Apple of my eye

 

It was believed as long ago as the ninth century that the pupil of the eye was a vital spot in the human anatomy.  Primitive medical curiosity about it caused the early healers to study the pupil as closely as they could.  They concluded that it was apple shaped and so it became popularly known as "the apple of the eye."

Because the pupil was considered as vital as life itself, it became customary for a gallant hero to call the object of his affection "the apple of my eye."