Which structure is known as the 'heart of the heart'?

The remnant of the embryonic aperture on the inter-atrial septum is called . . . . . .

Given below are two statements.
Statement $I$: Visceral layer of serous pericardium forms the outer wall of human heart.
Statement $II$: Epicardium is formed of a thick layer of cardiac muscles.
In the light of the above two statements,choose the correct answer from the options given below.

Mixing up of arterial and venous blood does not take place in a heart having

Blood to the heart is brought by the venous system by:

In humans,an artificial pacemaker is implanted due to defects in: