The transparent lens of the eye is held in position by a large number of strong but slender fibers, called suspensory ligaments, that extend inward from the ciliary networks of the eye. The body of the lens lies directly behind the iris and is composed of fibers that come from epithelial (hormone-producing) cells. In fact, the cytoplasm of these cells makes up the transparent substance of the lens. The lens capsule is a clear, membrane-like structure, that is quite elastic–a quality that keeps it under constant tension. As a result, the lens can assume a globular shape. However, the suspensory ligaments attached to the edges of the capsule are also under tension, and as they pull outward, the capsule and the lens inside are kept somewhat flattened. If the tension is relaxed, the elastic capsule rebounds, and the lens surface becomes more convex (rounded). Such changes occur in the lens when the eye is focused to view a close object. This adjustment is called accommodation, and is a function of the ciliary muscles.