Expansion
A wedge-shaped space between the anal canal and the ischium, containing fat and the pudendal canal
Expansion
Boundaries
- Medially - external anal sphincter and levator ani
- Laterally - obturator internus and the ischium
- Apex - where levator ani meets obturator internus
- Base - perianal skin
Contents
- Fat, which allows distension of the anal canal during defaecation
- Pudendal canal (Alcock’s canal) on the lateral wall, carrying the pudendal nerve and internal pudendal vessels
- Inferior rectal vessels and nerves crossing the fossa
The fat is poorly vascularised, so it is prone to infection. Abscesses arise from the anal glands opening at the dentate line, spreading into the intersphincteric plane and then into the fossa.
Because the two fossae communicate posteriorly behind the anal canal, infection can track across to form a horseshoe abscess requiring drainage on both sides.