Final answer:
The long vertical muscle of the abdominal wall is the rectus abdominis. It is part of the anterior abdominal wall muscles and is famous for forming the 'six-pack' appearance when well developed.
Step-by-step explanation:
The long vertical, strip-like muscle of the abdominal wall is the rectus abdominis. The anterior abdominal muscles include the medially located rectus abdominis, which is covered by a sheet of connective tissue called the rectus sheath. Each rectus abdominis muscle is a pair of long, linear muscles, commonly referred to as the "sit-up" muscles, because they are often exercised through sit-ups. These muscles originate at the pubic crest and symphysis, extending the length of the trunk. The rectus sheath encloses these muscles, joining at the anterior midline to form the linea alba, thus creating the appearance of "six-pack abs" when each muscle segment becomes pronounced.