What is the normal heart rate in a two-month-old infant?

Which part of the heart is responsible for initiating and maintaining its rhythmic activity?

Cardiac output is defined as the volume of blood:

$A$: The cardiac impulse which originates from $SA$ node in mammalian heart cannot spread directly from atria to ventricles.
$R$: In mammalian heart there is no continuity between cardiac muscle fibres of atria and those of ventricles except $AV$ bundles.

Purkinje fibers are found in:

Starling's law is related to