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) directly

- 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 that

bend the rays - refractive system of the eye

1

Refractive system of the eye

1. cornea

2. humor aquaeus

(in the anterior chamber)

3. lens

4. corpus vitreum

(vitreus humour)

http://themyopiaepidemic.com/index.php/2017/09/15/myopia-blog-65-website-updates-91517/

• 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 imaged into several points in retina

The refractive system of the eye bends the rays (a ray changes

its direction), so that object we gaze at is imaged on retina

•

sharp

blurred

Higher refractive power

Lower refractive power

• refractive power - the degree to which an optical system bends the rays

• refractive power is measured in diopters (D)

– 1 D=1/ focal length (m)

– focal length - the distance from an optical surface (lens) to the focal point

– focal point - the point where the rays of light focus (retina)

Focal length

Focal length

Total refractive power of the eye: 59 D (when looking at a distant object )

of that

- refractive power of the cornea 43 D

(interface of 2 media with different density, therefore higher refractive power)

- refractive power of the lens 16 D

- other parts of refractive system (humor aquaeus, corpus vitreum) have small

refraction and are not considered in the model of the eye (reduced eye)

Humor

aquaeus

Corpus

vitreum

retina

rays coming from distance (more than 5-6 meters) enter the eye as parallel (a)

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 (b)

in a system with the same refractive power they focus behind retina (because of the

law of refraction - the angle of incidence = the angle at of reflection)

if rays coming from a 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) (c)

(a)

(b)

(c)

Accommodation

- adaptation of the refractive power of the lens

to distance of the observed object

- in accomodation - the convexities

(curvatures) of the lens are increased

- result: focusing of the rays to 1 focal point on

the retina so that sharp image is produced

Mechanism of accommodation

• lens is attached to the ciliary muscle by radial zonule fibres (suspensory ligaments)

• 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

shape of the lens 1

• reflex activity – controlled by the autonomic nervous system

• the sequence of processes is summarized in the scheme below:

http://1.bp.blogspot.com/-8fSivFQeyV8/UxxNJeyMSmI/AAAAAAAACjw/9eN5tTm2njo/s1600/Accommodation.jpg

1

2

3

1

2

3

Punctum remotum – far point

• last point in the 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 with 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

Amplitude of accomodation and age (Donder´s table)

Age Amplitude (D) PP (cm) Age Amplitude (D) PP (cm)

10 14 7 45 3,5 28,5

15 12 8,3 50 2,5 40

20 10 10 55 1,75 57

25 8,5 11,8 60 1 100

30 7 14,2 65 0,5 200

35 5 20 70 0,25 400

40 4,5 22 75 0 infinity

https://www.slideshare.net/Eyenirvaan/prescribing-for-refractive-errors

• maximum gain in 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

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. presbyopia

4. astigmatism (aspherical ammetropia)

Myopia

http://www.hcballroom.com/957192-short-eye-fashion-women

- 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 (-)

- causes diverges the rays before they enter

the eye, so that they focus on retina

Hyperopia (hypermetropia)

https://patient.info/health/long-sight-hypermetropia

- 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 before they

enter the eye, so that thy focus on the retina

https://www.slideshare.net/Eyenirvaan/prescribing-for-refractive-errors

- 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

- symptoms show up at the age of 50, worsen with

aging

- cause: loss of elasticity of the lens due to a

decrease of water content + protein denaturation

- a form of hyperopia

- rays of light focus behind retina

- blurred image when looking at short distance

- correction of presbyopia – convex lens

Presbyopia

• normal cornea – shape of 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

Astigmatism

http://upload.wikimedia.org/wikipedia/commons/thumb/e/e7/Cylindrical_lens.svg/200px-Cylindrical_lens.svg.png

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

• correction – cylindical lens

but – even in a healthy eye the corneal curvature in

vertical plane is often slightly smaller than in

horizontal plane = normal astigmatism (less than 1 D)

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

- examination of the shape of cornea

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

http://spectacle.berkeley.edu/pics/clinic-exam-pics/keratoscope_topog260.jpg

https://encrypted-tbn3.google.com/images?q=tbn:ANd9GcRtOYl26qFqEWCnJOBsyoKFkd_sZHVK05Agmtx717DDbFMlj1AUMw

Task: Keratoscopy

Result

Normal:

- on the cornea are visible concentric circles – reflex of the keratoscope

Disorders:

- astigmatism – ellipsoid shape of circles

- injuries - scares on cornea -irregular shape of circles

Conclusion

is the result normal or abnormal?

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

• in an ophthalmologist´s office 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

http://www.eyes.arizona.edu/Teaching/MedStudents/FundOph.html

The eyeground (what you should see)

- 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)

• close to blind spot retinal vessels diverge, spread over retina, avoid macula lutea

• yellow spot - macula lutea

- dark orange colour (because retina is

thinner here, therefore the pigment

layer becomes visible)

• fovea centralis

- in the middle of yellow spot,

- place of the maximum visual acuity

- highest density of receptors

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

• an image of the network of retinal blood vessels that

is made visible in one's own eye

• can be seen by shining the beam of a small bright

light penlight through the pupil from the periphery of a

subject's vision

• retinal vessels - lie on the top of the retina

• (despite this we can see a complete visual field

because CNS completes the missing parts)

• the vessels permanently shade some

receptors, therefore they are normally not

illuminated by light

https://en.wikipedia.org/wiki/File:Purkinje_Tree_BM.jpg

light

receptors

vessel

• 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

• this results inperception of own retinal vessels

• often experienced during ophtalmoscopic

examination

light

receptors

vessel

light

receptors

vessel

receptor

potential

https://en.wikipedia.org/wiki/File:Purkinje_Tree_BM.jpg

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

https://www.slideshare.net/sevahakobyan/entoptic-phenomena

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?