This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
The Local Disease is Nature's Effort to Relieve by Derivation. "When the system is affected with some chronic disease which threatens to destroy vital organs and life itself, and which does not yield to the spontaneous efforts of the vital force, this endeavors to quiet the inner disease, and to avert the danger by substituting and maintaining a local disease on some external part of the body, whither the internal disease is transferred by derivation. In this way, the local affection for a time arrests the internal evil, without, however, being able to cure it, or to lessen it essentially. The fontanels of the old school have a similar effect, in the form of artificial ulcers upon external parts; they soothe internal chronic complaints, but without curing them".
Nevertheless, the local malady is never anything more than a part of the general disease, but it is a part which has become excessively developed in one direction by the vital force, and transported to the surface of the body where there is less danger, in order to lessen the internal morbid process. § 201, Organon.
The mental state and temperament of the patient are often of most decisive importance in the homoeopathic selection of a remedy, and should never escape the accurate observation of the physician, as the state of mind is always modified in so-called physical diseases.