Sclera.

“What’s the white bit in an eye called?”,
She asks me half asleep.
“It’s called the sclera, darling.
It’s job is to keep,
The rest of the eye protected,
With its tougher outer layer.
It helps the eye keep structure.
It’s job is to care.” 
“What's the blue bit called? She ponders”
“The iris.  The iris controls the light.”
“I’m the iris mum. You’re the white bit.”
“Oh my love, you might be right.”