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semen information
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semen information
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 deferential 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
liter (500 ml) of semen at ejaculation.
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