MANUAL PHYSIOLOGY PRACTICAL - Repository:The Medical ...

FM/UOJ
Experiment 2.3
Measuring the diameter of the optic disc
This experiment demonstrates the blind spot in the field of vision and uses the
blind spot to measure the diameter of the optic disc. The optic disc has no receptors on it
and light is perceived by the retina around it. So, the diameter of the disc is proportional
to the blind spot.
Method:
The subject stands about 150 to 200 cm away from a black board, one eye blind
folded. A mark is placed on the board at the eye level of the subject and the subject fixes
the gaze on the mark. The examiner moves a chalk from this point in the temporal fields
of vision horizontally along the board. The subject is requested to signal when the chalk
disappear form sight. As the signal is obtained, a mark is placed on the board at the point.
The chalk is moved further, until it comes into view again. Another mark is placed on the
board. The area between the two marks is the blind spot. The blind spot is mapped out by
moving the chalk in and out of the blind spot at all angles and the circle is completed.
Measure the distance of the board from the eye, the distance of the medial edge of
the blind spot from the mark and the diameter of the blind spot. The diameter of the optic
disc and the distance of it from the visual axis is calculated using the principle of visual
angle as follows:
The principal focal distance
Assume, that the distance of the subject form the board
Distance of the blind spot from the mark
Diameter of the blind spot
Distance of optic disc from visual axis
Diameter of the optic disc
= 1.7 cm.
= s cm
= v cm
= d cm
= V cm. and
= D cm
v/V = d/D = s/1.7
Calculate the Diameter of your optic disc and record it:…………………………………
Neurology
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