The rectum also has three additional lateral flexures formed by the submucosal transverse folds that protrude into the lumen of the rectum. These submucosal folds are also known as the valves of Houston, and they include the following:

Superior rectal valve - upper convexity on the left side of the rectum;
Middle rectal valve - intermediate convexity on the right side;
Inferior rectal valve - lower convexity on the left side of the intestine.

The typical large intestine characteristic features - taenia coli, haustra, semilunar folds and epiploic appendices - are not found in the rectum. Overall, the length of the proximal diameter of the rectum is similar to the sigmoid colon. However, the rectum enlarges distally and forms a dilation called the rectal ampulla. The rectal ampulla is a terminal portion of the rectum.

Anal canal

The anal canal is a continuation of the rectum. It is also the final portion of the large intestine and the entire gastrointestinal tract. The anal canal is a continuation of the rectum, extending between the anorectal junction and the anus. It is around 1 - 1.6 inches (2.5 - 4 cm) long. Like the distal third of the rectum, the anal canal is also a completely extraperitoneal organ.

Overall, the anal canal can be subdivided into two zones or segments - superior (upper) and inferior (lower). Both are separated by a demarcation line called the dentate or pectinate line. It is an irregular zig-zagging line that demarcates the superior two-thirds of the anal canal from its inferior one-third. Both segments between the dentate line have a different embryological origin, and therefore the neurovascular supply for each part is not the same.

Overall, the anal canal is responsible for defecation and fecal continence. Therefore, it is surrounded by a muscular sphincter system that is formed by two sphincters:

Anterior, the anal canal relates to the perineal body, which separates it from the root of the penis in men and the vagina in women. Lateral to the anal canal are the ischioanal (ischiorectal) fossae containing adipose tissue. And finally, posterior to it is the tip of the coccyx that is linked to the anal canal by the anococcygeal ligament.

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Rectum and anal canal (part 2)