Up to the end of the last century, the idea was widely held among ophthalmologists that the metabolism of the lens is insignificantly small. Interest in the physiology of the lens was therefore directed mainly to a consideration of its physical changes in accommodation. More recently, however, the chemical studies of Jess1and others have indicated that the lens has a definite protein metabolism, while experiments on the reduction of methylene blue2and on the consumption of dextrose and oxygen as measured by the Warburg apparatus3have shown that the lens possesses a small but definite carbohydrate metabolism. Since the lens is not supplied by blood vessels and is surrounded by a continuous membrane, the lens capsule, the degree of permeability of this membrane of necessity places certain limitations on the character and amount of the substances that can enter the lens as nutriments