A medical injector (10) comprising: a body (32) having a distal end (34) and a proximal end (36)； a displaceable plunger (12) disposed in said body, said plunger having a plurality of spaced-apart ratchet teeth (20) disposed along the length thereof； at least one guide (38) formed to engage said plunger, wherein said guide is configured to allow said plunger to move distally towards said distal end of said body but not proximally towards said proximal end of said body； and, an actuator (48) having an engaging portion (50) formed to engage one or more of said ratchet teeth, wherein said actuator is movable to a ready state, said engaging portion being displaced proximally relative to said plunger with said actuator being moved to said ready state, said guide preventing proximal movement of said plunger, thereby allowing said engagement portion to surround one or more of said ratchet teeth said actuator being moved to said ready state, and , wherein, said actuator may be displaced from said ready state to produce actuation of the medical injector, said displacement from said ready state producing the distal displacement of said engaging part with said engagement part engaging one or more of said ratchet teeth and causing distal displacement of said plunger with said engaging portion； characterized in that said injector further comprises: a frame-shaped rocker (62) pivotally connected to said actuator at a first end (64) of said rocker to rotate relative to said actuator, and pivotably connected to said body at said second end (66) of said rocker to rotate relative to said body, said rocker having said coupling part (50) arranged between said first and second ends, wherein said piston is arranged inside said rocker, and said guide and said engaging portion engages the same set of ratchet teeth.Un inyector médico (10) que comprende: un cuerpo (32) que tiene un extremo distal (34) y un extremo proximal (36)； un émbolo desplazable (12) dispuesto en dicho cuerpo, teniendo
