Sheat of recti abdominis

The sheath of the rectus abdominis is the fibrous envelope formed by the aponeuroses of the abdominal muscles (external abdominal oblique, internal abdominal oblique, and transversus abdominis), enclosing the rectus abdominis muscle along the ventral abdominal wall. The structure and composition of this sheath vary between species and between the cranial and caudal parts of the abdomen.

The sheath of rectus abdominis is divided into two part:

• External (superficial) lamina

• Internal (deep) lamina

These are formed by the fusion and passage of the abdominal muscles aponeuroses at different levels along the abdominal wall.

Variations

• Dogs and cats: Cranially to the umbilic, the outer layer is formed by the aponeuroses of the external and internal oblique muscles of the abdomen, while the inner layer is formed by the aponeurosis of the transverse muscle of the abdomen. Caudal to the umbilicus: the three muscle aponeuroses pass in front of the rectus abdominis muscle, so that the inner layer is absent. The sheath is therefore incomplete at the rear.

• Horse: In the cranial region, the arrangement is similar to that of the dog. In the caudal region, the inner layer is absent below the umbilicus. However, the aponeurosis of the transverse abdominal muscle may still partially contribute to this sheath-like role.

• Ox (Bovine): The sheath is complete along the entire length. The rectus abdominis muscle is surrounded cranially and caudally. The outer layer is formed by the external and internal oblique aponeuroses, and the inner layer is mainly formed by the aponeurosis of the transverse abdominis muscle.

• Pigs: The conformation of this structure is intermediate between that of horses and cattle. The sheath is generally complete, but its thickness and structure vary.