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.
This is a mild and local form of douching from a jug, or can, or large sponge, emptied along the spine as the patient sits on a board placed across the front of a bath. It may be continued from two to ten minutes, and sometimes should be commenced at 80°, and gradually cooled down, until after a few baths it can be borne wholly cold. If commenced too cold, in sensitive subjects it may give rise to headache or giddiness, and if continued too long may induce a rheumatic condition of the back-muscles, and is not free from risk to the kidneys. It would seem that the spinal cord is directly stimulated by the shock of cold water, and that the stimulus is reflected to peripheral and visceral nerves; hence, according to hydropathic authorities, "a divergent effect on the action of the heart, stomach, kidneys, uterus, etc." It is probable, also, that the sympathetic ganglia are directly stimulated, and this bath is useful in functional torpor, marked by numbness or slight paralysis of limbs, constipation, and phosphaturia, etc. Certainly, a very bracing effect and pleasant glow may be produced by a moderate cold wash to the spine, followed by manual friction, and the effect may be further developed by the use of a "rubbing wet sheet" to the whole surface for a few minutes afterward.