A long, somewhat wide muscle passing from the sternum or breast-bone in front to the pubis behind. It is situated between the broad tendons of the internal oblique and the transversalis muscles, and forms the principal part of the floor of the abdomen. It is marked by a number of transverse lines (lineae transversae), due to short tendinous intersections. It joins its fellow in the middle line of the belly.

Origin. - From the sternum, and cartilages of the ribs from the 5th to the 9th.

Insertion. - Into the anterior border of the pubis.

Action. - To flex the spine, and, by compressing the abdomen, to assist in expiration, urination, defecation, and parturition.