In the chemistry of vision in mammals,the photosensitive substance is called

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

Label the following parts in the given diagram of the human eye:
$(a)$ Aqueous chamber
$(b)$ Cornea
$(c)$ Lens
$(d)$ Retina
$(e)$ Vitreous chamber
$(f)$ Blind spot.

The optic nerve is the:

Choose the correct statement.

The area of the retina that is most sensitive to light is the: