pupil
Meaning of pupil: Not a structure but an absence: a hole whose width two opposing muscles adjust, and one of the most watched signs in an unconscious person.
Definition of pupil
The pupil is the opening at the center of the iris through which light enters the eye. It looks black because nearly all the light passing in is absorbed inside rather than reflected back out. Its width is set by two muscles within the iris, one closing it down in bright light and the other pulling it open in dim light or during alarm.
What controls its size?
Two sets of muscle fibers in the iris work against each other. A ring of fibers around the opening tightens like a drawstring and narrows it, driven by nerves that also slow the heart and settle digestion. A second set runs outward like spokes and pulls the opening wide, driven by the nerves that fire during fright, pain or exertion. Bright light triggers the first set within a fraction of a second, and light shone into one eye narrows both, because the signal crosses to serve each side. Focusing on something close narrows it as well, alongside the eyes turning inward.
Why do clinicians shine a light into it?
Because the response tests a circuit running from the retina, along the optic nerve, through the brainstem and back out to the eye, and it needs no cooperation from the person being examined. That makes it one of the few brain functions checkable in someone unconscious, sedated or too young to follow instructions. Sluggish, absent, uneven or fixed responses each point at a different part of that circuit. Swinging a light between the two eyes exposes a weak signal on one side, which paradoxically appears as the pupil widening when the light reaches the affected eye.
What do unequal pupils mean?
| Pattern | What it points toward |
|---|---|
| Small pupil with drooping lid and dry skin on that side | Interruption of the sympathetic supply |
| Wide pupil that will not narrow, lid down, eye turned out | Pressure on the third cranial nerve |
| Small difference, both reacting briskly, unchanged in dim light | A harmless lifelong difference |
| Both pinpoint but still reacting | Opioid effect, or a lesion in the pons |
Drops and dusts explain a fair share of one-sided wide pupils in people who are otherwise entirely well, since substances that block the narrowing nerve reach the eye easily from a finger. That possibility is considered before the frightening ones.
Used in a sentence
Both pupils were equal, round and reactive to light.
Good to know
A pupil that is wide and unreactive on one side, with a drooping lid and outward deviation, is treated as evidence of pressure on the third cranial nerve, while contact with substances that block the narrowing nerve produces a similar wide pupil in a person who is otherwise well.
Word origin
Where “pupil” comes from
From the Latin pupilla, meaning little doll, after the tiny reflected figure a person sees when looking into another eye.