The smooth muscles in it (ciliary muscles) contract to ease the tension on the suspensory ligament of the lens, which consists of fibrils that extend from ciliary processes to the lens.
The lens of the eye is a flexible, transparent, colorless, avascular body of epithelial cells behind the iris. The lens is held in place by the suspensory ligament of the lens and by the ciliary processes. The shape of the lens can be adjusted so that objects at different distances can be brought into focus on the retina. This mechanism is called accommodation. The lens loses much of its elasticity with aging, making it difficult to focus efficiently without corrective eyeglasses.
The anterior extension of the choroids is a thin muscular layer called the iris (gr. Rainbow) because it is the colored part of eyeball that can be seen through the cornea. In the center of the iris is an adjustable circular aperture, the pupil. It appears black because most of the light that enters the eye is not reflected outward. The iris, acting as a diaphragm, is able to regulate the amount of light entering the eye because it contains smooth muscles that contract or dilate in an involuntary reflex in response of the light available, causing the pupil to become larger or smaller. This mechanism is called adaptation.
The innermost layer of the eyeball is the retinal layer.
The retina
It is an egg-shaped, multi-layered, light sensitive membrane, containing network of specialized nerve cells. It is connected to the brain by a circuit of over million neurons in the optic nerves. The retina has a thick layer and a thin layer. The thick layer is nervous tissue, called the neuroretina that connects with the optic nerve. Behind it is a thin layer of pigmented epithelium that prevents reflection from the back of the retina. The function of the neuroretina is to receive focused light waves and convert them into nerve impulses that can be conveyed to the brain and converted into visual perceptions. The neuroretina doesn’t extend into anterior portion of the eyeball, where the light could not be focused on it.
The neuroretina consists of highly specialized photoreceptor nerve cells, the rods and cones (cones are color-sensitive, the rods only “see” black and white). The outer segment of a rod or cone consists most of the elements necessary (including light-sensitive photo pigments) to absorb light and produce a receptor (generator) potential. The inner segments contain mitochondria, Golgi apparatuses, endoplasmatic reticulum, the nucleus, and other structures necessary for generating energy and renewing molecules in outer segment.