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 posterior portion of the rectum has no peritoneal covering, the mesosigmoid ceasing opposite the third sacral vertebra, about 12.50 cm. (5 in.) from the anus. From this point the peritoneum slopes downward and forward, covering the sides and anterior surface of the rectum 5 cm. (2 in.) lower. The peritoneum is here reflected forward over the bladder in the male forming the rectovesical pouch and over the vagina and uterus in the female forming the pouch of Douglas It is within 7.5 to 8.5 cm. (3 to 3 1/2 in.) of the anus. This leaves 2.5 cm. (1 in. ) or more above the prostate which is not covered by peritoneum. It was through this space that the bladder was formerly tapped with a trocar to relieve it when distended. The peritoneum on the sides is less firmly attached to the rectum and pelvic colon than it is on its anterior surface.
The finger can palpate the anal canal and rectum for a distance of 10 cm. (4 in.) from the surface. Anteriorly as soon as the finger passes the sphincters the apex of the prostate can be felt; also the membranous urethra, particularly if it contains a bougie or sound. The prostate can be outlined and its size determined. If the prostate is not enlarged the base of the bladder above can be palpated and the tip of the finger will reach the rectovesical pouch. From the upper or posterior edge of the prostate and extending from near the midline upward and outward are the seminal vesicles, sometimes the seat of tuberculous disease. Just to the outer side of the upper end of the seminal vesicles are the lower ends of the ureters. Should a ureteral calculus become impacted at this point it might possibly be felt through the rectum. Posteriorly the coccyx and the hollow of the sacrum can be felt. The segments of the coccyx frequently are luxated or fractured and it is the seat of pain - coccygodynia - for which excision is done. These injuries cause either an ankylosis or a deformity of the coccyx which can often readily be detected by a finger internally and the thumb externally. Laterally the finger can explore the region of the spine of the ischium, the sacrosciatic foramina, and the tuberosities. If a patient is placed in the knee-chest position and a speculum is introduced the rectum immediately distends with air and its interior is visible as far as the promontory of the sacrum. By means of extra long tubes even the sigmoid loop can sometimes be seen. The valves of Houston are readily seen through the speculum.
In introducing tubes and bougies for examination or therapeutic purposes the greatest care is necessary, as death has not infrequently resulted from perforation into the peritoneal cavity.
This extends from the rectum to the anus or its opening on the skin, a distance of 2.5 to 4 cm. (1 to 1 1/2 in.). It begins at the level of the levator ani muscles and has the apex of the prostate directly in front of it and the tip of the coccyx behind and a little above. With the body vertical the anal canal has its axis inclining upward and forward toward the bladder; as soon as the sphincter ani is passed the axis of the rectum changes to upward and backward toward the hollow of the sacrum. In intruducing a speculum it should always be inclined first anteriorly and then posteriorly. Opposite the level of the levator ani the circular muscular fibres increase to form the internal sphincter. This extends down the anal canal for a distance of approximately 2.5 cm. (1 in.) and ends above the skin margin or, as it has been called, the "white line of Hilton." The external sphincter surrounds the lower part of the canal and stretches in a spindle shape from the tip of the coccyx to the central point or tendon of the perineum. Anteriorly it blends with the fibres of the levator ani and the other muscles of the perineum. It is a thick, powerful, voluntary muscle and extends outward from the white line of Hilton or mucocutaneous junction.
The upper half of the mucous membrane of the anal canal has six or eight longitudinal ridges or folds called the columns of Morgagni or Glis-son. Between the lower ends of these columns are small hollows called the crypts of Morgagni, and the free edges of the mucous membrane guarding the crypts are the anal valves.