Myocardium

A syncytium of branched and reticularly connected striated muscle cells forms the muscle tissue of the heart. It is a layer of predominantly longitudinally oriented muscle fibers and a spiral muscle fiber layer, whereby a concentric contraction of the hollow heart muscle is ensured. The wall thickness of the myocardium depends on the load, therefore it is three times stronger in the left ventricle than in the right. Anatomically and functionally, the working myocardium is distinguished from the excitation formation and conduction system.

https://en.wikipedia.org/wiki/Cardiac_muscle _([1])