Page 106 - Clinical Anatomy

ECA2 7/18/06 6:42 PM Page 91

The gastrointestinal tract 91

At an early stage rapid proliferation of the gut wall obliterates its lumen
and this is followed by subsequent recanalization.
The fore-gut becomes rotated with the development of the lesser sac so
that the original right wall of the stomach comes to form its posterior
surface and the left wall its anterior surface. The vagi rotate with the
stomach and therefore lie anteriorly and posteriorly to it at the oesophageal
hiatus.
This rotation swings the duodenum to the right and the mesentery of
this organ then blends with the peritoneum of the posterior abdominal wall
—this blending process is termed zygosis (see p. 98).
The mid-gut enlarges rapidly in the 5-week fetus, becomes too large to
be contained within the abdomen and herniates into the umbilical cord.
The apex of this herniated bowel is continuous with the vitello-intestinal
duct and the yolk sac, but this connection, even at this early stage of fetal
life, is already reduced to a fibrous strand.
The axis of this herniated loop of gut is formed by the superior mesen-
teric artery, which demarcates a cephalic and a caudal limb. The cephalic
element develops into the proximal small intestine; the caudal segment dif-
ferentiates into the terminal 2 feet (62cm) of ileum, the caecum and the
colon as far as the junction of the middle and left thirds of the transverse
colon.
Abud which develops on the caudal segment indicates the site of subse-
quent formation of the caecum; it may well be that this bud delays the
return of the caudal limb in favour of the cephalic gut during the subse-
quent reduction of the herniated bowel.
At 10 weeks this return of the bowel into the abdominal cavity com-
mences. The mid-gut loop first rotates anti-clockwise through 90° so that
the cephalic limb now lies to the right and the caudal limb to the left.
The cephalic limb returns first, passing upwards and to the left into the
space left available by the bulky liver. In doing so, this mid-gut passes
behind the superior mesenteric artery (which thus comes to cross the third
part of the duodenum) and also pushes the hind-gut—the definitive distal
colon—over to the left.
When the caudal limb returns, it lies in the only space remaining to it,
superficial to, and above, the small intestine with the caecum lying immedi-
ately below the liver.
The caecum then descends into its definitive position in the right iliac
fossa, dragging the colon with it. The transverse colon thus comes to lie in
front of the superior mesenteric vessels and the small intestine.
Finally, the mesenteries of the ascending and descending parts of the
colon blend with the posterior abdominal wall peritoneum by zygosis. This
embryological fusion of peritoneal surfaces is of major surgical importance.
Thus, in mobilising the right or left colon, an incision is made along this
avascular line of zygosis lateral to the bowel, allowing it to be mobilised
with its mesocolon and blood supply. In a similar fashion, the duodenum,
head of pancreas and termination of the common bile duct can be mobilised
bloodlessly by incising the peritoneum along the right border of the duode-
num—Kocher’s manoeuvre (see page 77).