Pupil
The pupil is the eye's aperture, controlling light entry.
The pupil is the opening at the center of the iris that lets light reach the retina. It looks black because incoming light is either soaked up by internal eye tissues or scattered inside the eye and fails to escape through the small hole. The term "pupil" dates back to ancient Roman times. In people, the pupil is round, but other animals have different shapes: vertical slits in some cats and reptiles; horizontal ovals in goats and sheep; and ring-like forms in certain catfish. While many foxes have vertical slit pupils, some species, like the arctic fox, have more rounded pupils.
- shapes
- Circular (humans), vertical slit (cats, some foxes), horizontal (goats, sheep), annular (some catfish), circular (emperor penguin), W-shaped (cuttlefish)
Lore & Background
The pupil is a hole in the center of the iris that allows light to strike the retina. Its size is controlled by two groups of smooth muscles: the sphincter pupillae (circular) and the dilator pupillae (radial). When the sphincter contracts, the pupil constricts; when the dilator contracts, it dilates. This is known as the pupillary light reflex, which is an important test of brainstem function. The sensory pathway from the retina involves rod and cone photoreceptors and melanopsin ganglion cells, with partial crossover of fibers between eyes.
Reader's Guide
The pupil's size varies with light, age, and cognitive activity. In bright light, it constricts to 1.5–4 mm; in darkness, it dilates to 3–8 mm, though maximal size varies widely among individuals. After age 25, average pupil size decreases. Pupil size can be affected by drugs: opioids cause constriction (miosis), while atropine, LSD, and cocaine cause dilation (mydriasis). Certain diseases, such as bene dilitatism from optic nerve damage, lead to chronically widened pupils. Some humans can voluntarily control their iris muscles to change pupil size. In animals, pupil shapes vary widely, including vertical slits in cats and foxes, horizontal rectangles in goats and horses, and square shapes in king penguins.
Did You Know?
- The term 'pupil' dates back to ancient Roman times.
- Solving simple multiplication problems can affect pupil size.
- Some humans can voluntarily dilate and constrict their pupils on command.
- The emperor penguin's pupil remains circular even when contracting in bright light.
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