If exogenous agents affect the cornea, it is easy to understand the localization of the changes produced, since they, as a rule, occupy the place where the obnoxious agent acted on the cornea. On the other hand, in the cases of endogenous diseases, localization often remains obscure. Why is it that in interstitial keratitis only the posterior layers of the cornea, and in keratitis nodosa the anterior ones, are affected, and in keratitis nodosa the periphery of the cornea is spared, while in other forms of dystrophies, as for instance, arcus senilis or marginal atrophy, it is exclusively involved? Therefore, one may distinguish localization according to the depth at which the changes occur (in the superficial or deep layers) from localization according to surface extension (central or peripheric)—topical localization. Different from this is the localization according to the affected histologic elements ; for instance, the stroma, the corneal corpuscles or