Human Eye General

Question 1

Where is the eye situated

Answer 1

• The eyeball fits into a bony socket situated in the cranium
• it is held in position by six external muscles which controls the external movement
• posteriorly fatty tissue cushions it

Question 2

How the eye functions

Answer 2

There are three stages involved in seeing:
A. Pathway of light rays and image info
B. Stimulation of photoreceptors
C. Pathway and destination of nerve impulses

Question 3

A. Pathway of light rays

Answer 3

• As light passes from air into the eye it passes through the entire thickness of the neutral layer of the retina to stimulate photoreceptors

Question 4

B. Stimulation of the photoreceptors

Answer 4

• in the retina the rods and cones are stimulated by light
• in both photoreceptors, photo pigments are broken down by light energy
• this breaking down generates and electrical impulse in the photoreceptors

Question 5

C. Pathway of nerve impulses

Answer 5

• the impulses from the photoreceptors travel along two layers of neurons
• the axons of the ganglia neurons form the optic nerve which leaves the eye at the blind spot and carries impulses to the cerebral cortex
• the impulses are interpreted as vision in the occipital lobe of the cerebral cortex

Question 6

What is binocular vision

Answer 6

The two eye’s visual fields overlap considerably. The brain combines this info to form a single, 3 dimensional image which helps to judge the distance, depth and size of the object being viewed.

Question 7

Accommodation of the eye

Answer 7

Is the process of producing a finely-focused image on the retina. It is carried out by the action of the ciliary muscles which change the shape of the lens.

Question 8

When does accommodation take place

Answer 8

It takes place when viewing objects nearer than 6 meters. Light from a close object diverges so the lens must bulge out to bend the light rays mire to focus the image on the retina

Question 9

How is accommodation brought about

Answer 9

1. The circular, ciliary muscles contract
2. This action pulls the ciliary body inwards towards the pupil
3. This causes the tension on the suspension ligament and on the lens to stretch
4. The lens bulges outwards, becoming more convex
5. This causes light rays to bend more
6. The image of a close object can now be focused clearly on the retina

Question 10

How do eyes adjust to view distant objects

Answer 10

From distant objects, the light rays travel and parallel lines and need little refraction. Distant vision is the natural state, where the ciliary muscle is relaxed, the suspension ligament is taut, exerting a pull on the lens so that it is thin

Question 11

Image formation

Answer 11

• light rays are bent, this is known as refraction. Light is refracted as it enters the cornea and on entering and leaving the lens. The cornea is responsible for most light refraction (but it’s curvature is constant so it cannot change its refractive power). The lens is highly elastic and it’s curvature can be actively changed to allow fine, sharp focusing of the image
• the light rays converge to form a focused imagine on the yellow spot of the retina
• the imagine will be a real image, it is smaller than the object, upside down and reversed from left to right