This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
A valuable influence may be exerted not only on the pelvic organs, but indirectly on the whole system, by sitting in water of varying depth and temperature (60° to 80° F.) and for a time varying from 5 to 30 minutes. "The immense network of nerves which supplies the whole of the organs contained within the hips and the lower belly, and the lower third of the spinal cord, terminating in great leashes of nerves, are the sensitive points upon which the sitz bath operates" (Gully). It certainly "draws blood from the brain," and is one of the best means of insuring sleep. It relieves fatigue, improves appetite and digestion, lessens headache and giddiness, regulates the bowels, and the uterine flow, and the action of the kidneys, often augmenting them when deficient, or lessening if excessive.
To obtain these good effects it is most important that thorough reaction be secured by friction or exercise, or warm clothing, otherwise shivering and depression come on. It may be advisable to commence with tepid water, or a more powerful stimulus may be given by a "running sitz," fresh cold water constantly entering the bath and circulating round the body. Or again, a local douche may be made to act upon the lumbar region, the perineum, the uterus, etc., whilst the ordinary sitz bath is in progress. The general effect of the cold sitz bath is sedative. It slows the pulse to the extent of 10 or 15 beats during the first five minutes - of 20beats in the course of half an hour (H. Johnson). It also lowers the respiration, though very slightly, and it lessens the body temperature. By its rubefacient effect, the sitz bath acts also as a derivative or counter-irritant.