The anal canal, lined by squamous mucosa, is prone to several groups of diseases, 
many of which are extensions of diseases in the rectum. 

Anal fissures (splitting of the mucosa with chronic inflammation) and fistulae
(a sinus extending from the anus or the rectum)are commonly seen in association with Crohn's disease,
but may also occur spontaneously.

Infections of the anal canal may be the result of sexually transmitted diseases, particularly 
chancres of syphilis, gonorrhoea and amoebic infections in homosexuals. Infection with human 
papillomavirus may cause condylomas in the perianal skin.

Tumours of the anal canal are most commonly squamous cell carcinomas. Less commonly, adenocarcinomas,
and small-cell carcinomas resembling oat-cell carcinomas occur.

Haemorrhoids, which are varicose dilatations of rectal veins, are very common.