.