The optic nerve is the:

Deficiency of vitamin $A$ causes ...... disease in human beings.

What is the primary function of the ciliary muscles in the human eye?

Answer briefly:
$(a)$ How do you perceive the colour of an object?
$(b)$ Which part of our body helps us in maintaining the body balance?
$(c)$ How does the eye regulate the amount of light that falls on the retina?

What is the function of ciliary muscles?

Chief functions of the rods and cones in the eye of a vertebrate are