Drawing the eye in cross-section
Draw a roughly circular shape. At the front, draw a curved transparent layer for the cornea, which refracts (bends) light. Behind it, draw the iris (the coloured part) with a gap in the centre for the pupil. Behind the iris, draw the lens: a biconvex (oval) shape held in place by suspensory ligaments attached to the ciliary muscles (a ring of muscle around the lens).
The space between the cornea and lens contains aqueous humour. The large space behind the lens contains vitreous humour. Line the back of the eye with the retina (containing light-sensitive cells). At the back, draw the optic nerve leaving the eye: the point where it connects is the blind spot. Mark the fovea (area of sharpest vision, packed with cone cells) slightly to one side.
Focusing: accommodation
To focus on a near object: ciliary muscles contract, suspensory ligaments become slack, the lens becomes thicker and more curved, refracting light more. To focus on a distant object: ciliary muscles relax, suspensory ligaments become taut, they pull the lens into a thinner, less curved shape, refracting light less.
Common mistake: students often state the ligaments pull the lens thicker: it is the opposite. When ligaments are taut, the lens is pulled thin.
The pupil reflex
In bright light, the circular muscles of the iris contract and the radial muscles relax. The pupil becomes smaller, reducing the amount of light entering and protecting the retina. In dim light, the radial muscles contract and the circular muscles relax. The pupil becomes larger, allowing more light in.