Final answer:
The diaphragm is the muscle that separates the abdominal and thoracic cavities and plays a key role in the breathing process as well as in the Valsalva maneuver.
Step-by-step explanation:
The broad sheet of muscle that separates the abdominal and thoracic cavities is known as the diaphragm. The diaphragm is a central player in the breathing process and is dome-shaped at rest. It is the primary muscle of breathing, contracting rhythmically to help expand the lungs and draw air in, and relaxing to push air out. Additionally, the diaphragm also has crucial roles in the Valsalva maneuver, which is a process involving the cooperation between the diaphragm and abdominal muscles during activities like defecating, urination, and childbirth.