Pupil
Pupilla
Definition
The pupil is the central opening of the iris, appearing as a circular black aperture in the centre of the eye. It is not a structure in itself, but rather a variable opening that allows light to pass through the iris to reach the retina.
Pupil size
The size of the pupil is dynamically regulated by the muscles of the iris.
Constriction (miosis) is produced by the sphincter pupillae,
While dilation (mydriasis) is mediated by the dilator pupillae.
These responses are controlled by the autonomic nervous system. Parasympathetic input causes constriction, whereas sympathetic input causes dilation.
Functional correlate
Functionally, the pupil regulates the amount of light entering the eye, optimizing visual acuity under different lighting conditions. In bright light, the pupil constricts to reduce light entry and improve image sharpness; in dim light, the pupil dilates to allow more light to reach the retina.
References
Standring, S. (Ed.). (2015). Eye (Chapter 42). In Gray's anatomy: The anatomical basis of clinical practice (41st ed., pp. 686696). Churchill Livingstone.
