FIGURE 17.31 • Drawing of the rectum and anal canal. The rectum and anal canal are the terminal portions of the large intestine. They are lined by the colorectal mucosa that possesses a simple columnar epithelium containing mostly goblet cells and numerous anal glands. In the anal canal the simple columnar epithelium undergoes transition into a stratified columnar (or cuboidal) epithelium and then to a stratified squamous epithelium. This transition occurs in the area referred to as the anal transitional zone, which occupies the middle third of the anal canal between the colorectal zone and the squamous zone of the perianal skin.
(inﬂammation of the appendix). The appendix is also a common site for carcinoid, a type of tumor originating from enteroendocrine cells of lining mucosa (see Folder 17.3).
The rectum is the dilated distal portion of the alimentary canal. Its upper part is distinguished from the rest of the large intestine by the presence of folds called transverse rectal folds. The mucosa of the rectum is similar to that of the rest of the distal colon, having straight, tubular intestinal glands with many goblet cells.
The most distal portion of the alimentary canal is the anal canal. It has an average length of 4 cm and extends from the upper aspect of the pelvic diaphragm to the anus (Fig. 17.31). The upper part of the anal canal has longitudinal folds called anal columns. Depressions between the anal columns are called anal sinuses. The anal canal is divided into three zones according to the character of the epithelial lining:  Colorectal zone, which is found in the upper third of the anal canal and contains simple columnar epithelium with characteristics identical to that in the rectum.
 Anal transitional zone (ATZ), which occupies the middle third of the anal canal. It represents a transition between the simple columnar epithelium of the rectal mucosa and the stratified squamous epithelium of the perianal skin. The ATZ possesses a stratified columnar epithelium interposed between the simple columnar epithelium and the stratified squamous epithelium, which extends to the cutaneous zone of the anal canal (Fig. 17.32 and Plate 64, page 626).
 Squamous zone, which is found in the lower third of the anal canal. This zone is lined with stratified squamous epithelium that is continuous with that of the perineal skin.
In the anal canal, anal glands extend into the submucosa and even into the muscularis externa. These branched, straight tubular glands secrete mucus onto the anal surface through ducts lined with stratified columnar epithelium.
Sometimes the anal glands are surrounded by diffuse lymphatic tissue. They often lead to the formation of pathologic fistulas (a false opening between the anal canal and the perianal skin).
Large apocrine glands, the circumanal glands, are found in the skin surrounding the anal orifice. In some animals, the