This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The incision is begun to the left of the median line 3 cm. (11/4 in.) in front of the anus and carried outward and back midway between the anus and tuberosity of the ischium. The knife is pushed steadily on until it enters the groove in the staff and thence backward into the bladder. The artery to the bulb is to be avoided by not going more than 3 cm. in front of the anus. The rectum is to be avoided by having it empty, by hooking the staff in the urethra well up to the pubic arch, thus drawing the urethra up, and by inclining the knife obliquely outward. The internal pudic artery is to be avoided by keeping away from the ramus of the ischium. Too free an incision of the prostate is bad because urinary infiltration is liable to occur in the pelvic fascia, also an accessory pudic artery, which if present may run along the side of the prostate, may thus be wounded. Usually the bleeding is slight and comes from the division of the superficial transverse perineal and branches of the inferior hemorrhoidal arteries and the prostatic plexus of veins. (For Perineal Prostatectomy see page 450 and Seminal Vesicles page 452).
The ischiorectal fossa is wedge-shaped, its base, extending between the tuberosity of the ischium and the anus, is about 2.5 cm. (1 in.) in breadth, and its apex extends up 5 to 7.5 cm. (2 to 3 in.), to the junction of the levator ani and internal obturator muscles. Its inner wall is formed by the levator ani and coccygeus muscles and its outer wall by the obturator internus muscle. Its deepest extreme posterior portion constitutes the posterior recess. This communicates superficially, beneath the coccygeal attachment of the external sphincter, with the fossa of the opposite side (see Fig. 475, page 473).
The anterior recess (pubic, Waldeyer) runs forward between the prostate gland internally and the ischiopubic ramus externally; the deep and superficial transverse perinei muscles and the deep layer of the triangular ligament are superficial to it.
The internal pudic vessels and pudic nerve lie on the internal obturator muscle and ramus of the ischium in a fibrous canal formed by the obturator fascia. It is called Alcock's canal and is 4 cm. (1 1/2 in.) above the tuberosity.
The inferior hemorrhoidal vessels and nerves enter the ischiorectal fossa at its posterior and outer side and run on the surface of the levator ani muscle toward the anus. The superficial perineal vessels and nerves enter the fossa anteriorly and immediately pierce the posterior edge of the superficial perineal (Colles's) fascia to supply the structures between it and the superficial layer of the triangular ligament.
The principal affection of the ischiorectal fossa is abscess. This is probably started by violence and infected from the rectum. It commonly tends to point through the skin or open into the rectum. On account of its tendency to burrow it is to be opened early. This is done by making an incision of ample size through the skin and then opening the abscess by blunt dissection in order to empty all pockets. Bleeding is usually slight because the vessels lie deep and escape being wounded. Should the abscess not break externally it may do so internally. If superficial it pierces the anal canal between the external and internal sphincters and makes an opening at about the white line. If it is very deep it may open into the ampulla of the rectum above the internal sphincter (see page 443) It is more common for pus to burrow down into the ischiorectal space through the levator ani than it is for it to burrow up from the ischiorectal fossa (Tuttle). Therefore in extensive ischiorectal abscesses communicating with the interior of the pelvis one should look for the origin of the trouble above. An abscess on one side is liable to be followed by one on the other and pus quite commonly crosses the median line posterior to the anus.