Three principal layers form the wall of the eye: The fibrous tunic (sclera & tunic), the vascular tunic (iris, cilliary body, and choroid), and the retina (pigmented layer & Neural layer). Sclera- the white outermost tunic for protection, maintenance, and serves as attachment site for extrinsic eye muscles. The anterior part of this coat is the transparent cornea, the convex shape of the cornea refracts (bends) light rays coming into the eye. The choroid, cells of the choroid are filled with pigment from the numerous melanocytes in this region. The melanin pigment is needed to absorb extraneous light that enters the eye, thus allowing the retina to absorb the remaining light rays and form a visual image. The ciliary body is composed of both ciliary muscles and ciliary processes. Ciliary muscles are bands of smooth muscle. Extending from the ciliary muscle to the capsule surrounding the lens are suspensory ligaments , which anchor the lens. Relaxation and contraction of the ciliary muscles change the tension on the suspensory ligaments, thereby altering the shape of the lens. The ciliary processes contain capillaries that secrete a fluid called aqueous humor. Iris- two layers of smooth muscle (sphincter pupillae and dilator pupillae) with a central opening called the pupil. Controls pupil diameter and thus the amount of light entering the eye. The retina is composed of an outer pigmented layer & an inner neural layer. The pigmented layer is composed of epithelial cells and supporting cells; absorbs extraneous light & provides vitamin A for photoreceptor cells. The neural layer is cpmposed of photoreceptor cells (rods, function in dim light situations and produce colorless vision & cones, function in bright light situations and produce color vision), bipolar cells, and ganglion cells; detect incoming light rays, then are converted to nerve signals and transmitted to the brain. Components of the retina: the optic disc contains no photoreceptors. This is because the ganglion axons toward the brain. It is commonly called the blind spot (no image forms there). Within the macula lutea is a depressed pit called the fovea centralis, which contains the highest proportion of cones and almost no rods. This pit is the area of sharpest vision. The Lens focuses incoming light onto the retina, and its shape determines the degree of light refraction. The internal space of the eye is subdivided by the lens into two separate cavities: the anterior cavity and the posterior cavity: the posterior cavity is occupied by vitreous humor , which helps maintain eye shape, supports the retina to keep if flush against the back of the eye, and transmits light from the lens to the retina. The iris subdivides the anterior cavity further into two chambers: the anterior chamber & the posterior chamber. The anterior cavity contains a fluid called aqueous humor ; it is a filtrate of blood plasma that resembles cerebrospinal fluid. The aqueous humor is continually resorbed into a vascular space, called the sclera venous sinus , located in the limbus between the cornea and the sclera. The scleral venous sinus drains into nearby veins. The circulation of aqueous humor provides nutrients and oxygen to both the lens and cornea and helps maintain the chemical environment within the anterior cavity in the eye. Lacrimal gland produces tears that lubricate the eye. Ora serrata separates retina from ciliary body.
