Action of cocaine on pupil compared with action on other structures containing smooth muscle.

Abstract

The mydriatic effect of cocaine is regarded by some to be due to a stimulation of the peripheral sympathetic nervous mechanism supplying the radial muscle of the iris. By others it is regarded as the result of a direct weakening of the circular muscle of the iris.

The first group of experiments here reported were performed in an attempt to demonstrate by actual measurements such a weakening of the circular muscle by cocaine. Accurate pupillary measurements were obtained by measuring the image of the pupil on a scale in the eye-piece of a measuring telescope. Series of measurements made in this way were employed for the plotting of curves showing the development of drug effects.

Albino rabbits were the animals employed. In the study of cocaine, two drops of a four per cent solution were placed in one eye only. After five to ten minutes there was placed in each eye a definite amount of solution of physostigmine sulphate. Frequent measurements were made of both pupils, and curves were plotted to show the effect exerted by the cocaine on either the degree of the physostigmine effect or on the rate of its development.

Thirty-six experiments were carried out, some on animals from which the superior cervical sympathetic ganglion on one side had been removed, and some on animals on which this operation had been performed on both sides. A few experiments were on unoperated animals. This work gave no evidence of a weakening of the circular muscle of the iris by cocaine.

The second group of experiments is a study of the effect of cocaine on the isollated sphincter nluscle of the iris, mounted in Locke-Ringer solution for the recording of its muscular activity as a kymographic tracing. Steers and large dogs were used.