Complex sentence: Cardiothoracic surgery is the field of medicine involved in surgical treatment of diseases affecting organs inside the thorax (the chest)--generally treatment of conditions of the heart (heart disease) and lungs (lung disease).
Split into simpler sentences: Cardiothoracic surgery is the field of medicine involved in surgical treatment of diseases affecting organs inside the thorax (the chest). Generally treatment of conditions of the heart (heart disease) and lungs (lung disease).