Complex sentence: The cardiac skeleton is made of dense connective tissue and this gives structure to the heart and forms the atrioventricular septum which separates the right from the left heart, and the fibrous rings which serve as bases for the four heart valves.
Split into simpler sentences: The cardiac skeleton is made of dense connective tissue and this gives structure to the heart. It forms the atrioventricular septum which separates the right from the left heart, and the fibrous rings which serve as bases for the four heart valves.