Muscles of the heart are

During human fetal development, the pulmonary artery is connected to the aorta by a narrow duct called the ductus arteriosus. What is this structure called after it closes and becomes a fibrous remnant in an adult?

Heartbeat is controlled by a nodal tissue which is made up of specialised cardiac muscles,called

Sino-atrial node is called the pacemaker of our heart. Why?

Analogy type questions:
$(1)$ Right atrium and right corner : $SA$ node :: Corner of right atrium : .......
$(2)$ First heart sound : $Lubb$ :: Second heart sound : ...........

Largest heart is found in