These waste products, together with the water containing them, are known as _urine_. The total amount of nitrogenous waste leaving the body each day is about twenty grams. It is passed through the _ureter_ to the _urinary bladder_; from this reservoir it is passed out of the body, through a tube called the _urethra_. After the blood has passed through the glomeruli of the kidneys it is purer than in any other place in the body, because, before coming there, it lost a large part of its burden of carbon dioxide in the lungs. After leaving the kidney it has lost much of its nitrogenous waste. So dependent is the body upon the excretion of its poisonous material that, in cases where the kidneys do not do their work properly, death may ensue within a few hours. [Illustration: Diagram of a section of the skin. (Highly magnified.)] Structure and Use of Sweat Glands.--If you examine the palm of your hand with a lens, you will notice the surface is thrown into little ridges. In these ridges may be found a large number of very tiny pits; these are the pores or openings of the sweat-secreting glands. From each opening a little tube penetrates deep within the epidermis; there, coiling around upon itself several times, it forms the sweat gland. Close around this coiled tube are found many capillaries. From the blood in these capillaries, cells lining the wall of the gland take water, and with it a little carbon dioxide, urea, and some salts (common salt among others). This forms the excretion known as _sweat_. The combined secretions from these glands amount normally to a little over a pint during twenty-four hours. At all times, a small amount of sweat is given off, but this is evaporated or is absorbed by the underwear; as this passes off unnoticed, it is called _insensible perspiration_. In hot weather or after hard manual labor the amount of perspiration is greatly increased.