Inside the Human Eye

In Human Vision we investigated our ability to see bright and dark objects, our wide field of vision and our excellent depth perception.

In this article we look at the different components of the human eye, excluding the retina, which all combine to enable us to see.

grey-eyes Deanern1 CC BY-SA 3.0

  • So how does our human vision compare with the vision of other animals such as cats?

Cats have a visual field of 200 degrees which is slightly wider than the average human visual field of 180 degrees.

field-of-vision-of-cats-compared-to-humans

Human color vision is better than the color vision of cats…

cat-sees-fewer-colours-than-humans

…but not as good as the color vision of humming birds….

ruby-throated-hummingbird-male-feeding-on-kalanchoe-flower

…. which can identify different species of flower by seeing them in the ultraviolet spectrum of light invisible to humans.

birds-see-ultraviolet-light-whilst-humans-do-not

The evolutionary advantage for having color vision is all too obvious below. Danger lurking in a rain forest is less easily seen in monochrome…

panther-seen-as-black-and-white-vision

…but easier to spot in color!

panther-in-colour-is-easier-to-spot

Our long distance vision is much better than that of cats….

visual-acuity-of-cats-compared-to-humans

….but our night time vision is less effective.

comparison-of-abilities-of-cats-and-humans-to-see-at-night

Our human vision is remarkably good despite the fact that our eyes are comparatively small in size. If we had the compound eyes of insects….

compound eyes of an insect

….our eyes would have to be far bigger to see with the same visual acuity!

human-compound-eye shown on top of a human head

  • So what are the different components of our eyes that combine to provide us with our ability to “see”?

eye-diagram-showing-labelled-parts-including-sclera-choroidretina-fovea-optic-nerve-iris-pupil-cornea-lens

Cornea

The cornea is the transparent part of the eye that covers the pupil (the opening at the center of the eye), the iris (the colored part of the eye), and the anterior chamber (the fluid-filled space next to the cornea).

cornea-shown-in-cross-section-in-a-human-eye

The cornea is responsible for refracting (bending) most of the light that enters the eye ball.

diagram-of-eyeball-showing-refractive-light-entering-the-eye-through-the-cornea

The cornea does not contain blood vessels, unlike other tissues of the human body. If blood vessels were present in the cornea they would interfere with the refraction of light and adversely affect vision.

With no nutrient-supplying blood vessels in the cornea it is the tears and watery fluid (‘aqueous humor’) in the anterior chamber that provide the cornea with all its nutrients.

anterior-chamber-cornea-iris-pupil-and-lens-of-a-human-eye

If the cornea should become misshapen…..

….and cloudy as a result of disease, vision would become very blurred.

hydrops-descemets_membrane_rupture_due_to_keratoconus

Pupil

The pupil is the round hole located in the center of the iris that allows light to enter the eye.

human-iris

If you gaze into someone’s eye, you will not see any light reflected out of the pupil. As a consequence of the fact that no light escapes, the pupil appears black. You didn’t know that us humans have our very own black holes, did you?!

black-hole-compared-to-pupil-of-the-eye-where-no-light-escapes

The constriction or dilation of the pupil according to the amount of light entering the pupil is a ‘reflex action’-an immediate response to a stimulus for which we have no conscious control.

pupillary-reflex-showing-pupil-dilation-and-contraction

  • So how does the pupillary ‘reflex action’ take place?

In the below diagram a torch is shone directly into one eye. Information, in the form of nerve impulses, is then transmitted to the mid-brain via the optic nerve along the ‘afferent’ pathway.

pupillary-reflex-explained-including-diagram-of-third-cranial-nerve-ciliary-ganglion-and-optic-nerve

The decision that both pupils should constrict, even though a torch is being shone into only one eye, is made in the mid-brain and sent back to the eyes along the ‘efferent pathway’ via the third cranial nerve.

Iris

It is the muscles of the iris which control the contraction or dilation of the pupils.

pupil-consticts-as-circular-muscles-of-iris-contract-and-pupil-dilates-as-radial-muscles-of-iris-contract

Lens

Whilst about 80% of the refraction of light takes place at the cornea, most of the remaining 20% of refraction occurs as light passes through the inner crystalline lens.

human-eye-showing-how-lens-focuses-light-on-the-retina

In a healthy eye the lens can change shape to accommodate rays of light from both distant and close objects.

Distant vision and close vision in human eye

It is the ciliary muscles that control the flattening and thickening of the lens. When viewing distant objects the lens flattens, the ciliary muscles relax and the suspensory ligaments tighten.

distant-vision-of-human-eye-showing-ciliary-muscles-relaxed-and-suspensory-ligaments-taut

When viewing objects close up the rays of light need to bend more. To ensure that light does bend (refract) more the ciliary muscles contract, the lens thickens and the suspensory ligaments loosen.

close-vision-of-human-eye-showing-ciliary-muscles-contracted-and-suspensory-ligaments-slackened

In a healthy eye the lens is flexible enough to ensure that light focuses on the retina, no matter what the distance of the object being viewed.

normal-eye-light-focused-on-retina

Blurred vision, in the form of ‘myopia’, occurs when light entering the eye is focused in front of the retina.

myopia-light-focused-in-front-of-retina

‘Myopia’ can be corrected by wearing a pair of glasses fitted with concave lenses.

myopia-or-shortsightedness-corrected-with-provision-of-concave-lens

Blurred vision also occurs when the focal point of light is behind the retina.

hyperopia-or-hypermetropia-light-is-focused-behind-the-retina

‘Hyperopia’, or far sightedness, can be corrected by wearing a pair of glasses fitted with convex lenses.

hyperopia-or-hypermetropia-is-corrected-with-a-convex-lens

Older people typically develop age related far sightedness. The lenses in older people stiffen and become less flexible. As a consequence images form behind the retina causing blurred vision.

presbyopia-or-age-related-farsightedness-shown-on-diagram-of-the-eye

Sclera

The sclera, also known as the ‘white of the eye’, comprises the protective outer layer of the eye. It is composed of dense, fibrous tissue and forms the supporting wall of the eyeball. In humans the whole sclera is white, visibly contrasting with the colored iris. In other mammals the visible part of the sclera more closely matches the color of the iris.

sclera of human and chimpanzee compared

The sclera is covered by the conjunctiva, a clear mucus membrane that helps lubricate the eye.

diagram of two eyes showing normal and inflammed conjunctiva

The choroid

The choroid is the pigmented, highly vascular layer…

diagram of the eye showing vascular choroid

….lying between the sclera and the retina.

In addition to arteries the choroid contains veins.

The choroid supplies nutrients and oxygen to the retina; it is the choroidal blood flow that regulates the temperature of the retina.

Any impairment of the blood flow from the choroid to the retina may cause such conditions as ‘age related macular degeneration’. This is where the photo receptive cells within the macula part of the retina, with its high density of cone photo receptors, slowly die off.

diagram of the eye showing fovea and macula

The pigment of the choroid absorbs excess light entering the eyeball so avoiding light reflection inside the eye. The “red eye”phenomenon seen on photos, which becomes apparent when flash photography is used, is caused by light reflecting off the choroid’s numerous blood vessels.

Vitreous humor

The vitreous humor is the gel-like substance that occupies the vitreous chamber, the space between the lens and the retina.

Diagram of eye showing vitreous chamber between the lens and the retina

Along with providing vital support to the lens, the outward pressure of the vitreous humor helps maintain the shape of the vitreous chamber. It is essential that the eye maintains the correct shape to ensure that any light passing through the pupil can focus onto the retina.

The vitreous humor is transparent and has refractive properties similar to that of water.

index-of-refraction-showing-water

The vitreous humor is also stagnant (immobile) and is not served by any blood vessels. As a consequence it is not actively regenerated or replenished.

Any unwelcome build up of fluid pressure (‘intraocular’ pressure) within the vitreous chamber risks damaging the optic nerve.

eyeball showing inraocular pressure with risk to optic nerve

Retina

Rays of light passing through the vitreous chamber strike the retina, the light sensitive half millimeter thick layer of tissue which lines the inside of the eye ball.

Light passes through layers of specialised, sensory nerve cells (neurons) before striking and activating the photo receptive cells- the so called ‘rods’ and ‘cones’.

schematic diagram of eye showing nerve cells and neurons

In the next article we will look closer at the workings of the retina including the function of the photo receptive rods, cones and neurons.

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