A medical injector is provided herein having a body with a displaceable plunger. The plunger includes a plurality of spaced-apart ratchet teeth disposed along the length thereof. At least one indexer is provided formed to engage the plunger, wherein the indexer is configured to allow the plunger to displace distally towards a distal end of the body but not proximally towards a proximal end of the body. The medical injector also includes an actuator having an engagement portion formed to engage one or more of the ratchet teeth. The actuator is displaceable to a ready state, the engagement portion being displaced proximally relative to the plunger with the actuator being displaced to the ready state. The indexer prevents proximal movement of the plunger thereby allowing the engagement portion to by-pass one or more of the ratchet teeth with the actuator being displaced to the ready state. In addition, the actuator is displaceable from the ready state to cause actuation of the medical injector. The displacement from the ready state causes distal displacement of the engagement portion with the engagement portion engaging one or more of the ratchet teeth and causing distal displacement of the plunger with the engagement portion. Advantageously, the subject invention provides a plunger for a medical injector which may be controllably advanced with a minimum number of cooperating parts.
