Final answer:
The name of the muscular sac on the floor of the pelvic cavity, inferior to the peritoneum and posterior to the pubic symphysis, is the pelvic diaphragm.
Step-by-step explanation:
The muscular sac situated beneath the peritoneum, posterior to the pubic symphysis in the pelvic cavity, is called the pelvic diaphragm. This crucial structure provides support to pelvic organs and helps control bowel movements. Its position below the peritoneum and behind the pubic symphysis highlights its key role in maintaining the integrity of the pelvic region. Understanding the pelvic diaphragm is vital in discussions about pelvic anatomy and function, particularly in the context of its significance in supporting organs and contributing to bodily functions.