*Quantity of Excretory Products.*—If the weight of the normal body be taken at intervals, after growth has been attained, there will be found to be practically no gain or loss from time to time. This shows that materials are leaving the body as fast as they enter and that the tissues are being torn down as fast as they are built up. It also shows that substances do not remain in the body _permanently_, but only so long perhaps as is necessary for them to give up their energy, or serve some additional purpose in the ever changing protoplasm. The excretory organs then remove from the body a quantity of material that is equal in weight to the materials absorbed by the organs of digestion and respiration. This is estimated for the average individual to be about five pounds daily. The passage of waste from the body is summarized in Table III. TABLE III. THE PASSAGE OF WASTE MATERIALS FROM THE BODY Materials State How Formed Condition in How Removed in the Body the Blood from the Blood Carbon Gas By the Dissolved in Separated dioxide oxidation of the plasma from the the carbon and in loose blood at the of proteids, combination alveoli of carbohydrates, with salts the lungs and fats. in the and then blood. forced through the air passages into the atmosphere. Urea Solid By the Dissolved in Removed by oxidation in the plasma. the the liver of uriniferous nitrogenous tubules of compounds. the kidneys and to a small extent by the perspiratory glands. Water Liquid By the As water. Removed by oxidation of all the the hydrogen organs of of proteids, excretion, carbohydrates, but in the and fats. largest Amount formed quantities