The action of the depressor nerve on the pupil

Stimulation of the depressor nerve in white rabbits, narcotized by the subcutaneous injection of 5-10 milligrams of morphine sulphate per kilo, usually causes a definite diminution of the pupil. This contraction in typical cases is composed of two stages: a sharp, prompt, short, initial contraction followed by a slower gradual one. Often only the initial contraction is observed, at other times only the slower gradual contraction.

The initial contraction, when present, is obtained as soon as the nerve is stimulated, before the blood pressure begins to fall. The slower contraction occurs while the bloodpressure is falling, and the iris blanches at the same time.

Stronger stimuli are necessary to cause this contraction of the pupil than suffice to bring on the characteristic drop of bloodpressure. A strong fall of bloodpressure due to a moderate depressor stimulation does not cause any alteration of the pupil.

Stimulation of one depressor may cause a contraction of the pupil on the opposite side.

This pupillary effect cannot be obtained with the same certainty as the fall in bloodpressure. After several successful trials, the pupil usually fails to respond for a while.

The two depressors vary in their pupillary effect; one may yield excellent pupillary contractions, the other one none at all.

The stimuli used were rarely longer than three to five seconds; the strength 100-150 mm. coil distance (Petzold coil).

Section of the sympathetic nerve, or extirpation of the superior cervical ganglion, the depressor of the same side being stimulated several days later, exerts no appreciable effect on the result. The reflex therefore seems to act on the third nerve chiefly, if not entirely.

In addition to this pupillary effect, depressor stimulation at times causes a short wink or a more or less prolonged retraction of the bulbus.