Retractable Injection Port

A cannula, for delivering a bolus of a pharmaceutically active agent to the posterior chamber of the eye, having an orifice disposed at the distal tip. The cannula is slidably disposed within a hypodermic needle. The sliding action of the cannula within the needle provides a means for trimming a formed bolus of a pharmaceutically active agent from the orifice of the cannula. In addition, the double wall formed by the cannula within the needle provides insulation from the cooler temperatures within the eye to allow for the delivery of phase transition substances.