Cardiac muscles are ......... in nature.

The correct pathway of the impulse of heartbeat through the heart is ...........

In humans,the sinus venosus is considered to have merged with the wall of which chamber?

The ion that always keeps the cardiac muscle unit in a contracting state is

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.

Differentiate between Atrium and Ventricle.