The wall of the colon is divided into four layers: the serosa, external muscular region (muscularis externa), submucosa and mucosa (Figure 7.2). The squamous epithelium of the serosa is covered with adipose tissue which forms distended fat pouches of peritoneum, known as appendices epiploicae. These are larger and more numerous in the distal half of the colon and are one of its distinguishing features. The serosa is absent from the rectum and anal canal.
The muscularis externa consists of an inner circular muscle layer and an incomplete outer longitudinal layer composed of three separate 0.8 cm wide, longitudinal strips known as teniae coli. These bands converge in the caecum at the root of the appendix. They travel the length of the colon and eventually widen and join to form a continuous outer longitudinal muscle layer which covers the rectum. Between the teniae coli is a thin layer of longitudinal muscle which allows the inner circular muscle layer to bulge outwards. This outward bulge is interrupted at intervals by contractions of the circular muscle, giving the colon its characteristic sacculated appearance. These sacculae are also known as haustra and are more pronounced in the proximal half of the colon. Their size and shape varies with the contractile activity of the circular muscle.
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