👉 The term "vitreum" is a medical term used to describe the inner lining of the eye, which contains the lens and other structures that help the eye focus light. The vitreum is made up of three layers of cells: the outer layer, called the choroid; the middle layer, called the cornea, which is a clear, watery tissue; and the inner layer, called the retina, which is responsible for seeing in low light conditions.