Variations in the composition of the human rectus sheath: a study of the anterior abdominal wall.

Three modes of construction of the rectus sheath above the arcuate line were found in a study of 40 cadavers. The classical description occurred most frequently, in 24 cases. The other constructions conformed to those described in earlier studies although the incidences were different. The accessory internal oblique muscle was found in none of the 40 cadavers, and pyramidalis muscle occurred on one or both both sides in only 3 out of 56 specimens. The shape and position of the arcuate line were neither symmetrical nor constant, and neither was the arrangement of the nerve supply to the rectus abdominis muscle or to the overlying skin.