Papillary Muscles

Small protrusions of the working myocardium which hold the Chordae tendineae and tighten the sail flaps shortly before the systole.

They are connected to the [flaps] of the heart by sinewy threads and keep the flaps closed when the pressure in the heart increases.

https://de.wikipedia.org/wiki/Papillarmuskel _{(Wikipedia CC-by-sa-3.0)}