The cornea is the transparent, dome-shaped structure at the front of the eye. It has five layers - the epithelial layer, Bowman's layer, the stromal layer, Descemet's membrane, and the endothelial layer. The stromal layer makes up around 90% of the cornea's thickness and contains collagen fibrils that are arranged in a precise lattice structure, which is responsible for the cornea's transparency. The cornea focuses light, acts as a protective barrier, and withstands changes in intraocular pressure.