The Eye I (Basic Anatomy and Optics)

Question 1

What are the three layers (tunics) of the eye and what do they contain?

Answer 1

Fibrous tunics (sclera - insertion of ocular muscles; cornea - focuses light)
Vascular tunic (choroid layer - pigmented, absorbs light; iris - gives eye colour, circular and radial smooth muscle controls pupil diameter; ciliary body - ciliary muscle (with ciliary processes = epithelial folds), zonular fibres (attach ciliary muscle to lens), changes shape of lens)
Neural tunic (retina - contains photoreceptors and neurons)

Question 2

What are the differences between radial and circular muscles in the iris?

Answer 2

Pupillary dilator muscles = radial (contract to dilate the pupil)
Pupillary constrictor muscles = circular (contract to constrict the pupil)

Question 3

Parasympathetic and sympathetic activity and the iris

Answer 3

Parasympathetic activity constricts the pupil (=bright light)
Sympathetic activity dilates the pupil (=dim light)

Question 4

What is the vitreous humour?

Answer 4

Jelly-like fluid that fills vitreous cavity (posterior cavity of eye)

Question 5

What is the aqueous humour?

Answer 5

Watery liquid that fills the anterior and posterior chambers, providing nutrient and waste exchange for the cornea and lens

Question 6

Describe the flow of the aqueous humour

Answer 6

Produced in posterior chamber –> circulates to anterior and vitreous chambers –> passes through trabecular meshwork at base of cornea –> enters canal of Schlemm which drains into ocular veins

Question 7

What does the amount of refraction of light depend on?

Answer 7

Density of medium it passes through and the angle at which the light strikes the surface

Question 8

What is the role or the cornea and lens in light refraction?

Answer 8

The cornea plays the largest role in refraction and bend the light coming in
The lens fine-tunes the refraction and focuses the light on the retina

Question 9

What is the lens and how does it work? (i.e., muscular accommodation, etc.)

Answer 9

A naturally elastic, cellular structure that wants to be round
The ciliary muscle is a ring of circular muscle
When the ciliary muscle is relaxed, it forms a large diameter ring which puts tension on the zonular fibres - this flattens the lens
When the ciliary muscle is contracted, it forms a smaller diameter ring which puts less tension on the zonular fibres - this makes a rounder lens
Rounder lenses bend more light

Question 10

What is the purpose of accommodation? How does the lens accommodate for near and far objects?

Answer 10

Light from close objects strikes the eye at greater angles than light from far objects - goal is to focus light on the retina
Accommodation means making the lens rounder for nearer objects
Need a ROUND lens for NEAR objects, so CONTRACT the ciliary muscles
Need a FLAT lens for FAR objects, so RELAX the ciliary muscles

Question 11

What is presbyopia?

Answer 11

When the lens loses elasticity and has a diminished capacity to accommodate for near objects

Question 12

What is myopia? What happens with this? How is it fixed?

Answer 12

Problem focusing on distant objects (nearsightedness)
The eyeball is too long for focusing power of lens with far objects, lens cannot flatten enough to focus image on retina and image is focused in front of retina
With near objects, the image is focused on the retina with a flatter lens than normally required
Accommodate by wearing concave lenses

Question 13

What is hyperopia? What happens with this? How is it fixed?

Answer 13

Problem focusing on near objects (farsightedness)
The eyeball is too short for focusing power of lens - lens is not round enough to focus image on retina and the light is not reflected enough, focusing the image behind the retina
Far objects are focused on retina with rounder lens than normally required (don’t need to accommodate for this)
Convex lens is used to accommodate for close objects