Final answer:
Below the arcuate line, the anterior rectus sheath is formed by all three abdominal wall muscle fascias. Above this line, the posterior sheath is formed by the transversus and half of the internal oblique fascia.
Step-by-step explanation:
Below the arcuate line, all three abdominal wall muscle fascias form the anterior rectus sheath. Above the arcuate line, the transversus and half of the internal oblique fascia form a posterior rectus sheath fascia. The anterior abdominal muscles include the external oblique, the internal oblique, and the transversus abdominis.
These muscles along with the rectus abdominis, enclosed by rectus sheaths, protect internal organs and assist in movements of the vertebral column. The linea alba is a prominent midline structure made from these sheaths, known for its contribution to the 'six-pack abs' appearance.