654 A MANUAL OF ANATOMY 



it is parallel with the inner part of Poupart's ligament. Its 

 length is from i inch to i| inches, and its breadth at the base about 

 ^ inch. It is triangular, the base being formed by part of the 

 pubic crest, and the apex being directed upwards and outwards. 

 The margins of the ring are called pillars or columns. From the 

 oblique position of the ring one pillar is external and inferior, the 

 other being internal and superior. The internal or superior pillar, 

 which is thin and straight, passes to be attached to the front of the 

 symphysis pubis. The external or inferior pillar is at first thin, 

 but it soon becomes thick and prismatic, and is fixed to the pubic 

 spine. It presents superiorly a concavity which lodges the sper- 

 matic cord in the male and the round ligament of the uterus in 

 the female. The intercolumnar or external spermatic fascia is 

 attached to both pillars, and it here lies upon the spermatic cord. 

 Underneath the cord this fascia is attached along the pubic crest 

 between the pillars of the ring. These attachments of the fascia 

 explain why urine, extravasated into the perineum, does not find 

 its way into the inguinal canal. Within the lower and inner part 

 of the ring is the triangular fascia, and directly behind the ring is 

 the conjoined tendon, which strer^thens what would otherwise be 

 a weak part of the abdominal wall. The ring is smaller in the 

 female than in the male. 



The linea alba has been already described (see p. 639). 



The triangle of Petit is only present when the latissimus dorsi 

 and external oblique do not meet. It is situated immediately 

 above the centre of the iliac crest, and is bounded in front by 

 the posterior border of the external oblique, behind by the anterior 

 border of the latissimus dorsi, and below by the central portion 

 of the iliac crest. It is covered only by skin and fascia, and its 

 floor is formed by a part of the internal oblique. In this situation 

 a lumbar hernia may occur, or a lumbar abscess may find its way 

 to the surface. 



Obliquus Internus Abdominis — Origin. — (i) The deep or abdo- 

 minal surface of Poupart's ligament over its outer half; (2) the 

 summit of the iliac crest over its anterior two-thirds ; and (3) the 

 posterior aponeurosis of the transversalis abdominis. 



Insertion. — (i) The lower borders of the cartilages of the lower 

 three or four ribs ; (2) the lower borders of the cartilages of the 

 seventh and eighth ribs, and the side of the ensiform process of 

 the sternum ; (3) the linea alba in its whole length ; (4) the front 

 of the pubic crest ; and (5) the pectineal portion of the ilio-pectineal 

 line for \ inch. 



Nerve-supply. — This is similar to that of the external oblique, 

 with the addition of twigs from the inguinal nerve and hypogastric 

 branch of the ilio-hypogastric, both of which are derived from the 

 lumbar plexus, more particularly the first lumbar nerve. 



Action. — This is similar to that .of the external oblique. When 

 the right internal oblique acts simultaneously with the left external 

 oblique, the trunk is rotated to the right side, and vice versa. To 



