Anatomy and Physiology: Special Senses – The EYES
TUNICS of the EYEBALL
- Fibrous Tunic – this is the outermost tunic and is also known as the sclera, the thick and white connective tissue. The fibrous tunic (sclera) is seen anteriorly as the “white part of the eye.” The central portion of this tunic is modified so that it is crystal clear. The cornea is the transparent “window” through which light enters the eye and is well supplied with nerve endings. This is the main reason why blinking and increased tearing occur when the cornea is touched because most nerve endings found here are pain fibers.
- Vascular tunic – this is the middle coat of the eyeball and has three distinguishable regions namely:
- Choroid – this is located posteriorly and lies between the sclera and the retina. It contains the blood vessels that provide nourishment to the outer layers of the retina. It is composed of layers of blood vessels that nourish the back of the eye. The choroid is opaque and deeply pigmented with melanin to absorb excessive light; else internal reflection would form multiple images on the retina. It is less vascular where the retina is thin. The choroid connects with the ciliary body toward the front of the eye and is attached to edges of the optic nerve at the back of the eye.
2. Ciliary Body – the ciliary body is made up of ciliary muscles and ciliary processes. It lies just behind the iris. This is the structure to which lens are attached by a suspensory ligament called the ciliary zonule and then the iris. The pigmented iris has a rounded opening, the pupil, through which light passes. Nourishment for the ciliary body comes from blood vessels which also supply the iris. Ciliary processes are short, black tissues arranged radially. They secrete aqueous humour.
One function of the ciliary body is the production of aqueous, the clear fluid that fills the front of the eye. It also controls accommodation by changing the shape of the crystalline lens. When the ciliary body contracts, the zonules relax. This allows the lens to thicken, increasing the eye’s ability to focus up close. When looking at a distant object, the ciliary body relaxes, causing the zonules to contract.