BB physiology 2

Question 1

what is the fundus of the eye and what does it include

Answer 1

• the fundus of the eye is the interior surface of the eye and includes the retina, optic disc, macula and fovea

Question 2

what is an ophthalmoscope

Answer 2

The opthalmoscope is an instrument through which an observer can look into another person’s eye and view the retina

Question 3

describe how an ophthalmoscope works

Answer 3

• When you are focused on a distant object the rays of light entering your eye are approximately parallel and brought to a focus on your retina.
• Conversely if your retina emitted light the rays of light leaving your eye would be parallel so that someone else looking into the distance along the line of the parallel rays from your fundus would focus your retina on theirs i.e. would see it.
• Infact you do not need any lenses to see someone else’s fundus; only, for example a mirror at 45º with a hole in it to reflect light into the subject’s eye.
• The “O” on an opthalmoscope is infact a hole- there is no lens.

Question 4

how can you see the eye of the myopic person

Answer 4

• Someone who is short sighted (myopic) cannot focus on a distant object (their eye is too long, or as if they could not relax accommodation) i.e. rays of light from their illuminated retina are converging as they leave the eye to make the rays parallel, in order to see the fundus of a myopic person a concave (or “minus”, red) lens must be placed before the eye.
• Similarly a convex (or “plus”, green) lens is needed for a long-sighted person.
• Thus the value of the lens needed to get a clear view of the fundus gives some indication of how long or shortsighted the subject (or observer) is but only if both are focused for distance.

Question 5

what does myopic mean

Answer 5

means short sighted

Question 6

how do you use an ophthalmoscope

Answer 6

• To use an opthalmoscope the subject usually sits down and looks at a distant object. - The examiner turns on the opthalmoscope light, makes sure he is looking through the correct side of the instrument and looks through the aperture at the subjects eye from about two feet away.

Question 7

what will you see in an ophthalmoscope

Answer 7

• You will see a red glow which is called the “red reflex” this is the reflection of the light from the blood in the vascular choroidal layer of the eye, through the almost most transparent retina.
• Now approach the subject, still keeping the red refle in view, but stare into the distance, do not focus on the subjects eye. As you get close, say 4 inches, you will start to see details and when as close as is proper you will see the fundus oculi of the subject.

Question 8

describe how the pupillary light reflex

Answer 8

The pupillary light reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retina of the eye, thereby assisting in adaptation to various levels of darkness and light, in addition to retinal sensitivity

Question 9

what is the blind spot

Answer 9

The optic disc (where the retinal ganglion cell axons leave the eye) is white because there is no chorodial layer or retinal layer covering this region. Light falling on this area is not perceived and so it is a blind spot. You are not conscious of it when one eye is closed; it is not black or white or coloured - there is no sensation at all to make you aware of it.

Question 10

How do you test for the blind spot

Answer 10

Set up the blackboard one meter from the subject. Cover the left eye. Place a white headed pin to the left side of the board (this is the fixation point). The subject should look steadily at this pin. The 2mm target (a white disc) is now moved smoothly away from the central fixation point. It will suddenly disappear from view. This point should be marked with a pin.

Now starting IN the blind spot move the target in any direction until it reappears. From these manoeuvres you can map out the blind spot. Measure its diameter in mm. Sketch its appearance in the results section. Given that the effective length from the lens to the retina is 16.67mm, you can work out how wide is your blind spot (optic disc).

Question 11

how do you measure convergence

Answer 11

Use a device consisting of a printed card sliding along a meter rule. Starting with a card at the far end of the rule and looking at it with both eyes open, slide it along your nose. Bring it as close as possible and note the point at which the card appears to double

Question 12

how do you measure accommodation

Answer 12

• same process as convergence
• now repeat the last procedure but with one eye occluded and note the distance at which the card becomes blurred. This latter is the near point of accommodation

Question 13

how do you measure visual acuity

Answer 13

use a snellen chart

• Using the test chart.
• Stand 6 meters from the chart and record your visual acuity. What is the line of the lowest letters you can read? Do this with and without glasses if you have them. Record for each eye separately and both together.

Question 14

what is the average foveal cone diameter

Answer 14

30μm

Question 15

what isa he distance between the lens and the eye and the retina

Answer 15

the effective distance between the lens and the eye and retina is 16.7mm