1. The Perineum-II (Anal Triangle)By
Prof. Dr. Imran Qureshi

2. The Posterior (Anal) Triangle
This is the triangle lying between the ischial tuberosities on either side and the coccyx posteriorly.
It comprises of the anus with its superficial sphincters, levator ani and, on either side, the ischiorectal fossa.

3. The Ischiorectal / Ischio-anal fossa
Is of great surgical importance because of its tendency to become infected.
Its boundaries are:
Laterally: the extrapelvic fascia over obturator internus (i.e. the side wall of the pelvis)
The pudendal vessels and nerve run in a fascial tunnel called the pudendal canal contained within this fascial wall. They give off the inferior rectal vessels and nerve respectively, which supply the external sphincter and perianal skin, then pass forward to supply the perineal tissues

4. The Ischiorectal / Ischio-anal fossa
Medially: The extrapelvic fascia over levator ani and the external anal sphincter
Posteriorly: The sacrotuberous ligament covered posteriorly by gluteus maximus
Anteriorly: The urogenital perineum
Floor—skin and subcutaneous fat.

5. Clinical Considerations
The content of the fossa is coarsely lobulated fat, which permits distention of the median anal canal.
Since the ischiorectal fossae communicate with each other behind the anal canal—infection in one can pass easily to the other.
Infection of this space may occur from boils or abrasions of the perianal skin, from lesions within the rectum and anal canal, from pelvic infection bursting through levator ani or, rarely, via the bloodstream.
The fossa contains no important structures and can, therefore, be incised when infected.
The pudendal nerves can be blocked in the pudendal canal on either side to give regional caudal anaesthesia.

6. Recesses
These are narrow extension of the fossa beyond its boundaries.
Anterior recess: Extends forwards above the urogenital diaphragm almost up to the posterior surface of the body of pubis.
Posterior recess: It is smaller than anterior recess. It extends deep to sacrotuberous ligament.
Horse shoe recess (deep post anal space/Post Sphincteric space). It Connects the two fossae behind the anal canal.

7. The Ischiorectal / Ischio-anal fossa

8. The Ischiorectal / Ischio-anal fossa

9. The Ischiorectal / Ischio-anal fossa

10. The Ischiorectal / Ischio-anal fossa

11. Potential spaces around the anorectum
Perianal space:
The Perianal fascia is in the form of septa which pass laterally from the lower end of the intersphincteric longitudinal fibers to the pudendal canal.
It separates a shallow subcutaneous Perianal space from the deep Ischiorectal space.
The Perianal space surrounds the anal canal below the white line of Hilton and contains subcutaneous part of external sphincter, external rectal venous plexus, terminal branches of inferior rectal vessels and nerve.
The fat in the Perianal space is tightly arranged in small loculi.
The infection in this space is therefore very painful due to tension caused by swelling.

12. Potential spaces around the anorectum
Sub mucous space:
Above the white line of Hilton between mucous membrane and internal sphincter. Contains internal rectal venous plexus and lymphatic.

13. The Ischiorectal fossa
 It is a wedge spaced space situated one on either side of the anal canal below the pelvic diaphragm.
The base is directed downwards towards the skin.
It is 5 to 6 cm deep, anterioposteriorly 5 cm, and 2.5 cm side to side , lying below the levator ani muscles and on either side of anal canal.
Post anal space connects the two fossae posteriorly by a horse shoe path.
The space is filled with loose areolar tissue and loosely arranged large loculi of fat.
The infection of this space leads to abscess formation and are least painful because swelling can occur without tension.

14. The Ischiorectal fossa
Boundaries:
Base: Formed by the skin.
Apex: Formed by meeting of Obturator fascia with the inferior fascia of the pelvic diaphragm (anal fascia). The line corresponds to the origin of levator ani from the lateral pelvic wall.
Anteriorly: The fossa is limited by the posterior border of perineal membrane.
Posteriorly:
Lower border of the gluteus maximus and
Sacrotuberous ligament.

15. The Ischiorectal fossa
Lateral wall: Vertical and is formed by:
Obturator internus with Obturator fascia, and
Medial surface of ischial tuberosity below the attachment of Obturator fascia.
Medial wall: Slopes upwards and laterally and is formed by:
External anal sphincter with fascia covering it in the lower part and
Levator ani with anal fascia in the upper part.

16. The Ischiorectal fossa
Recesses: These are narrow extension of the fossa beyond its boundaries.
Anterior recess: Extends forwards above the urogenital diaphragm almost up to the posterior surface of the body of pubis.
Posterior recess: It is smaller than anterior recess. It extends deep to Sacrotuberous ligament.
Horse shoe recess (deep post anal space/Post Sphincteric space): Connects the two fossae behind the anal canal.

17. The Ischiorectal fossa
Contents of the Ischiorectal fossa: 
Ischiorectal pad of fat.
Inferior rectal nerve and vessels.
Posterior scrotal / labial nerve and vessels.
Pudendal canal and its contents.
Perineal branch of fourth sacral nerve.
Perforating cutaneous branch of S2 , S3 nerve .

18. Pudendal Canal
 It is a fascial tunnel present in the lower part of the lateral wall of the Ischiorectal fossa , just above the sacrotuberous ligament .
Pudendal canal is formed by splitting of the fascia lunata.
The fascial wall of the canal is fused laterally to the Obturator fascia, medially to the perineal fascia and inferiorly with the sacrotuberous ligament.
It contains pudendal nerve and the internal pudendal vessels.