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 apex rests on the posterior layer of the triangular ligament I to 2 cm. (1/2 to 3/4 in.) behind and a little below the subpubic angle and just inside the upper end of the anal canal. This is about 3 to 4 cm. (1 1/4 to 1 1/2 in.) above the white line of Hilton and the prostate is immediately felt by the finger as soon as it enters the rectum. The prostate lies on the rectum, so that it is readily accessible. Its apex being about 3 cm. (1 1/4 in.) from the mucocutaneous white line, its upper edge would be 6 cm. (2 1/2 in.) and the rectovesical pouch 8.75 cm. (3 1/2 in.) above this line. Thus all these structures are usually within reach of the finger. In the median line, extending to each side, the vasa deferentia and seminal vesicles, if diseased, as they sometimes are in tuberculous affections, can readily be felt, but when healthy are too soft to be easily distinguished. On each side is the levator ani muscle, which embraces the prostate as far forward as the membranous urethra, where it practically blends with the deep transverse perineal and compressor urethrae muscles (see recto-urethralis muscle - Perineum, page 475).
The greater portion of the prostate is composed of unstriped muscular tissue, which is not only arranged peripherally but sends prolongations inward, forming spaces in which the glandular tissue is lodged. There is also a layer surrounding the vesical opening of the urethra. The action of this latter muscle is probably to act as a true sphincter to retain the urine in the bladder. It also by its contraction prevents the regurgitation of the semen into the bladder.
In the urethral and vesical portions of the prostate are numerous veins. These in the old become varicose, hence the frequency of bleeding in old prostatic cases. Around the anterior portion of the prostate and laterally posteriorly lies the prostatic venous plexus. Into it anteriorly empties the dorsal vein of the penis; from above it receives the vesical veins, and in those advanced in age it communicates also with the hemorrhoidal plexus posteriorly. Fenwick has shown (Jour, of Anat. xix. 1885) that in the young these veins possess valves which become incompetent as age supervenes. The prostatic plexus unites in a single large vein on each side which empties into the internal iliac vein.
This is the most common affection of the prostate. According to Mansell Moullin it always begins in the glandular elements. It is of two kinds, fibrous and glandular. Both start as glandular but the former in some cases predominates and the glandular element atrophies and leaves a comparatively small hard fibrous prostate. The glandular character of median growths has already been explained on page 449 as originating from the prespermatic and subcervical groups of Albarran.
Glandular hypertrophy of the lateral lobes forms the ordinary large prostates for which prostatectomy is performed. The bleeding, which is so common in these cases of enlarged prostate, is due to the varicose condition of the veins around the posterior portion of the urethra and vesical mucous membrane.