Anatomy
Surgical
anatomy
The rectum has an ill-defined anatomical
beginning, but surgically the
rectosigmoid junction lies opposite the sacral promontory. From here the rectum
follows the curve of the sacrum to end at the anorectal junction. At this point,
the puborectalis muscle encircles the posterior and lateral aspects of the
junction, creating the anorectal angle (normally 1200). The rectum has three
lateral curvatures: the upper and lower are convex to the right, and the middle
convex to the left: on the mucosal (lumen) aspect these three curves are marked
by semicircular folds (Houston’s valves) (Fig. 60.1). That part of
The
lowest part of the rectum is separated by a fascial condensation —
Denonvilliers’ fascia — from the prostate in front, and behind by another
fascial layer — Waldeyer’s fascia —from the coccycx and last two sacral
vertebrae (Table 60.1). These fascial
layers are surgically important as they are a barrier to malignant penetration,
and are valuable guides at operation.
Blood supply
The superior
rectal artery is
the direct continuation of the inferior mesenteric artery and is the main
arterial supply of the rectum. Opposite the third sacral vertebra, the artery
divides again behind the lower third of the rectum into two
The middle
rectal artery arises
on each side from the internal iliac artery (Fig.
60.2) and passes to the
rectum in the lateral ligaments. It is usually small and breaks up into several
terminal branches.
The inferior
rectal artery arises
on each side from the internal pudendal artery as it enters Alcock’s canal.
It hugs the inferior surface of the levator ani muscle as it crosses the roof of
the ischiorectal fossa to enter the anal muscles (Fig.
60.2).
Venous
drainage
The superior haemorrhoidal veins draining the upper half of the anal
canal above the dentate line pass upwards to become the rectal veins: these
unite to form the superior rectal vein which later becomes the inferior mesenteric vein. This forms part of the portal venous
system, and ultimately drains into the splenic vein. Middle rectal veins exist,
but are small, unimportant channels unless the normal paths are blocked.
Lymphatic
drainage
The lymphatics of the mucosal lining of the
rectum interchange freely with those of the muscular layers. The usual
drainage flow is upwards, and only to
a limited extent laterally and downwards. For this reason, surgical ablation
of malignant disease concentrates mainly on achieving wide clearance of
proximal lymph nodes. However, if the usual upwards routes are blocked (e.g. by
carcinoma) flow can reverse, and it is then possible to find metastatic lymph
nodes on the side walls of the pelvis (along the middle rectal vessels) or even
in the inguinal region (along the inferior rectal artery).
Superior rectal nodes
These are an important group of nodes on the
back of the rectal ampulla above the levator ani muscle (Fig.
60.3), also known
as the pararectal lymph glands of Gerota.
Middle rectal nodes
These lie close to the middle rectal arteries
and pass to lymph nodes around the internal arteries. The Japanese have stressed
the importance of removing these lymph glands when operating on rectal cancer.