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

Match the following columns:

Column-$I$	Column-$II$
$P$. Interatrial septum	$I$. Thick fibrous tissue
$Q$. Interventricular septum	$II$. Thin wall
$R$. Atrioventricular septum	$III$. Thick wall

Assertion : Muscle fibres of $SA$ node possess the lowest rhythmicity among all cardiac muscles.
Reason : Due to this fact,it can initiate excitatory waves at the highest rate.

Ventricular contraction is initiated by which of the following?

The 'Bundle of His' is a muscular tissue found between:

What is the heart rate of a rabbit?