The wall of the human heart is thick due to:

Purkinje fibers primarily help in the contraction of which part of the heart?

The pericardium and the pericardial fluid help in

Select the correct option:
$(1)$ The pericardium is a double-layered membrane surrounding the heart.
$(2)$ The $SA$ node has a higher rate of impulse generation compared to the $AV$ node.

Match the valves of the human heart in column $I$ with the respective opening they guard in column $II$ and select the correct option.

Column $I$	Column $II$
$i$. Eustachian valve	$a$. Opening of pulmonary aorta
$ii$. Thebesian valve	$b$. Left atrioventricular aperture
$iii$. Mitral valve	$c$. Opening of inferior vena cava
$iv$. Semilunar valve	$d$. Opening of coronary sinus

Pericardial fluid is present between . . . . . . of heart.