A tiny flap of skin that covers and protects the eye is known as an eyelid. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outer world and so allowing vision. This might happen deliberately or involuntarily. Involuntary blinking is important for maintaining good vision because it removes dust and debris from between the lens and the retina. The three main types of eyelids are: naked, hairy, and browed. Naked eyelids are only covered by skin and no hair or bone is present below the eye. Hairy eyelids have small hairs that cover the face except for around the eyes where they are worn away by tears and sweat. Browed eyelids have a bone called the zygomatic arch under the skin near the eye that keeps the eye socket open. This allows more space for the brain and helps sound travel out of the ear. Blinking is controlled by a nerve called the oculomotor nerve. It sends signals to different parts of the brain when there is need for more blood flow to the eye to help it process more information. If the signal reaches the part of the brain that controls facial muscles, the eyelid will close automatically. If however, the signal goes to another part of the brain, then this tells the body not to blink.