Question

The size of the pupil is controlled by the
A. Ciliary muscles
B. Suspensory ligaments
C. Cornea
D. Iris muscles

Answer 1

The organs of sight are a pair of eyes. The eye is a hollow, spherical organ, about 2.5 cm in diameter and about 6 to 8 gm in weight. A muscle regulates the size of the pupil allows light to pass only through the center of the lens.

Complete answer: At the junction of the sclera and the cornea, the vascular coat sharply bends into the cavity of the eyeball to form a thin, colored partition. This partition is called iris. It is perforated at the middle by an aperture called the pupil. The iris contains two sets of smooth muscles: sphincters and dilators, of ectodermal origin. These muscles regulate the amount of light entering the eyeball by varying the size of the pupil. The sphincter muscles are arranged in rings. Their contraction makes the pupil smaller in bright light so that less light enters the eye. The dilator muscles are arranged in a radial manner. Their contraction widens the pupil in dim light to let in more light. Iris by regulating the size of the pupil allows light to pass only through the center of the lens, which is optically the most effective part. The parasympathetic fibres constrict the pupil and the sympathetic fibres dilate it.
So the correct answer is option D-Iris muscles

Additional information:
The wall of the eyeball consists of three layers. The external layer of the eye consists of dense connective tissue called the sclera. The anterior portion of this layer is called the cornea. The middle layer choroid contains many blood vessels and looks bluish in colour. The eyeball contains a transparent crystalline lens which is held by the ciliary body.

Note: The main function of ciliary muscles is changing the shape of the lens which occurs during the accommodation reflex. Suspensory ligaments of the lens are series of fibres that connect the ciliary body of the eye with the lens, holding it in place and cornea is the part of the eye that covers the front of the eye when light enters the cornea it bends or refracts the incoming light onto the lens.