The kidney is not the only organ by which wastes are removed; the liver has a part in this as well. The action of the liver is not, however, simply to assist the kidney; there are certain special substances which are taken care of by the liver and which are never gotten rid of through the kidneys. The first of these are the broken-down blood pigments. We have already seen that after the red corpuscles become worn out they are decomposed in the spleen and the hemoglobin carried to the liver where the iron and other usable parts are saved and the useless remainder is passed on into the bile to be conveyed out of the body. Bile is green in vegetable-eating animals and golden brown in flesh-eating; the difference depends on the so-called bile pigments which are the waste remains of hemoglobin. The difference in color is the result of a slight chemical difference in the pigments which in turn depends on the diet, so that in man the bile is either green or golden brown according to whether the diet is vegetarian or chiefly of flesh. The liver also takes out from the blood a waste substance which has the formidable name of _cholesterin_. The interesting thing about this substance is that unless the bile is altogether normal chemically it will not be dissolved and so will be thrown down in the form of little grains or lumps. So long as these grains are small they can pass down the bile duct to the small intestine and be carried out of the body, but after they become bunched together into lumps it is difficult or even impossible for them to be carried down the narrow bile duct. These lumps make up what are commonly called gallstones. The acute pain known as gallstone colic is due to the stretching of the bile duct by the passage of one or more of these stones down it. At the present time no way is known to prevent the formation of gallstones in persons in whom it starts, nor of softening them after they have once been formed; the only relief is through surgery, which, fortunately, is entirely adequate, so that thousands of persons are living in great comfort to-day who would be dead or living in acute suffering if the surgeon had not been able to remove the accumulation of gallstones from the passages in their livers.