The kidneys, whose office it is to secrete urine, are located on either side of the lumbar vertebrae, below the last false-rib. The urine is conveyed from each kidney by a small tube, called ureter, to the bladder. The bladder is the reservoir for the urine, and is located behind the pubis, and above and before the lower part of the rectum. The passage by which the urine passes from the bladder, is called urethra.

We are now very briefly to consider that part of the system by which oxygen is introduced into the body and the elements of life conveyed to every part of it.