This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
To whatever part opium may be applied, if vital, it is capable of producing its general effects. Injected into the rectum, or sprinkled upon the skin deprived of the epidermis, it operates with almost as much certainty as when swallowed. In the lower animals, when introduced into the cellular tissue, it acts with great energy; in the peritoneal cavity, produces convulsions and death; and in the cavity of the heart, weakens or suspends its action. Through the skin, protected by the epidermis, it acts very slowly and feebly upon the system.
Upon the part itself with which it is brought into contact, it produces a series of effects analogous to those upon the system generally. The first effect is slightly to stimulate, the second to diminish sensibility and the power of action. Thus, when in contact with the conjunctiva, the mucous membrane of the nose and urethra, the blistered skin, or the surface of wounds or ulcers, it first occasions heat, pain, and some degree of inflammation; and it has already been stated that, when long chewed. it will sometimes blister the mouth; but, alter a time, the pain ceases, and the several surfaces become less sensible to ordinary impressions upon them. Even through the cuticle it is capable of producing some anodyne influence.