What muscle forms the bottom of the thoracic cavity?

The diaphragm is the muscle that forms the bottom of the thoracic cavity. It is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity and plays a crucial role in respiration. When the diaphragm contracts, it moves downward, creating a negative pressure in the thoracic cavity that allows air to fill the lungs. This is essential for breathing and is a key function of the respiratory system.

Understanding the diaphragm's role is important for anyone in a medical or chiropractic context, as it not only serves as a separator between the thoracic and abdominal cavities but also significantly influences breathing mechanics. The diaphragm is innervated by the phrenic nerve and is vital for maintaining airflow and overall respiratory function.

Other muscle groups mentioned, such as the rectus abdominis and intercostal muscles, are also important in the context of the thoracic cavity and respiration, but they do not serve as the bottom boundary of the cavity. The latissimus dorsi is a broad muscle contributing to movements of the upper body but has no role in forming the floor of the thoracic cavity. Thus, the diaphragm is the correct answer as it specifically fulfills the anatomical and functional requirements stated in the question.