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Senses 3
The optics of the eye
Accommodation of the eye
Ammetropias
The eyeground
Visual field
Practical tasks
• Purkinje´s images
• Keratoscopy
• Ophthalmoscopy
• Purkinje´s flash figure
• Determination of the puntum proximum
• Examination of the visual field - perimetry
Vision • sense organ – eye Sensory receptors • rods and cones in retina
Adequate stimulus • light – electromagnetic waves with wave length 400 – 760 nm • absorption of light stimulates the sensory receptors
Rays of light that enter the eye
come from
-light sources (sun, bulb)
-mostly are reflected from
surrounding objects
Vision provides 80% of all sensory information to a human
– important for communication (written text, non-verbal communication)
• the receptors are in retina – lines the inner surface of the eye
• before light rays reach retina they pass through several layers of the
eye (refractive system of the eye)
1
1
2
3 4
http://www.internal.schools.net.au/edu/lesson_ideas/optics/images/eye_structure.gif
The major parts of the human eye
Refractive system of the eye
• consists of dioptric media
1. cornea
2. humor aquaeus
3. lens
4. corpus vitreum
(vitreus humour)
1
- if light rays strike an interface that is perpendicular (right
angle) to the beam, the rays enter the second medium
without deviating from their course
- if the rays pass through an angulated interface, the rays
bend (= refraction)
http://www.internal.schools.net.au/edu/lesson
_ideas/optics/images/eye_structure.gif
• the ability to bend rays = refractive power
• measured in diopters (D)
– 1 D=1/ focal length (m)
– focal point - the point where the rays of light focus (retina)
– focal distance - the distance from an optical surface (lens) to the focal point
• total refractive power of the eye 59 D
• of that
– refractive power of cornea 43 D*
– refractive power of lens 16 D
– other parts of refractive system have small refraction
and are not considered in the model of eye (reduced eye)
* a ray changes direction (bends) as it travels from a medium of one refractive
index to another medium that has a different refractive index (density)
* cornea – interface air/solid
• image at the retina – reversed,
diminished
• the upper part of visual field is imaged at
the bottom part of retina (left at right, etc.)
• the image is processed by brain and it is
perceived in the upright position
•
visual
target
focal point in retina
•
• • • •
sharp image – if rays from a point of visual target
are focused into 1 point in retina
blurred image – if rays from 1 point of the visual
target are focused into several points in retina
Law of reflection
- the angle at which light is incident on the
surface = the angle at which it is reflected
• rays coming from distance (more than 5-6
meters) enter the eye as parallel
• refraction system of the eye bends the rays
and they are focused on retina
• rays coming from closer distance (less than 5
- 6 m) are divergent (smaller angle of
incidence!)
• if rays coming from closer distance are to be
focused on retina they have to be bent more
(otherwise they will not focus on retina and
result in blurred image)
angle of
incidence
angle of
reflection
angle of incidence
angle of reflection
Accommodation
• adaptation of refractive power of the lens
to distance of the observed object
• in accomodation - the convexities
(curvature) of the lens is increased
• the greater the convexity of lens – the
higher the refractive power, i.e. the ability
to bend rays
• purpose – focusing of the rays to 1 focal
point on the retina so that sharp image is
produced
lower
refractive power
focal distance
higher
refractive power
lens
not accommodated accommodated
eye
musculus ciliaris relaxed contracted
tension of the zonule
fibres
high low
lens - curvature smaller bigger
Mechanism of accommodation
• lens is attached to the ciliary muscle by radial fibres
(zonula Zinnii, zonule fibres)
• fibres pull the lens edges to the outer circle
• m. ciliaris acts as a sphincter, its tone regulates the
tension of zonula Zinnii and thus that of lens
• reflex activity - controlled by parasympathetic
nerves
ciliary muscle zonula
Zinnii
lens
• maximum increase of refractive power of lens +14 D (in a child)
• the amplitude of accommodation reduces with age
• refractive power of lens is decreasing (lens becomes less elastic – less water
content + protein denaturation)
• the ability to focus near objects becomes lower
Punctum remotum – far point
• last point in distance that can be seen sharply without accommodation (it is in
distance 5 – 6 m)
Punctum proximum – near point
• last point, that is seen sharply in maximum accommodation
Accommodation area – distance between close and distant point (in meters)
Accommodation width – change in the refractive power of the lens when
measured from punctum remotum to punctum proximum (D)
sharp
with accomodation
sharp without
accomodation
blurred,
max accomodation
PR PP
Decrease of the refractive power of lens by age
age punctum proximum (m) accomodation ability(D)
10 0,07 14
20 0,10 10
30 0,12 8
40 0,22 4,5
50 0,40 2,5
60 1,0 1,0
70 4,0 0,25
80 infinity 0
Presbyopia
- is a condition in which the lens of the eye diminished its ability to
accommodate in that extent that comfortable reading at normal distance is no
longer possible (blurred image when looking at short distance)
- symptoms show up in age of 40/50 , worsen with aging
- correction of presbyopia – convex lens
Refractive diorders
Emmetropia
• normal function of the refraction system
of the eye
• condition for which the eye (without
accommodation) images a distant
object onto the retina
Refractive disorders (ammetropia)
• when the eye fails to bring into focus
(on retina) the image of a distant object
causing blurred vision
Ammetropias
1. myopia – shortsightedness
2. hyperopia – farsightedness
3. astigmatism (aspherical ammetropia)
4. presbyopia
• •
Myopia – short sightedness
- parallel rays are bent too much
- the focal point is in front of the retina
- image at the retina is blurred
Causes:
- eyeball is too long (spherical aberration)
- refracive system of the eye is too strong
(refractive aberration)
Correction:
- concave lens (diverges the rays)
http://www.unmc.edu/physiology/Mann/pix_7/errors.gif
Hyperopia – farsightedness
- parallel rays are not bent sufficiently,
they focus behind retina
- at the retina a point is imaged into
several points – image is blurred
Causes:
- eyeball is too short (spherical
aberration)
- the refractive system is too weak
(refractive aberration)
Correction:
- convex lens - causes convergence of
the rays
http://www.unmc.edu/physiology/Mann/pix_7/errors.gif
Task: Purkinje´s images
• part of the light rays directed towards the
eye do not reach retina, but are reflected
• reflection takes place on the
1. cornea - 1st Purkinje´s image
2. anterior surface of the lens - 2nd Purkinje´s image
3. posterior surface of the lens - 3rd Purkinje´s image
Procedure
• work in a dark room
• hold the candle in front of the patient´s eye in safe distance (10 - 20 cm)
• observe the Purkinje´s images – reflexes of the flame
1st Purkinje´s image (cornea)
- image is upright
- when moving the candle, image moves in the same direction
2nd Purkinje´s image (lens – anterior surface)
- image is upright and less pronounced
- when moving the candle, image moves in the same direction
3rd Purkinje´s image (lens – posterior surface)
- image is reversed
- moves in the opposite direction to movement of the light source
Result and conclusion
- describe and explain your observation
Task: Keratoscopy
Astigmatism
• refractive error of the eye - the eye shows different
powers at different meridian planes
• results from larger curvature in one plane of the lens
•
• light rays are incorrectly focused on the retina
• a point is imaged in one plane in several
points causing blurred vision
• normally curvature in vertical plane is often
slightly smaller than in horizontal plane
= normal astigmatism (normally less than 1 D)
• examination of the shape of cornea
• normal cornea – a slice of a ball
• curvatures in all planes are the same
• in all planes the rays are focused to 1 point
http://www.nei.nih.gov/health/errors/images/astigmatism-image.jpg
Procedure
• the patient is seated backwards to daylight
• put the keratoscope in front of his eye
• through an opening in the centre of the keratoscope
observe the reflection of concentric circles in
patient´s cornea
Result
- normal: on the cornea are visible concentric circles –
reflex of the keratoscope
- disorders:
• astigmatism – ellipsoid shape of circles
• corrected by cylindrical lenses – bend light rays
only in one plane
• injuries - scares on cornea -irregular shape of circles
Conclusion
• is the result normal or abnormal?
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e7/Cylindrical_lens.svg/200px-Cylindrical_lens.svg.png
cylindrical lens
http://spectacle.berkeley.edu/pics/clinic-exam-pics/keratoscope_topog260.jpg
https://encrypted-tbn3.google.com/images?q=tbn:ANd9GcRtOYl26qFqEWCnJOBsyoKFkd_sZHVK05Agmtx717DDbFMlj1AUMw
Task: Determination of the punctum proximum
Scheiner´s optometer
• a wooden stick with cm scale
• 2 pins fixed in a marker that can be moved
• metal piece with the openings for observation of the pin
Procedure
• the examinee is sitting and looking through an opening in a metal piece of the
Scheiner´s optometer and
• he/she focuses on the head of the pin fixed to a marker of the optometer
• the pin is located at the beginning of the optometer close to examinee´s eye -
the examinee does not see it sharply
• the examiner moves the pin away from the examinee´s eye
• when the examinee starts to see the pin head sharply, read the distance from
examinee´s eye = punctum proximum
Result
- distance of the punctum proximum
- calculate the refractive power of the lens (1/distance in m)
Conclusion: is the result normal?
Task: Examination of the eyeground - Ophtalmoscopy
• image of the retina observed through the pupil by an ophtalmoscope
• Direct ophtalmoscopy
– examiner examines the background face to face to the patient
– a detailed 16-times magnified image - upright
• Indirect ophtalmoscopy
– a lens (16 D) is put between the ophtalmoscope and the eye
– image is reversed and and 4-times magnified
– examinee is in larger distance from the examiner
Procedure
• examine in a dark room, both examiner and examinee sit
• switch the ophtalmoscope on, examine the patient´s right eye with your right eye
• observes the retina through the optic of the ophtalmoscope
• neither the doctor nor the patient accommodate during the examination
• if the doctor or the patient wear glasses, the ophtalmoscope must be adjusted to
their diopters (patient´s + doctor´s)
e.g. if the sum of diopters is 4 – adjust to the value -4
The eyeground - round shape, orange colour
Structures to observe:
• blind spot (optic disc, optic nerve head) - area where axons of retinal ganglion cells converge and form the optic nerve (lighter spot in nasal part)
• yellow spot - macula lutea
- dark orange colour – thinner retina, the pigment layer becomes visible
• close to blind spot retinal vessels diverge, spread over retina, avoid macula lutea
• fovea centralis
- in the middle of yellow spot,
- place of the maximum visual acuity
- highest density of receptors
• normally the examination is performed
after dropping atropine into the
patient´s conjunctival sack
• atropine causes paralysis of m.
constrictor pupillae
• mydriasis occurs – diameter of the
pupil is increased
Diabetic retinopathy
- aneurysms
- bleeding
- neovascularization
Hypertension Intracranial hypertension
– swollen papilla n. optici
• Examination of eyeground
is part of examination in
patients with e.g.
– Hypertension
– Diabetes
– Brain disorders
(intracranial hypertension)
• typical abnormalities –
help the staging of the
disease
Purkinje´s flash figure
• sensation of the vessels in the own retina
• retinal vessels
– located in front of the retina
– therefore permanently shade some receptors -
they are normally not illuminated by light
• (despite this we can see a complete visual field because CNS completes the
missing parts)
• unilluminated receptors are adapted to darkness and therefore more sensitive to
light
• if a strong light stimulates these receptors (e.g. if a light comes from unusual –
lateral direction), they generate a stronger receptor potential than receptors
„used to“ to the light
• the individual has a sensation of his/her own vessels
http://t1.ftcdn.net/jpg/00/08/85/86/400_F_8858656_Jwv2Gheg
EpyxaykPJcn7xi8nMXXRossx.jpg
Procedure
• switch the ophtalmoscope on
• put the ophtalmoscope to the lateral
side of the eye
• look straight forward, do not
accommodate (look into the distance)
• direct the light rays into the eye in such
an angle that the image of retinal veins
occurs (it appears as an image of dry
soil, or a spider´s network)
Result and conclusion
• describe (and draw) your observation
Visual field
• space that we see when focusing the eye at one point
Range
• temporal direction 90°
• nasal direction 60 °
• upwards 60 °
• downwards 70 °
• monocular visual field
• binocular visual field
• visual fields of both eyes partially
overlap
60 °
70 °
Perimetry
• the examinee is sitting in front of the perimeter, his head is fixed
• the non-examined eye is covered
• the examined eye is focusing on a cross in the middle of the semicircular arm
• the semicircular arm is positioned to horizontal plane
• the doctor rotates a knob in the back of the arm, by rotating the knob a light beam is moving along the semicircular arm (a light dot)
• the patient is required to announce
– when he notices the dot in his visual field
– when the dot disappears from his visual field
• the examination is repeated in other positions of the semicircular arm
• record the results (point on a sheet)
• move the arm to other positions (5)
• examine other planes
• in horizontal plane the blind spot should be
found (the dot disappears from the visual field
for a moment – close to the centre of the arm)
Result:
- connect the points with lines – visual field
- compare the visual field with normal field
Conclusion:
- is the result normal?
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