A housing (212) receives a syringe and includes a return spring (226) forbiasing the syringe from an extended position in which its needle (218)extends from the housing to a retracted position in which it does not. A drivespring (230) acts on a first drive element (232) and a second drive element(234) acts upon the syringe to advance it from its retracted position to itsextended position and discharge its contents through the needle. The firstdrive element is capable of movement relative to the second once a nominalcoupling position has been reached. A release mechanism is activated when thefirst drive element (234) is further advanced to a nominal release position,to release the syringe (214) from the action of the drive spring, whereuponthe return spring restores the syringe to its retracted position.
