The transitions toward the ends 21, 22, as well as to the area 25, can be rounded off.This embodiment offers particularly large passages for fluff, dust or other dirt, which are released in the direction toward the needle back 12.While the latch springs 19 of the embodiments of FIGS. 2 and 3 are each designed symmetrical in respect to their longitudinal axis, FIG. 4 illustrates an embodiment which