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 rectum extends from the level of the third sacral vertebra to where it pierces the levator ani muscle, 3.7 cm. (1 1/2 in.) in front of the tip of the coccyx, but at a lower level, and opposite the lower and anterior part of the prostate. It is 8.75 cm. (3 1/2 in.) long and passes into the anal canal; this latter is 2.5 to 4 cm. (1 to 1 1/2 in.) long, and extends to the skin border (Fig. 445).
When collapsed the rectum appears as a nearly straight tube following the curve of the sacrum, but when distended it becomes distinctly sacculated It possesses an external longitudinal and internal circular layer of muscular fibres. The longitudinal fibres are continuous with those on the colon but instead of being composed of three bands are fused together into two bands, anterior and posterior. On the sides the longitudinal fibres are not so abundant. The circular fibres are continuous on the anal canal as the internal sphincter. For the distance of 4 cm. (1 1/2 in.) between the tip of the coccyx and its termination, the rectum lies on the two levator ani muscles, which join in the median line. The lower portion of the rectum is larger than the upper and is called the ampulla.
Fig. 445. - Rectum and anal canal.
The anterior surface of the rectum at the ampulla lies against the posterior surface of the prostate but is not intimately adherent to it. At the apex of the prostate the anterior rectal wall makes a more or less sharp turn backward. At this part the rectum and the prostate are embraced by the fibres of the levator ani muscle, which practically blend with the compressor urethrae muscle and surround the membranous urethra. The muscular fibres passing from the longitudinal layer of the rectum to the membranous urethra have been called by Proust the recto-urethralis muscle; they keep the lower extremity of the ampulla of the rectum in close approximation to the apex of the prostate. This is the part of the rectum which has been frequently wounded in the operations of perineal lithotomy and prostatectomy. In the latter operation division of this band allows the rectum to be pushed back and exposes the apex of the prostate.
The sacculation of the rectum is produced by three creases or crescentic folds, called the rectal valves or valves of Houston (Dublin Hospital Reports, 1830). Of these the middle is the largest. It springs from the right anterior quadrant about 5 to 6 cm. (2 to 2 1/4 in.) above the margin of the anal canal. The superior and inferior valves spring from the left posterior quadrant a short distance above and below the middle valve. At the juncture of the rectum and sigmoid flexure there is another fold on the anterior wall which tends to obstruct the view in making examinations. These valves are composed of connective tissue and circular muscular fibres covered with mucous membrane.