MANUAL PHYSIOLOGY PRACTICAL - Repository:The Medical ...

FM/UOJ
Experiment 2.4
Measurement of intra ocular pressure
The aqueous humour of the eye is formed by the ciliary body, flows through the
pupil and absorbed into the canal of Schlemm and from there to the venous plexus. The
circulation of this fluid provides the pressure in the eye which helps to keep the spherical
structure. When the formation is reduced as in severe dehydration the pressure reduces
and when the drainage is affected as in glaucoma, the pressure increases.
The pressure can be clinically judged by pressing the eyeballs over the eyelid. The
same principle is use to measure the pressure more precisely by a Tonometer.
Experiment 2.5
Visual acuity
The degree to which the details and contours of objects are perceived is the visual
acuity. It is usually defined by the minimum separable distance between two lines. It is
the shortest distance between two lines which can be identified as two lines. Defects in
visual acuity may occur due to neural or refractive errors.
Snellen chart
Snellen chart is commonly used to test visual acuity. The letters on it are prepared
so that the width of the lines in the letter subtend 1 minute arc and the minimum distance
between the lines is also 1 minute and the width of the whole letter subtends 5 minute arc
from the distances marked below them. An individual with normal visual acuity could
read the first row of letters from a distance of 60 meters (200 feet). Similarly, the row of
letters marked 6 meters (20 feet) could be read from that distance.
Method:
The snellen chart is placed at the eye level and illuminated well. The subject is
placed 6 meters (20 feet) from the chart. One eye is blind folded. The subject is asked to
read from top. The last line that is clearly read is noted. The procedure is repeated for the
other eye.
The visual acuity is determined by dividing the distance from which the chart is
read by the distance from which a normal person can read the last row read by the subject.
If the subject has read the row marked 6 meters, his visual acuity is 6/6 (or 20/20). If the
subject read up to the row marked 18 meters, the acuity is 6/18- less than normal. If the
subject goes on to read the row marked 5 meters, then the acuity is 6/5 which is better
than normal.
If the visual acuity of the subject is reduced, the test is repeated allowing the
subject to read through a pin hole. The pin hole eliminates the function of the refractive
mechanisms and the performance improves if the subject had refractive error. If the
reduction of visual acuity was due to neural problems, pin hole does not improve the
acuity.
Neurology
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