abdominal cavity, in the region of the loins, one on each side of the
spine. The convex side of each kidney is directed outwards, and the
concave side is turned inwards towards the spine. From the middle of the
concave side, which is termed the _hilus_, a long tube of small caliber,
called the _ureter_, proceeds to the bladder. The latter organ is an
oval bag, situated in the pelvic cavity. It is composed principally of
elastic muscular fibers, and is lined internally with mucous membrane,
and coated externally with a layer of the _peritoneum_, the serous
membrane which lines the abdominal and pelvic cavities. The ureters
enter the bladder through its posterior and lower wall, at some little
distance from each other. The openings through which the ureters enter
the bladder are oblique, hence it is much easier for the secretion of
the kidneys to pass from the ureters into the bladder than for it to get
the other way. Leading from the bladder to the exterior of the body is a
tube, called the _urethra_, through which the urine is voided.

The excretion of the kidneys, termed the _urine_, is an amber-colored or
straw-colored fluid, naturally having a slightly acid reaction, and a
specific gravity ranging from 1,015 to 1,025. Its principal constituents
are _urea_ and _uric acid_, together with various other animal matters
of less importance, and saline substances, held in solution in a
proportionately large amount of water. The composition of the urine and
the quantity excreted vary considerably, being influenced by the
moisture and temperature of the atmosphere, by the character of the food
consumed, and by the empty or replete condition of the alimentary tract.
On an average a healthy man secretes about fifty ounces of urine in the
twenty-four hours. This quantity usually holds in solution about one
ounce of urea, and ten or twelve grains of uric acid. In the amount of
other animal matters, and saline substances, there is great variation,
the quantity of these ranging from a quarter of an ounce to an ounce.