This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
Cocaine is an alkaloid obtained from the leaves of Erythroxy-lon coca, or of Erythroxylon truxillense (Fam. Erythroxylacea). The coca shrub is extensively cultivated at an elevation of 3500 to 6000 feet in the mountains of Peru, Bolivia, and Ecuador, and to some extent also in Mexico and the East and West Indies. It has been estimated that 100,000,000 pounds of the leaves are used annually in South America. They yield cocaine and several other alkaloids, all compounds of ecgonine. Cocaine is the methyl-benzoic acid compound; cinnamyl-cocaine is the cin-namic acid compound, and truxilline is the truxillic acid compound.
Coca itself is used to some extent in the form of a wine, but the only pharmacopoeial representatives are cocaine, soluble in oil, and cocaine hydrochloride, soluble in 0.4 part of water and 3.2 of alcohol, but insoluble in oil. This alkaloidal salt is decomposed at a temperature of about 98° C, so its aqueous solution cannot be sterilized by boiling. Its solutions are not antiseptic, and frequently show a growth of mold. This mold development may be retarded by the addition of boric acid. The following formulae show the close relation between atropine, cocaine, and tropacocaine.
Cocaine is a general protoplasmic poison, capable of irritating and destroying cells, or of stopping the motions of leukocytes, amebae, and ciliated cells. Solutions above 5 per cent, in strength injected hypodermatically may result in death of tissue, which shows either as a necrotic area of the skin or as a sterile abscess; the application to the eye may, for the same reason, result in cloudiness or ulceration of the cornea. This effect is not usually seen, but it occurs often enough to be of importance.
From application to mucous membranes or injection beneath the skin there promptly follows complete abolition of pain, from depression of the ends of the sensory nerves or their ad- jacent nerve-fibrils. In addition, there is local constriction of the arterioles from stimulation of both muscle and vasoconstrictor nerve-endings. The constriction of the vessels is not so great as that from epinephrine. The anesthesia and constriction come on in one to four minutes and last from fifteen minutes to one hour.
The drug cannot penetrate the unbroken skin. The author kept a finger for fifteen minutes in a 20 per cent. aqueous solution of cocaine hydrochloride, and it showed neither anesthesia nor blanching, though one drop of the liquid on the tongue was quickly followed by loss of sensation. But cocaine is readily absorbed through mucous membranes or the moist parts of the vulva. After the injection or application there may be a momentary irritation, but very quickly there is complete loss of the sense of pain, with shrinkage and paling of the part from comparative bloodlessness. Any mucous membrane to which the drug can be directly applied becomes shrunken and anesthetic in this way, e. g., membranes of the nose, throat, mouth, esophagus, stomach, rectum, vagina, urethra, bladder, and conjunctiva. In the hypodermatic use the drug is injected just beneath the epidermis, and its action is prolonged and intensified by the addition of epinephrine. This further constricts the vessels and prevents the too ready removal of the cocaine by the circulation. For the same reason it tends to make the skin incision bloodless. In a finger or toe prolonging of the local action may result from the application of a tourniquet or band to impede the venous return flow.
In the anesthesia, though the sense of pain is promptly lost, the sense of touch is not so readily abolished, and the temperature is scarcely affected, if at all; hence the touch of an instrument or the heat of a cautery may be felt, though pain is absent. The drug at first tastes bitter, but the taste for bitter soon becomes completely abolished, while that for sweet and sour merely becomes dulled, and taste for salt is not affected. If applied in the nose, the sense of smell is abolished.
It has been found that anesthesia is produced if the drug is applied to any part of the nerve, from the nerve-ending to the posterior root; so anesthesia in therapeutics may be obtained -
(a) by the application of the solution to a mucous membrane;
(b) by its injection beneath the mucous membrane or skin; (c) by its injection into the nerve; or (d) by its injection into the spinal canal, so that it may reach the posterior roots. This last method is known as "spinal analgesia" or "spinal anesthesia." Cocaine has not the marked selective action of atropine, but from 10 c.c. of 1 to 3 per cent. solution Ritter (1909) obtained in dogs a general anesthesia lasting from fifteen to thirty minutes. The dogs were fully awake, but quiet and indifferent and insensitive to pain. Meltzer, Kast, and Meyer obtained similar effects in animals.
The drug affects sensory nerves very readily, but not so readily the motor nerves. If both sciatics of a frog be exposed high up in the thigh, and a little cocaine injected into the substance of one of them, an electric stimulus to the nerve on the uncocainized side (or above the cocainized area on the other side) produces the usual reflex results, notably contraction of the splanchnic arteries. But no such results follow the electric stimulation of the cocainized nerve below the area of injection. Evidently, then, the afferent impulses on the cocainized side are blocked and do not pass the cocaine.
But the electric stimulation of the sciatic above the cocainized area produces the usual muscular contraction in the leg below, so that motor impulses are not blocked by the cocaine. There is, perhaps, a slight hindrance to the passage of motor impulses, as mentioned by Crile.