As all poisons formed in, or entering, the body are brought to the liver for destruction, it is in an extremely exposed position, and very liable to break down under the attack of these poisons, whether of infectious diseases, or chloroform, or alcohol, or those formed by putrefaction in the stomach and intestines. This is why those who have lived long in the tropics and suffered from malaria, dysentery, and other infectious diseases, and those who drink too much alcohol, or have chronic indigestion, or dyspepsia, are likely to have swollen and inflamed livers. The Gall Bladder. The liver has on its under side a little pear-shaped pouch called the _gall bladder_, in which the bile is stored before it is poured into the bowel. If this becomes inflamed by disease germs, or their poisons, in the blood, little hard masses will form inside it, usually about the size of a grain of corn, known as _gall stones_. So long as they stay in the gall bladder, they give little trouble, but if they start to pass out through the narrow bile duct into the intestine, they cause severe attacks of pain, known as "gall-stone colic," and, by blocking up the duct, may dam up the flow of the bile, force it back into the blood again, and stain all our tissues, including our skin and our eyes, yellow; and then we say we are _jaundiced_. Jaundice may also be caused by colds or other mild infections which attack the liver and bile ducts and clog the proper flow of the bile. The Kidneys. The kidneys are another form of blood-filter, which deal chiefly with waste stuffs in the blood left from the proteins, or Meats, of our food--meat, fish, milk, cheese, bread, peas, beans, etc. These waste-stuffs, called _urea_ and _urates_, are formed in the liver and brought in the blood to the kidneys. These lie on either side of the backbone, opposite the small of the back, their lower ends being level with the highest point of the hip-bones, nearly six inches higher than they are usually supposed to be. When you think you have a "pain across the kidneys," it is usually a pain in the muscles of the back much lower down, and has nothing to do with the kidneys at all. A very large artery carries the blood from the aorta to each side of the kidney, and a large vein carries the purified blood back to the vena cava and heart. Two smaller tubes about the size of a crow quill, the waste pipes of the kidneys (the _ureters_), carry the water containing urea and other waste substances strained out by the kidneys and called urine, down into a large pouch, the _bladder,_ to be stored there until it can be got rid of. [Illustration: THE URINARY SYSTEM _K_, kidneys; _U_, ureters; _B_, bladder; _A_, artery; _V_, vein.]