Page 75 - Clinical Anatomy

ECA2 7/18/06 6:42 PM Page 60

60 The abdomen and pelvis

(a)◊Above the costal margin, the anterior sheath comprises the external
oblique aponeurosis only; posteriorly lie the costal cartilages.
(b)◊From the costal margin to a point half-way between umbilicus and
pubis, the external oblique and the anterior part of the internal oblique
aponeurosis form the anterior sheath. Posteriorly lie the posterior part of
this split internal oblique aponeurosis and the aponeurosis of transversus
abdominis.
(c)◊Below a point half-way between umbilicus and pubis, all the
aponeuroses pass in front of the rectus so that the anterior sheath here com-
prises the tendinous expansions of all three oblique muscles blended
together. The posterior wall at this level is made up of the only other struc-
tures available—the transversalis fascia, (the thickened extraperitoneal
fascia of the lower abdominal wall), and peritoneum.
The posterior junction between (b) and (c) is marked by the arcuate line
of Douglas, which is the lower border of the posterior aponeurotic part of the
rectus sheath. At this point the inferior epigastric artery and vein (from the
external iliac vessels) enter the sheath, pass upwards and anastomose with
the superior epigastric vessels which are terminal branches of the internal
thoracic artery and vein. The rectus sheaths fuse in the midline to form the
linea alba stretching from the xiphoid to the pubic symphysis.
The lateral muscles of the abdominal wall comprise the external and
internal oblique and the transverse muscles. These correspond to the three
layers of muscle of the chest wall— external, internal and innermost inter-
costals, and, like them, have their neurovascular bundles running between
the second and third layer. They are clinically important in making up the
rectus sheath and the inguinal canal, and also because they must be divided
in making lateral abdominal incisions.
Their attachments can be remembered when one bears in mind that
they fill the space between the costal margin above, the iliac crest below,
and the lumbar muscles covered by lumbar fascia behind. Medially, as
already noted, they constitute the rectus sheath and thence blend into the
linea alba from xiphoid to pubic crest.
The obliquus externus abdominis (external oblique) arises from the outer
surfaces of the lower eight ribs and fans out into the xiphoid, linea alba, the
pubic crest, pubic tubercle and the anterior half of the iliac crest.
From the pubic tubercle to the anterior superior iliac spine its lower
border forms the aponeurotic inguinal ligament of Poupart.
The obliquus internus abdominis (internal oblique) arises from the lumbar
fascia, the anterior two-thirds of the iliac crest and the lateral two-thirds of
the inguinal ligament. It is inserted into the lowest six costal cartilages,
linea alba and the pubic crest.
The transversus abdominis arises from the lowest six costal cartilages
(interdigitating with the diaphragm), the lumbar fascia, the anterior two-
thirds of the iliac crest and the lateral one-third of the inguinal ligament; it is
inserted into the linea alba and the pubic crest.
Note that the external oblique passes downwards and forwards, the
internal oblique upwards and forwards and the transversus transversely.