Abstract: A liquid-injection instrument without needle comprises an elongated body in two separable parts placed end to end. The first, so-called medical part, comprises at least one cavity receiving a liquid-tight cell which contains the injection liquid and has a movable base which, when displaced, forces liquid from the cell towards at least one variable-volume dosing chamber in the medical part. This communicates with a nozzle provided with an injector capable of forming a narrow jet of liquid following forcible expulsion of liquid from the dosing chamber. The second, so-called motor part, comprises a pneumatically controlled mobile assembly which effects alternating movements. The mobile assembly comprises a working piston (52-233-420) and a control and propulsion spring (62-241-444) which acts on the piston when gradually compressed by the movement of a plunger (52-276-445) caused by the pressure of the compressed air in a gas-tight chamber (51-287) in the body.