General shape, size and position of the eye

Question 1

What is photoreception?

Answer 1

the process by which light energy from the environment produces changes in specialised nerve cells in the retina- the rods and cones

the changes result in nerve action potentials which are relayed to the optic nere and then to brain where information processed and consciously appreciated as vision

Question 2

What are all the structures within the eye other than the photoreceptors (rods and cones) for?

Answer 2

• may be aprt of the system for focusing and transmitting light onto the retina (cornea, lens, iris, ciliary body)
• may be necessary for nourishing and supporting tissues of eye (choroid, aqueous outflow system, lacrimal apparatus)

Question 3

What is the approximate shape and size of the eye?

Answer 3

approximately a sphere 2.5cm in diameter with volume of 6.5ml

in reality more of the parts of two spheres - smaller one anteriorly, cornea, and the sclera which constitutes large sphere

Question 4

What proportion of the circumference of the globe is formed by the cornea?

Answer 4

one sixth

Question 5

How does the curvature of the cornea compare with the sclera?

Answer 5

the cornea has a greater curvature than the sclera

Question 6

What is the raidus of the cornea?

Answer 6

7.8mm

Question 7

What is the radius of the sclera?

Answer 7

11.5mm

Question 8

What is the average axial lenngth of the globe of the eye?

Answer 8

24mm (range 21-26mm varies between individuals)

Question 9

What is the diameter of the globe?

Answer 9

23mm

Question 10

What is the horizontal length of the globe?

Answer 10

23.5mm

Question 11

Are small eyes hyperopic (hypermetropic) or myopic?

Answer 11

hyperopic (aka hypermetropic)

Question 12

What is the cut off axial length of the globe to be consider a small eye?

Answer 12

<20mm

Question 13

Are large eyes hyperopic (hypermetropic) or myopic?

Answer 13

myopic

Question 14

What is the the cut off axial length of an eye to be consiered a large eye?

Answer 14

26mm - 29mm

Question 15

Where within the orbit is the eye situated?

Answer 15

anterior portion, closer to the lateral than the medial wall and nearer the roof than the floor

Question 16

What is another name for the layers/ coats that make up the eye?

Answer 16

tunics

Question 17

What are the 3 coats (tunics) that make up the eye?

Answer 17

1. Fibrous (corneo-scleral) coat
2. Uvea or uveal tract (composed of choroid, ciliary body and iris)
3. Neural layer (retina)

Question 18

What are the three things that the uveal tract is composed of?

Answer 18

1. choroid
2. ciliary body
3. iris

Question 19

What do the three coats of the eye surround?

Answer 19

the orbital contents: the lens and the transparent media (aqueous humour and vitreous body)

Question 20

What are 2 key functions of the cornea and sclera?

Answer 20

1. form tough fibrous envelope that protects the ocular tissues
2. fibrous coat provides important structural support for intraocular contents and attachment of the extaocular muscles

Question 21

Name the following on the eye diagram:

• corneal epithelium
• cornea
• iridocorneal angle
• limbus
• canal of schelmm
• pupil iris
• conjunctiva
• posterior chamber
• lens zonules
• cilia body
• anterior chamber
• lens

Answer 21

Question 22

Identify the three tunics of the eye on the diagram (fibrous coat, uveal tract, neural layer).

Answer 22

neural = purple

uveal tract = red

fibrous = blue

Question 23

Label the following on the eye diagram:

• vitreous
• hyaloid (Cloquet’s) canal
• optic disc
• choroid
• retinal pigment epithelium
• retina (neural layer)
• optic nerve
• dura mater
• subarachnoid spae
• lamina cribrosa
• fovea
• sclera
• parsa plana
• ora serrata
• rectus muscle tendon and belly

Question 24

What is the name of the point at which the cornea meets the sclera?

Answer 24

limbus (or corneoscleral junction)