406 THE MrSCLES 



Those above, which form the inner pillar, as it is called, of the ring, run clown- 

 wards and inwards to be attached to the front of the symphysis pubis; those 

 below, which form the external pillar of the ring, form a thin edge at first, but 

 thicken just before their altachnicut to the spine of the pubes and the inner ex- 

 tremity of the ilio-pectineal line, for at this point the external i)illar is identical 

 with Pou})art's ligament. Upon the surface of the oblicjuus externus, close to the 

 external al)dominal ring, the obliiiue lilires of tlie ajtoneurosis are fastened together 

 by some transversely directed fibres, the intercolumnar fibres, which run upwards 

 and inwards from Poupart's ligament, limiting and rounding off the upper and 

 outer end of the external abdominal ring. A thin membrane, the intercolumnar 

 fascia, is prolonged from the edges of this opening over the spermatic cord and 

 round ligament, of which it forms the external envelope. 



Nerve-supply. — From the anterior branches of the lower thoracic nerves, by 

 means of numerous filaments which, passing through the internal oblique, enter 

 the muscle on its deep surface. 



Action. — (1) The curved muscular fibres in their contraction tend to become 

 straiglit and so compress the viscera which lie in their concavity; they act in 

 defecation, micturition, parturition, and all expiratory efforts; (2) the two obli(jui 

 externi acting together will draw upwards the front part of the pelvis, and so fiex 

 it upon the thorax; (3) the muscle of one side, acting alone or in conjunction with 

 the internal oblique of the opposite side, Avill rotate the pelvis and the lower part 

 of the body to the same side; (4) it will tend by its posterior fibres to draw the 

 crest of the ilium upwards towards the low^er ribs, and will thus act as a lateral 

 flexor of the pelvis upon the thorax; (5) acting from below, the muscles of the two 

 sides will draw the thorax downwards and forwards and flex the lumbar and lower 

 part of the thoracic spine; (6) it wall rotate the tliorax upon the pelvis so as to 

 turn the thorax and the upper part of the body to the ojjposite side; (7) the 

 posterior fibres wall flex the thorax laterally. 



Relations. — Superficially, the integuments, and for a short space behind the 

 latissimus dorsi; deeply, the lower ribs, tlieir cartilages, the intercostal muscles 

 between them, and the internal oblique; and below, the spermatic cord or round 

 ligament in the inguinal canal. 



Variations. — The oblique externus may rise from more or fewer ribs. Occasionally a deeper 

 plane is separated from the vest of the muscle. In one case, the anterior part of the aponeurosis 

 was observed to be wanting. 



2. OBLIQUUS INTERNUS ABDOMINIS 



The obliquus internus abdominis (fig. 266) — named from its relation to the 

 preceding muscle and the direction of its fibres — is an irregular cpiadrilateral curved 

 sheet, partly fleshy and partly aponeurotic. 



Origin. — (1) The outer half of Poupart's ligament; (2) the anterior two-thirds 

 of the space intervening between the inner and outer li})s of the crest of the ilium; 

 (3) the outer and posterior aspect of the aponeurosis of the transversalis abdominis 

 (which aponeurosis is also called the lumbar fascia). 



Insertion. — (1) For about one inch (2 "5 cm.) into the inner extremity of 

 the ilio-pectineal line; (2) the anterior border of the crest of the pubes; (3) the 

 whole length of the linea all>a; (4) the lower borders of the cartilages of the 

 last three ribs. 



Structure. — Arising by fleshy and short tendinous fibres intermingled, a fleshy 

 sheet is soon formed, the fibres of which diverge; the anterior passing forwards 

 and doAvnwards, the middle forwards and upwards, and the posterior directly 

 upwards to their insertion, which is by means of a broad aponeurosis. At the front 

 of the lower intercostal spaces the fibres run parallel to, and in the same plane 

 with, the internal intercostals. The position of the change from fleshy to apo- 

 neurotic fibres may be indicated by two lines at right angles to each other; one 

 passing upwards and a little outwards from the middle of Poupart's ligament, the 

 other horizontally forwards below the tip of the last rib, and near the edges of the 

 lower rib cartilages. The aponeurosis is blended with that of the external oblique, 



