This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
These operate mainly by distension. They are applicable when there is special reason to avoid irritating the rectum; and should be used habitually in preference to more active substances, when they are found to answer the purpose. Flaxseed tea, barley-water, or infusion of slippery elm, may be used. When great bulk is wanted, warm water will answer as well. It is not always that these injections will operate. Indeed, in the ordinary quantity of a pint, they are very apt to be retained, and undergo absorption; in which case there is usually an increased flow of urine. To produce much effect, they must be given freely, so as to distend the bowels, and in some instances reach even the small intestines. Two evils are to be guarded against in their use; one, injury to the bowel by physical over-distension, the other, paralysis of the muscular coat from the same cause. The temperature should be slightly above that of the body, so that it may act as a stimulant. The liquid is best introduced by one of the self-injecting syringes; and it should be prevented from escaping, by a towel twisted around the pipe, and held firmly against the fundament. The cases to which these injections are most applicable, are those in which there is feculent accumulation to remove, spasmodic contraction to relax, and mechanical obstructions to be overcome. Cold water may sometimes be successfully used in obstinate spasms of the bowels, which have resisted other measures. It seems, by the sudden shock on the nervous centres, to surprise the spasm into relaxation. But this requires caution; for, in feeble persons, whose systems might not have the energy to react, dangerous prostration might be induced.