Anatomy of the Colon and Rectum
The large intestine is a muscular tube which starts at the termination of the ileum, and ends at the anal canal. It is divided into two parts; the colon and the rectum (Figure 1.1). The colon is further divided into five parts; the caecum is the first part and is a blind-ended sac, located in the right iliac fossa, from which the appendix arises. The caecum leads to the ascending colon, which ascends the right side of the abdomen to turn just under the liver (hepatic flexure) to become the transverse colon. This crosses the abdomen transversely in the epigastric/umbilical regions to turn inferiorly at the splenic flexure to become the descending colon. This descends the left side of the abdomen where it becomes tortuous as the sigmoid colon, which in turn becomes the rectum at the pelvic brim. The rectum descends the pelvis in the midline to end at the anus. The rectum is divided into three equal parts, the upper, middle and lower thirds. The colon is fixed to the posterior abdominal wall by layers of peritoneum in its ascending and descending parts, but is relatively more mobile in the remaining portions. The rectum is adherent to the posterior pelvic wall by further folds of peritoneum (the mesorectum).
Figure 1.1 Anatomy of the Large Intestine
The proximal colon up to a point two-thirds of the way along the transverse colon is derived from the mid-gut and all parts distal to this are derived from the hind-gut. Development of the large intestine begins in the sixth week of gestation, and the colon comes to occupy its characteristic location in the abdomen during the tenth week of gestation as the gut rotates anti-clockwise through 270 degrees.
The large bowel is a muscular tube lined by a mucous membrane (Figure 1.2). The innermost layer is the mucosa, which is comprised of an epithelial lining, the lamina propria and the muscularis mucosae. The epithelial cells are arranged into a distinct pattern of closely packed, straight glands of cells specialised for water absorption (Figure 1.3). The sub-mucosa contains larger blood vessels lymphatics and nerves. The muscularis propria consists of smooth muscle in two layers. The inner layer comprises muscle fibres arranged in a circumferential pattern, whilst the outer layer is made of longitudinally arranged muscle fibres. These are condensed into three distinct bands (taenia coli) throughout the length of the colon, but become continuous again in the rectum. The outermost layer is the adventitia, comprised of connective tissue.
Figure 1.2 Cross Section of the Colonic Wall
Figure 1.3 Histology of the Colon
Vascular, Lymphatic and Nerve Supply
The colon and rectum are supplied with arterial blood from the arteries of the primitive mid- and hind-gut; namely the superior and inferior mesenteric arteries. The vessels anastomose to form a marginal artery which runs longitudinally near to the bowel wall. The ‘watershed’ area between the two arterial inflows is at the point two-thirds of the way along the transverse colon. The distal rectum is also supplied with blood from the inferior rectal arteries, which are branches of the internal iliac artery. Lymphatic drainage follows the arterial inflow into the superior and inferior mesenteric lymph nodes. The nerve supply is divided into two parts. The parasympathetic supply is from the vagi in the midgut portions, and from pelvic parasympathetic nerves (S2-S4) in the hindgut portions. The sympathetic supply is derived from spinal cord segments T10-L2. Pain fibres accompany these vasoconstrictor fibres, to produce umbilical pain from the midgut, and hypogastric pain from the hindgut.