What are the abdominal muscles?

Question

My professor told us that there are 5 different abdominal muscles and gave us 4 of them and challenged us to find the 5th. I've looked all over the net and can't seem to find it. Does anyone know?

We've been given the following:

Rectus Abdominis

External Oblique

Internal Oblique

Transversus Abdominis

Answer 1

The pyramidalis is a small and triangular muscle, anterior to the Rectus abdominis, and contained in the rectus sheath. It originates at the pubic bone and is inserted into the linea alba half way up to the umbilicus