semen

semen consists of sperm from the testes, and seminal fluid secreted by the accessory glands of the male reproductive tract; these include the seminal vesicles, prostate gland, and the bulbo-urethral glands (known as ‘Cowper's glands’ after a seventeenth-century French anatomical plagiarist who was not in fact the first to describe them). The seminal fluid secreted by these glands provides a medium in which sperm can be carried to the female reproductive tract, and may provide nutritional and protective support, but we know that seminal fluid is not essential for fertilization, because sperm taken directly from the vas deferens is capable of fertilization in its absence.

Semen is produced during the complex genital reflexes which culminate in ejaculation. Upon sexual stimulation, the muscular wall of each vas deferens contracts, pushing sperm towards its widened end, the ampulla, where it joins with the ducts from the seminal vesicles to form the ejaculatory duct. This duct then passes into the prostate gland, which lies below the exit of the bladder, and joins with the urethra — the final common tube for the excretion of urine and the passage of semen. The small, pea-sized bulbourethral glands lie on either side of the base of the prostate gland, and their ducts open into the urethra. As the sperm is pushed from the two vasa deferentia into the urethra during sexual stimulation (the emission phase), simultaneous contractions of the accessory glands add fluid to the sperm.

The bulk of seminal fluid is secreted by the seminal vesicles, which were previously thought to store sperm (storage is now known to be at the end of the long coiled tube of the epididymis in the testis). These two glands release a viscid, yellowish fluid into the ejaculatory duct. The seminal secretion from the prostate gland is a thin, milky fluid, which is responsible for the characteristic smell of semen, while the bulbo-urethral glands secrete mucus, which acts as a lubricant prior to ejaculation. In man the volume of semen produced at each ejaculation is about 3–4 ml, containing about 300 million sperm. This is a very conservative volume compared with the wild boar which can produce up to half a litre (500 ml) of semen at ejaculation.

Saffron Whitehead

See also coitus, ejaculation; sperm; testes.