This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Cocaine, when applied locally to the eye, has several actions. It produces local anaesthesia, dilatation of the pupil, and relaxation with more or less complete paralysis of the ciliary muscle. When two to three drops of a 4-per cent. solution are applied to the eye at intervals of five minutes, such complete local anaesthesia of the cornea, conjunctiva, and iris is produced in twenty to thirty minutes as to allow operations to be performed on the eye. At the same time the cocaine causes constriction of the superficial vessels, producing blanching of the conjunctiva. The dilatation of the pupil is great, is quickly attained, and differs from that produced by atropine in the fact that the cocainised pupil reacts to light and accommodation. The mydriasis is probably due to stimulation of the ends of the sympathetic in the iris, for cocaine will not produce any mydriatic effect after the cervical sympathetic has been cut for such a length of time as to allow degeneration of the peripheral ends to occur, nor has stimulation of the cervical sympathetic any effect in increasing the ad maximum cocaine mydriasis. That the third nerve is not paralysed is shown by the fact that stimulation of it produces contraction in the cocainised pupil. A similar effect follows local stimulation of the sphincter pupillae. That the action of cocaine is exerted on the peripheral ends and not on the centres of the sympathetic is shown by the fact that section of the cervical sympathetic does not alter the pupil which is fully dilated by cocaine, and cocaine induces mydriasis in an exsected eye.2