The term excretion is commonly used to denote a gland fluid the,.chief constituents of which are manufactured by other tissues, and are of no use in the economy, but, on the contrary, require to be continually removed in order that their accumulation in the blood may not give rise to injurious consequences. These effete matters are the outcome of the various chemical changes in the tissues, whence they are collected by the blood and carried to the glands which preside over their elimination.

The next group of cutaneous glands is commonly arranged among the excretory organs, though its more important function, as will hereafter appear, is to supply surface moisture for the purpose of regulating the temperature.