Ptosis is a drooping or falling of the upper eyelid. The skin of the forehead, upper eyelid and orbit of the eye may be involved. Symptoms generally include a rash of the forehead with swelling of the eyelid. The eyelash grows at the edges of the eyelid and protects the eye from dirt. Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally. However, if accompanied by ptosis of the upper eyelid, this may indicate Horner's syndrome. Occasionally they can even lodge in the conjunctiva under the eyelid. The surgeon will begin with an incision on the eyelid. Demodicosis may also cause itching, swelling, and erythema of the eyelid margins. Staphylococcal blepharitis is diagnosed by examining erythema and edema of the eyelid margin. Blepharospasm is any abnormal contraction or twitch of the eyelid. Common side effects are double vision, droopy eyelid, overcorrection, and no effect. Distichiasis is the presence of extra eyelashes along the eyelid. Larger chalazia are removed through an incision in front of the eyelid. An anatomical variation in humans occurs in the creases and folds of the upper eyelid.