Biology preliminary examations today ! ?

Well, both are true but the second answer is a more detailed account of what might happen. The cornea is part of the eye and sand would have to first enter the eye to scratch the cornea badly enough to actually enter it. The irritation would cause tears to be produced to try and flush away the sand. The cornea is living tissue, but it contains no blood vessels. It gets oxygen directly from the atmosphere and surrounding tissue. I don't know if it has a nerve supply and can register pain. Corneal surgery appears to involve very little anesthetic so I suppose it has a minimal nerve supply. Sand embedded in it would probably not be painful and might not trigger the secretion of tears. Because it has no blood supply, there can be no direct responce to infectious agents from either the circulatory or lymph systems. The eye does secrete a powerful antibacterial compound, and this serves to protect the eye from infections.