Straight muscle of the abdomen

Musculus rectus abdominis

Compresses the abdomen, supports its floor and flexes the loins, drawing the pelvis toward the chest.

Where it runs from and to

A muscle is told apart from its neighbours by what it takes hold of at each end, which is also what decides what it does when it shortens.

Origin: The costal cartilages of the fourth to ninth ribs and the sternum.
Insertion: The prepubic tendon on the pubis.
Action: Compresses the abdomen, supports its floor and flexes the loins, drawing the pelvis toward the chest.
Nerve: Ventral branches of the caudal thoracic and the lumbar nerves.

Written for Vetomica in its own words from the standard reference anatomy of the horse; the books it was checked against are listed below.