This is the outermost and the largest of the abdominal muscles. It is broad behind and narrow in front, and gives off a wide aponeurotic tendon, which passes obliquely downward and backward.

Origin. - From the outer surface of the thirteen or fourteen posterior ribs and from the tendon of the latissimus dorsi.

Insertion. - Its fibres take an oblique direction downward and backward, and give off a wide aponeurotic tendon, which becomes inserted into (1) the linea alba in the centre of the belly, (2) the pubis, (3) the external angle of the ilium or haunch-bone. Between the two last named points of insertion the tendon forms an arch known as "Pou-part's ligament". Near the pubis its fibres divide and form an elliptical opening, the external ring for the passage of the testicle into the scrotum.

Action. - To flex and arch the back and to assist in expiration. By forcible compression of the abdomen it becomes an active agent in urination, defoecation, and parturition.