In the cheese world, the word eyes refers to the holes in cheese, a characteristic that makes Swiss cheeses Swiss. Eyes are formed by carbon dioxide gas (CO), which is produced by natural and harmless bacteria that's added during the initial stage of cheesemaking. Eyes (also simply called holes) can range in size from that of a pinhead to as large as a walnut.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.