The anal canal

The anal canal is 1.5in (4 cm) long and is directed downwards and backwards from the rectum to end at the anal orifice. The mid-anal canal represents the junction between endoderm of the hind-gut and ectoderm of the

Fig. 63 Sagittal section of the rectum and its related viscera in the female.

Anal Canal Mucocutaneous Junction

cutaneous invagination termed the proctodaeum. Failure of breakdown of the separating membrane results in imperforate anus.

1 The lower half is lined by squamous epithelium and the upper half by columnar epithelium; the latter presents vertical columns of mucosa (the columns of Morgagni) connected at their distal extremities by valve-like folds (the valves of Ball). A carcinoma of the upper anal canal is thus an adenocarci-noma, whereas that arising from the lower part is a squamous tumour.

2 The blood supply of the upper half of the anal canal is from the superior rectal vessels, whereas that of the lower half is the blood supply of the surrounding anal skin, the inferior rectal vessels, which derive from the internal pudendal, and ultimately the internal iliac vessels. The two venous systems communicate and therefore form one of the anastomoses between the portal and systemic circulations.

3 The lymphatics above this mucocutaneous junction drain along the superior rectal vessels to the lumbar nodes whereas, below this line, drainage is to the inguinal nodes. A carcinoma of the rectum which invades the lower anal canal may thus metastasize to the groin nodes.

4 The nerve supply to the upper anal canal is via the autonomic plexuses, the lower part is supplied by the somatic inferior rectal nerve, a terminal branch of the pudendal nerve (see Fig. 99b). (The lower canal is therefore sensitive to the prick of a hypodermic needle, whereas injection of an internal haemorrhoid with sclerosant fluid, by passing a needle through the mucosa of the upper part of the canal, is painless.)

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