Although the eye's pupil is indeed a hole in the front surface of the eye, this hole is covered in the front by a strong, transparent coating called the cornea and in the back by a fibrous, transparent object called the lens. Also, the space between the cornea and the lens is filled with a gel-like fluid called the aqueous humor. All of these layers form a watertight barrier that seals off the inside of the eyeball from the outside world. Fortunately, all of these layers are transparent, so light can still enter our eyes through the pupils, thereby allowing us to see. To ensure that they are transparent, the cornea and lens do not contain any blood vessels. Therefore, nutrients must flow to these tissues through simple diffusion from the aqueous humor.