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 rectum that lies below the middle valve has a much wider diameter than the upper third, and is known as the ampulla of the rectum. The adult rectum is approximately 18—20 cm in length and is conveniently divided into three equal parts: the upper third, which is mobile and has a peritoneal coat except near to the middle third where the peritoneum only covers the anterior and part of the lateral surfaces; the middle third, which is the widest part of the rectum and is confined within the diameter of the bony pelvis; and the lowest third, which lies within the muscular floor of the pelvis and has important relations to fascial layers.

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 an anterior and a posterior branch. The arteries and their accompanying lymphatics are kept applied to the back of the rectum by dense connective tissue (the rnesorectum or ‘rectal fascia’).

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.