When a small foreign body falls in the eye and causes irritation it is usually situated either on the cornea or stuck on the inner surface of the upper eyelid. When the eyes are kept open we tend to blink at least 15 to 20 times a minute.
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Every time the eyelid moves over the cornea, both the lid and the cornea get irritated and the discomfort is quite severe. If the eye is kept closed, there is no movement of the eyelid or cornea and the irritation is much lesser.
The cornea (black part of the eye) is the most sensitive part of the external eye and is very rich in nerve fibres. When the foreign body is either on the cornea or on the eyelid, the corneal nerve fibres are irritated and the resultant discomfort is very severe. On the other hand, if the foreign body is lodged in the conjunctiva (white part of the eye) the irritation is much less severe because compared with the cornea, the conjunctiva is less sensitive. Rubbing the eye vigorously when a foreign body is present in the eye can cause damage to the delicate epithelium of the cornea.
A very small foreign body lodged in the cornea can, under rare conditions, be covered by the epithelium of the cornea. In such cases, there will be no irritation at all. For this to occur, the foreign body should be very small, it should be inert, and should be smooth. Under these conditions, the corneal epithelium can grow over it.