Together with cutaneous excretion should be mentioned the continuous and extensive loss all over the surface of the body, from the casting off of the superficial layers, of the dried horny cells of which the outer part of the skin is composed.
The way in which the cells of the mammary gland produce their important secretion is by their protoplasm adopting a peculiar method of fat manufacture, while all the strength of its nutritive powers is devoted to the elaboration of the constituents of milk. In a similar way the cells of the epidermis devote their nutritive activity to the production of a certain material - keratin - which cannot be called a secretion in the ordinary acceptation of the term, but which is certainly elaborated as the result of the nutritive changes going on in the protoplasm of the cell during its life history, just as many other substances are produced as the result of the nutritive activity of gland cells.
The work of the epidermal cells supplies - not a peculiar chemical reagent, as do some of the gland cells of the digestive tract, nor yet a nutrient fluid, like milk - but an insoluble, impervious, tough coating, for the exterior of the body, which, though thin and elastic, is very strong and resisting.
The nearest analogy among the secretions to the keratin in the epidermal cells is the production of mucin in the cells of the epithelial lining of the mucous membranes. Both substances may be looked upon as excretions, as they do not reenter the system, being cast off, but each of them perform a definite function, and is produced by special protoplasmic elements, like the secretions more generally recognized as such.
The amount of nitrogenous substances thus excreted cannot well be reckoned, but having regard to the great extent of surface from which they are derived, it must be considerable.