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The dorsal cavity includes the cranial cavity, which contains the brain, and the vertebral canal, which protects the spinal cord. The ventral cavity comprises the thoracic and abdominopelvic cavities; the diaphragm separates these regions and is the chief muscle of respiration. The thoracic cavity contains the lungs and heart, while the abdominopelvic cavity is divided into abdominal and pelvic regions. Serous membranes line closed ventral cavities and cover organs. Their parietal and visceral layers are separated by a space containing friction-reducing fluid. The pleura, pericardium, and peritoneum are the principal serous membranes. Mucous membranes line passages that open to the exterior, whereas synovial cavities in joints contain synovial fluid.