Bile as secreted by the liver is a clear, limpid fluid of low specific gravity; but during its stay in the gall-bladder it is concentrated by absorption of water, and mucin is added to it. It contains “bile-salts” of complex constitution. These salts favour the solution of certain by-products of cell-metabolism, cholesterin and lecithin; substances which are formed in many cells, both in animals and plants. Cholesterin occurs most abundantly in nerve-tissue and in blood-corpuscles. Lecithin also is a by-product of the metabolism of nerve-tissue. Protoplasm appears to be incapable of oxidizing these substances, as it does other products of metabolism. Other substances of equally complex constitution are reduced to urea if they contain nitrogen; to water and carbonic acid if nitrogen be absent. Cholesterin and lecithin have to be eliminated without further change. Some of the cholesterin is excreted by the sebaceous glands of the skin. It is the chief constituent of “lanoline” prepared from sheep’s wool; an unguent which owes its valuable properties to the resistance which cholesterin offers to cell action, and therefore to the action of living ferments. Bacteria cannot turn it rancid. The sebaceous glands have the power of directing metabolism into a channel in which cholesterin is the chief product, but apparently all cells make it in small quantity. The bile-salts carry cholesterin and lecithin into the alimentary canal, from which they are not reabsorbed. Some of the bile-salts are lost to the body, but the remainder re-enter the circulation, and recommence their work as vehicles for these inoxidizable and insoluble substances. In the gall-bladder cholesterin is apt to separate out from the bile in the form of gall-stones; but whether this is due to an excess of cholesterin in the bile, or to an abnormal, inflammatory condition of the lining membrane of the gall-bladder, is still an open question. Bile also contains bile-pigments. Their colour varies in different animals, and changes according as the bile is exposed to the air, or subject to the action of reducing agents. If oxidized, the colour is green (biliverdin); if reduced, brownish-yellow (bilirubin). Bile-pigment is formed from hæmoglobin, the colouring matter of the blood, after the removal of its iron. Worn-out red blood-corpuscles are destroyed in the spleen, in the manner already described, but it is uncertain whether the conversion of the hæmoglobin thus set free into bilirubin occurs in the spleen, or whether this chemical change is reserved for the liver. Physiologists incline to the view that the liver is the seat of the change.