1. An infusion monitor comprising a cartridge, sensing means operatively associated with said cartridge for monitoring the flow of liquid in a transparent infusion tube assembly, said sensing means comprising a sensing beam for sensing a liquid being infused through said transparent infusion tube assembly, means for directing said sensing beam from said cartridge, a movable clamp assembly pivotly mounted on said cartridge, reflecting means on said clamp assembly for re-directing said sensing beam back toward said cartridge, said cartridge having openings therein for said sensing beam, said reflecting means being opposite said openings, an emitter and a receiver for said sensing beam within said cartridge adjacent said openings, said clamp assembly being movable relative to said openings to reflect said beam and to permit the cartridge to be mounted on said infusion tube assembly and support said cartridge on said infusion tube assembly.