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.
Fistula in ano may start as an ischiorectal abscess which perforates internally into the rectum or anal canal and externally through the skin. It may also start as an ulcer of the mucous membrane of the rectum or crypts of Morgagni and then produce an ischiorectal abscess which finally opens on the skin. The most common site of the internal opening is just above the anus and below the insertion of the levator ani. This is in the groove between the external and internal sphincters. Sometimes, however, the fistula pierces the levator ani and opens into the ampulla of the rectum. As the external opening is usually to the outer side of the external sphincter this latter is divided in operating, as is also a part or all of the internal sphincter if the opening is high up. Incontinence of faeces is usually avoided if the sphincter is only divided at one place and at right angles to its fibres, not obliquely.
Fig. 448. - Hemorrhoids.
The rectum can be removed either by the perineal or sacral route. In the perineal operation the incision is made from near the base of the scrotum to the coccyx, surrounding the anus. If the incision is made near the white line the external sphincter is saved and turned to each side with the skin flap. The external sphincter is split anteriorly as far as the central point of the perineum and posteriorly to the coccyx. The rectum being drawn forward the levator ani muscle is cut through on its sides and posterior surface about 4 cm. (1 1/2 in.) above the anus, the coccyx, if necessary, being excised. The rectum is then drawn back, the finger slipped beneath the anterior portion of the levator ani, which is farther from the surface than the posterior, and it is divided. These fibres practically constitute the recto-urethralis muscle of Proust. This is near the apex of the prostate; from here up to the peritoneal reflection or rectovesical pouch the rectum is loosely attached but at that point it is necessary to divide the rectal fascia (a part of the rectovesical fascia, p. 435) on the sides, after which the rectum can be drawn still further down. If it is desired to go still higher the peritoneum may be pushed up off the rectum or it may be opened and the mesorectum detached close to the sacrum so as not to injure its vessels. The detached rectum is then brought down, cut off, and its divided end sutured to the skin.
In approaching the rectum by the sacral route an incision is made across the sacrum opposite the third sacral segment and from its right extremity (Tuttle) down to beyond the tip of the coccyx. The bone is chiselled through opposite the fourth sacral foramina and the flap turned down (Fig. 449). The lateral and middle sacral arteries may have to be ligated. The peritoneum, which is visible in the upper portion of the wound, may then be incised close to the rectum to avoid wounding the ureters, and the mesorectum detached close to the sacrum. This loosens the rectum, which can then be brought out and the opening in the peritoneum sewed shut. As much of the rectum as is desired is removed and the cut ends united by a Murphy button or end-to-end suture.
Fig. 449. - Excision of the rectum. The sacrum has been divided and turned aside. The rectum is drawn to the left, exposing the ureter and vas deferens and seminal vesicle, and the peritoneal cavity has been opened above.
In carcinoma enlarged lymph-nodes may be found in the mesorectum or hollow of the sacrum and should of course be removed.