Anatomy of the Colon

The endoscopist views the colon from a perspective unlike that of any other visualizing technique, viewing the inner relief of the "intestinal skeleton," which is made up of three straplike bands of longitudinal muscles (tenia coli) and numerous half-moon-shaped cross-folds (semilunar folds) which give rise to the pouchlike haustra between them (Fig. 1.6).

Structure of the intestinal wall. The intestinal wall can be divided microscopically into four layers, the structure of which does not vary significantly from the six macroscopic segments of the colon (rectum, sigmoid colon, descending colon, transverse colon, ascending colon, and cecum) (Figs. 1.7,1.8). The en-doscopic forceps biopsy usually takes samples limited to the mucosa. In order to collect deeper proportions of the intestinal wall (e.g., submucosa) it is practical to use a snare.

Colon segments. The division of the colon into segments is based primarily on anatomical rather than functional aspects. Only the rectum and cecum are unlike the other segments in that they function as reservoirs.

Cecum. Passage from the ileum to the cecum is restricted by the Bauhin valve that prevents the backward flow of the contents of the intestine out of the colon into the small intestine. The Bauhin valve is made up of two lips with a reinforced circular muscle layer that permit the opening of a narrow slit, the ileoce-cal valve, and that merge into two membrane folds at the front and back (Fig. 1.9).

The cecum is normally located intraperitoneally in the iliac fossa of the lower right abdomen. In the final months of pregnancy, the beginning of the large intestine grows up the inside of the right side of the abdomen. A "displaced" cecum can result if it remains at the level of the liver.

The three tenia of the cecum converge in a star shape at the end of the (vermiform) appendix that is not intubated during colonoscopy.

Layers Colon Wall
Fig. 1.6 Schematic structure of the colon wall. Tenia (1), pliquae semilunares (2), and haustra (3).
Sigmoid Colon Tenia
Fig. 1.7 Anatomy of the colon.

Fig. 1.8 Microscopic structure of the intestinal wall (with the kind >

permission of Prof. Dr. H. Arnholdt, Pathological Institute, Klinikum

Augsburg).

(1) Mucosa: deep, close together crypts with numerous goblet cells and enterocytes, high brush border;

(2) Submucosa: loose connective tissue layer, latticelike formation of collagen fibers, blood, and lymph vessels, vegetative plexus submu-cosus;

(3) Muscularis propria: strong inner circular muscle layer, outer longitudinal layer, reinforced tenia, plexus myentericus;

(4) Serosa: peritoneal covering.

Colonic Muscularis Mucosae Layer
Bauhin Valve
Fig. 1.9 Detailed view of the cecum and Bauhin valve.

Fig. 1.10 Rectum and anal canal.

Table 1.5 Mesenterial fixation of the colon

Intestinal segment

Mesentery

Meaning for endoscopy

Sigmoid colon

Persistent, mobile mesentery

Makes endoscope passage difficult due to mobility

Transverse colon

in abdominal cavity

Descending colon

Retroperitoneal fixation of the mesentery

Endoscopic passage easier

Ascending colon

Cecum

Rectum

Primarily retroperitoneal

Good maneuverability of the endoscope

Mesentery. The mesentery is a double layer of peritoneum, which, during the embryonic phase, attaches the ascending colon and the descending colon to the back wall of the abdomen, creating a retroperitoneal fixation; the mesentery persists as a free attachment for the transverse colon and sigmoid colon so that they remain mobile (Tab. 1.5). Because of this, the passage of the endoscope can result in colon movements and even looping in the abdominal cavity (see Chapter 5).

Rectum and anal canal. The 15-20-cm-long rectum is closed off to the outside by the hemorrhoidal zone, where the anal columns containing arterial and vascular bundles are located. Together with the reinforced muscle layers of the internal and external anal sphincter muscles, the hemorrhoidal zone supports bowel continence. The epidermis extends 2-3 cm into the anal canal. Cranially limited by a transverse fold (the Kohlrausch fold), the rectal ampulla is a highly expandable area that functions as a reservoir (Fig. 1.10).

Attention

► Precise knowledge of anatomy is essential for the management of colonoscopy, correct description of pathological findings, and understanding the clinical features of intestinal diseases.

References

See Chapter 2.

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Responses

  • lauren
    How deep is sigmoid colon?
    8 years ago
  • sinit semhar
    How to relax the sigmoid colon?
    7 years ago
  • christina
    Is there a valve between the sigmoid and descending colon?
    5 years ago
  • michelle pfeiffer
    Is rectum made up of folds of skin?
    4 years ago

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