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The Male Reproductive System / The Male Reproductive System
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The prostate, seminal vesicles, and bulbourethral glands
 These structures are the male accessory reproductive organs and provide secretions to form the bulk of the seminal fluid of an ejaculate. The prostate is in the lesser or true pelvis, centred behind the lower part of the pubic arch. It lies in front of the rectum. The prostate is shaped roughly like an inverted pyramid; its base is directed upward and is immediately continuous with the neck of the urinary bladder. The urethra traverses its substance. The two ejaculatory ducts (see below) enter the prostate near the upper border of its posterior surface. The prostate is of a firm consistency, surrounded by a capsule of fibrous tissue and smooth muscle. It measures about four centimetres across, three centimetres in height, and two centimetres front to back (about 1.6 by 1.2 by 0.8 inch) and consists of glandular tissue contained in a muscular framework. It is imperfectly divided into three lobes. Two lobes at the side form the main mass and are continuous behind the urethra. In front of the urethra they are connected by an isthmus of fibromuscular tissue devoid of glands. The third, or median, lobe is smaller and variable in size and may lack glandular tissue. There are three clinically significant concentric zones of prostatic glandular tissue about the urethra. A group of short glands that are closest to the urethra and discharge mucus into its channel are subject to simple enlargement. Outside these is a ring of submucosal glands (glands from which the mucosal glands develop), and farther out is a large outer zone of long branched glands, composing the bulk of the glandular tissue. Cancer of the prostate is almost exclusively confined to the outer zone. The glands of the outer zone are lined by tall columnar cells that secrete prostatic fluid under the influence of androgens from the testis. The fluid is thin, milky, and slightly acid.
 The seminal vesicles are two structures, about five centimetres (two inches) in length, lying between the rectum and the base of the bladder. Their secretions form the bulk of the seminal fluid. Essentially, each vesicle consists of a much-coiled tube with numerous diverticula or outpouches that extend from the main tube, the whole being held together by connective tissue. At its lower end the tube is constricted to form a straight duct or tube that joins with the corresponding ductus deferens to form the ejaculatory duct. The vesicles are close together in their lower parts, but they are separated above where they lie close to the deferent ducts. The seminal vesicles have longitudinal and circular layers of smooth muscle, and their cavities are lined with mucous membrane, which is the source of the secretions of the organs. These secretions are ejected by muscular contraction during ejaculation. The activity of the vesicles is dependent on the production of the hormone androgen by the testes; castration causes atrophy of the seminal vesicles. The secretion is thick, sticky, and yellowish: it contains the sugar fructose and is slightly alkaline.
The bulbourethral glands, often called Cowper's glands, lie on the underside of the urethra between the prostate and the bulk of the penis. They are hardly larger than a pea. Each has a slender duct that runs forward and toward the centre to open on the floor of the spongy portion of the urethra. These glands are poorly developed in man. Their secretion is liberated during sexual excitement and may help to lubricate and coat the urethra to assist passage of the ejaculate.
The two ejaculatory ducts lie on each side of the midline and are formed by the union of the duct of the seminal vesicle, which contributes secretions to the seminal fluid, with the end of the ductus deferens at the base of the prostate. Each duct is about two centimetres (about 0.8 inch) long and passes between a lateral and the median lobe of the prostate to reach the floor of the prostatic urethra. This part of the urethra has on its floor (or posterior wall) a longitudinal ridge called the urethral crest. On each side is a depression, the prostatic sinus, into which open the prostatic ducts. In the middle of the urethral crest is a small elevation, the colliculus seminalis, on which the opening of the prostatic utricle is found. The prostatic utricle is a short diverticulum or pouch lined by mucous membrane; it may correspond to the vagina or uterus in the female. The small openings of the ejaculatory ducts lie on each side of or just within the opening of the prostatic utricle. The ejaculatory ducts are thin-walled and lined by columnar cells.
The following reflexes may be active when there is malfunction of the male reproductive system.
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