Orbit and Cavernous Sinus - LO 5
5. What are the three parts of the eyeball? Describe the supporting apparatus of the eyeball and its various components.
The fibrous layer of the eye is important in providing structure and strength to the eyeball. This layer consists of the sclera and cornea. The sclera covers the majority of the eyeball, is often referred to as the “white of the eye.” It is the insertion site for extra-ocular and intrinsic eye muscles. The cornea covers the anterior portion of the eye (approximately ⅙th of the anterior eye - Moore et al., 7th edition), is completely avascular, sensitive to touch, and is the main refractive index of the eye.
The vascular layer of the eye (uvea) is composed of 3 structures: choroid, ciliary body, and iris. The iris is a diaphragm located on the anterior surface of the lens. The pupil, an aperture important in transmitting light, is located centrally in the iris. The iris is the portion of the uvea with color, while the pupil is black. The pupil is controlled by two muscles: sphincter pupillae (contracts pupil, parasympathetically-controlled) and dilator pupillae (dilates pupil, sympathetically-controlled).
The inner (nervous) layer of the eye is composed of the neural retina. The retina is composed of 10 layers that contain specialized receptors (rods and cones) as well as interneurons, glia, and ganglion cells. The axons of the ganglion cells exit the retina as the optic n. (CN II). The main source of blood supply to the retina is the central retinal a., which travels within the optic n. to the retina, and is a branch of the ophthalmic a.