Final answer:
The diaphragm is the major skeletal muscle that separates the thoracic cavity from the abdominal cavity. It plays a vital role in breathing and bodily functions like defecation, urination, and childbirth through the Valsalva maneuver.
Step-by-step explanation:
The major muscle that separates the abdominal cavity from the thoracic cavity is known as the diaphragm. This skeletal muscle is dome-shaped at rest and plays a crucial role as the primary muscle in breathing. The diaphragm forms the floor of the thoracic cavity and the roof of the abdominal cavity, participating in critical bodily functions.
During processes such as defecation, urination, and childbirth, there is cooperation between the diaphragm and the abdominal muscles, an action referred to as the "Valsalva maneuver". When the abdominal muscles contract and the pressure cannot displace the diaphragm upward, it increases pressure within the intestinal, urinary, or reproductive tracts, aiding in these processes.
The diaphragm's ability to contract and relax alters the volume of the thoracic cavity during respiration. The superior surface of the diaphragm is convex, contributing to the formation of the elevated floor of the thoracic cavity, while its inferior surface is concave, shaping the curved roof of the abdominal cavity.