eonardo da Vinci, Sagittal and Horizontal Sections of the Human Head with Layers of the Head compared with an Onion. K/P 32r (ca. 1490). On this page Leonardo outlined in a sagittal and transverse plane the 'cell doctrine' concept. The brain cavity is depicted as an empty space with three balloon-like expansions connected to the eye by dura-like coverings connecting with the posterior component of the eye. Using the onion analogy Leonardo comments on the layers of the scalp, eye and brain coverings. The eye is shown in sagittal section with the traditional spherical lens and the frontal sinus for the first time.