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.

1. General Structure

The sheath of rectus abdominis is divided into:

• 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.

2. Species Variations

Dog and Cat:

• Cranial to the umbilicus: The external lamina is formed by the aponeuroses of the external and internal abdominal oblique muscles; the internal lamina is formed by the aponeurosis of the transversus abdominis.

• Caudal to the umbilicus: All three muscle aponeuroses pass in front of the rectus abdominis, so the internal lamina is absent. Thus, the sheath is incomplete posteriorly.

Horse:

• Cranial region: Similar arrangement to the dog.

• Caudal region: Internal lamina is absent caudal to the umbilicus. However, the aponeurosis of the transversus abdominis may still contribute partially.

Ox (Bovine):

• Complete sheath throughout: The rectus abdominis is enclosed both cranially and caudally. The external lamina is formed by the external and internal oblique aponeuroses, and the internal lamina is mainly formed by the transversus abdominis aponeurosis.

Pig:

• Intermediate between the horse and ox. The sheath is generally complete, but with variability in thickness and structure.