It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens.

[15] They spread tears on the eyes, which contains substances which help fight bacterial infection as part of the immune system.

Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above and below water.

Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision.

Voluntary blinking of the eyelids in succession while making eye contact is considered a feminine flirtatious behaviour.

Long and prominent lashes are seen as a sign of beauty and are almost universally coveted for emphasizing the eyes and drawing attention to them.

Although most animals have some degree of binocular vision the amount of overlap largely depends on behavioural requirements.

The structure of the mammalian eye owes itself completely to the task of focusing light onto the retina.

This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain.

The ipRGCs enable entrainment to the Earth's 24-hour cycle, resizing of the pupil and acute suppression of the pineal hormone melatonin.

The fovea gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires staring at things.

Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (averted vision) where rods also exist, and where the light is sufficient to stimulate cells, allowing an individual to observe faint objects.

The breakdown results in the activation of Transducin and this activates cyclic GMP Phosphodiesterase, which lowers the number of open Cyclic nucleotide-gated ion channels on the cell membrane, which leads to hyperpolarization; this hyperpolarization of the cell leads to decreased release of transmitter molecules at the synapse.

The light information from these cells is not involved in vision and it reaches the brain not directly via the optic nerve but via the retinohypothalamic tract, the RHT.

They exercise control over the size of the pupil, and they lead to acute suppression of melatonin secretion by the pineal gland.

The purpose of the optics of the mammalian eye is to bring a clear image of the visual world onto the retina.

To make images clear for objects at different distances from the eye, its optical power needs to be changed.