edge of the flexible wall follows fairly precisely the shape of the lower jaw arch in addition at the front teeth the starting angle in relation to the vertical plane can be greater than elsewhere thereby ensuring that it does not press the gum excessively fig1 to 4 show details of the favourable application form of the invention the cavities 15 formed in the front part of the u shaped arch in the direction of the sagittal plane can be separately mentioned the cavities go through the walls of the arch the cavity holes help breathing particularly when the patient is sleeping the u shaped arch has been made of silicon polymer or of plasticized polyvinyl chloride it is particularly advantageous to use liquid silicon as the raw material a series can be made of several different sized devices according to the invention in the light of the measurements shown in graphics 5 a and 5 b this kind of series can contain for example 13 orthodontic activators having the dimensions specified in table 1 the relative dimensions of the smaller devices intended to be used mainly in the milk tooth phase can be somewhat different from the other devices in the series nevertheless all the devices in the series still contain contours according to the essential characteristics of the invention and relative dimensions the measurements presented in table 1 are exemplary u201c approximate u201d measurements but they do describe the essential dimensions of the device according to one favourable application form the series of devices.