Page 103 - Clinical Anatomy

ECA2 7/18/06 6:42 PM Page 88

88 The abdomen and pelvis

•◊◊prehepatic — e.g. thrombosis or congenital obliteration of the portal
vein;
•◊◊hepatic—e.g. cirrhosis of the liver;
•◊◊posthepatic—e.g. congenital stenosis of the hepatic veins.
If obstruction from any of these causes occurs, the portal venous pres-
sure rises (portal hypertension) and collateral pathways open up between the
portal and systemic venous systems.
These communications are:
1◊◊between the oesophageal branch of the left gastric vein and the
oesophageal veins of the azygos system (these oesophageal varices are the
cause of the severe haematemeses that may occur in portal hypertension);
2◊◊between the superior rectal branch of the inferior mesenteric vein and
the inferior rectal veins draining into the internal iliac vein via its internal
pudendal tributary;
3◊◊between the portal tributaries in the mesentery and mesocolon and
retroperitoneal veins communicating with the renal, lumbar and phrenic
veins;
4◊◊between the portal branches in the liver and the veins of the abdominal
wall via veins passing along the falciform ligament from the umbilicus
(which may result in the formation of a cluster of dilated veins which
radiate from the navel and which are called the caput Medusae);
5◊◊between the portal branches in the liver and the veins of the diaphragm
across the bare area of the liver.
A striking feature of operations upon patients with portal hypertension
is the extraordinary dilatation of every available channel between the two
systems which renders such procedures tedious and bloody.

Lymph drainage of the intestine (Fig. 68)
The arrangement of lymph nodes is relatively uniform throughout the
small and large intestine. Numerous small nodes lying near, or even on, the
bowel wall drain to intermediately placed and rather larger nodes along
the vessels in the mesentery or mesocolon and thence to clumps of nodes
situated near the origins of the superior and inferior mesenteric arteries.
From these, efferent vessels link up to drain into the cisterna chyli.
The lymphatic drainage field of each segment of bowel corresponds
fairly accurately to its blood supply. High ligation of the vessels to the
involved segment of bowel with removal of a wide surrounding segment of
mesocolon will, therefore, remove the lymph nodes draining the area. Divi-
sion of the middle colic vessels and a resection of a generous wedge of
transverse mesocolon, for example, would be performed for a growth of
transverse colon.

The structure of the alimentary canal
The alimentary canal is made up of mucosa demarcated by the muscularis
mucosae from the submucosa, the muscle coat and the serosa — the last being
absent where the gut is extraperitoneal.