CSEC Biology: The Human Eye

Your eyes are probably your most important sense organs. I mean, think about it: which of your 5 senses do you use the most? Definitely sight! Looking at something is one of the first ways we as humans try to perceive new things. After all, you have to see it to believe it.

As you might have guessed, your eyes are very complex organs with a lot more going on inside of them than you might think. You could say that they're more than meets the eye. Puns aside, CSEC requires that you understand the structure of the eye and the functions of each part, as well as a few of the many defects of the eye.

(Section B 7.9 and 7.10 in the Biology syllabus)

The following diagram shows the structure of the eye:

You've likely seen a diagram like this several times before, but there might be a few structures you're unfamiliar with.

Sclera- tough, white outer coating of the eyeball meant to protect the eye from damage

Choroid- Layer containing blood vessels which keep the retina supplied with a source of oxygen and food materials as well as melanin (to prevent reflection within the eye)

Retina- Layer containing light-sensitive rod and cone cells on which the image is formed

Vitreous humour- Mostly water-based jelly-like fluid which maintains the round shape of the eyeball

Fovea- Most light-sensitive area on the retina containing only cone cells. Point at which most light rays are focused. This point in the eye is responsible for detailed colour vision.

Hyaloid canal (also called Cloquet's canal)- contains lymph, and decreases and increases in volume alternate to the accommodation of the lens to maintain a constant volume in the eye. That is, when the lens increases in volume, the hyaloid canal decreases in volume, and when the lens decreases in volume, the hyaloid canal increases in volume.

Optic Disc/Blind Spot- Point on the retina where the optic nerve leaves the eye. No rods or cones are present, so it is a 'blinds pot' in the vision of each eye. However, each eye compensates for the blind spot of the other, so the blind spot is not noticeable in normal binocular vision (vision with both eyes).

Optic nerve- carries impulses from the cells of the retina to the visual cortex in the rear of the brain for processing.

Blood vessels- supply tissue in the eye with blood

Ciliary body- Contains ciliary muscle, a circular muscle which adjusts the size of the lens during accommodation

Iris- Circular, coloured, muscular disc which controls how much light enters the eye through the pupil

Pupil- Hole in the center of the iris which allows light to enter the eyeball

Aqueous Humour- Colourless, water-based fluid which maintains the dome shape of the cornea

Cornea- Transparent anterior section of the sclera which refracts light rays onto the retina

Suspensory Ligament- Ligament attaching the ciliary body to the lens

Lens- Elastic biconvex structure which changes shape during accommodation to fine tune the focusing of the image on the retina

Rods and Cones

The retina comprises two types of light-sensitive cells known as photoreceptors, the rods and the cones.

Rods are responsible for vision in low light (scotoptic vision).

They only detect images in black and white.

They are the more numerous cells in the retina- about 120 million of the total 125 million photoreceptors in the retina are rods.

They are more sensitive to the brightness of light, and have low visual acuity.

They have only one pigment, rhodopsin.

They are cylindrical in shape (like rods).

Cones only become active at higher light intensities (photopic vision).

They are capable of detecting colours and fine details.

This fine detail detection is due to a higher visual acuity.

As the name suggests, cones are conical in shape.

Accommodation of the Lens (Focusing Light onto the Retina)

If you've ever focused on something near to you against a distant background, you'll know that everything far away will become blurry behind the object of your focus, which will appear sharp. The opposite applies when focusing on the distant backdrop; the distant landscape will become sharp and clear while the nearby object becomes blurry. You can try this by holding your finger up to your eye and focussing on your finger and then what's behind your finger.

Interestingly, if you try switching between them quickly enough, you might feel a bit of a strain inside your eye. These are your ciliary muscles contracting and relaxing to pull and push your lens into a shape which allows the light rays to focus.

For distant objects, the ciliary muscle relaxes, the suspensory ligaments are pulled tight, and the lens are pulled thin. This bends the light only slightly.

For nearby objects, the ciliary muscle contracts, the suspensory ligaments become loose, and the lens is allowed to bulge. This causes the light rays to be bent a lot.

Light Entering the Eye

You've probably been spending the better part of a year indoors due to our current pandemic, so your eyes are usually accustomed to lower light intensities. When you occasionally step outdoors to remind yourself what the sun looks like, you'll go through a couple seconds of agony before your eyes adjust. This is the time taken by your iris to adjust the size of the pupil so that less sunlight enters your sensitive little eyes.

Your iris comprises two sets of muscles, the (inner) circular muscles and the (outer) radial muscles. Each set of muscles does the opposite of what the other set does to either increase or decrease the size of the pupil:

In B, the eye is exposed to bright light, so the circular muscle contracts and the radial muscle relaxes. The result is a smaller pupil to restrict the amount of light passing into the eye. This is called miosis.

In C, the eye is exposed to dim light, so the circular muscle relaxes and the radial muscle contracts. The result is a larger pupil to allow more light in. This is called mydriasis.