The Reproductive System 459
Acrosomal (head) capAcrosome
HeadCondensed
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NeckMitochondriaMiddle pieceFlagellum (^)
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Figure 19- 4 The anatomy of a sperm cell.
The epididymis is a comma-shaped structure located
along the posterior border of a testis, consisting principally
of a tightly coiled tube called the ductus epididymis. The
epididymis is the site where the sperm cells continue to
mature. The tube is about 20 feet long and it takes the
sperm about 20 days to move through this tube. The tube
stores the maturing sperm cells while they develop their
swimming capability via their flagella. The tube propels the
sperm toward the next part of the system during
ejaculation, when the smooth muscle of the wall of the tube
contracts by peristalsis.
The next part of the duct system is the ductus deferens
(DUCK-tus DEF-er-enz) or the vas deferens. Within the
tail of the epididymis, the ductus epididymis becomes less and
less convoluted. At this point, it is now referred to as the
ductus or vas deferens. It is also commonly called the seminal
duct. It is approximately 18 inches long and ascends the
posterior border of the testis, penetrates the inguinal canal, and
enters the pelvic cavity where it loops over the side and down
the posterior surface of the urinary bladder (see Figure 19-1).
The tube is enclosed in a connective tissue sheath along with
nerves and blood vessels. This sheath is called the spermatic
cord. The end
of the ductus or vas deferens has a dilated terminal portion
known as the ampulla. Each ductus deferens empties into
its ejaculatory duct, the next part of the system. When a
vasectomy (vas-EK-toh-mee) is performed for birth
control, the physician performs this minor operation by
making an incision into the scrotal sac to cut or cauterize
the ductus or vas deferens. The testes still produce sperm
but they are now unable to make their way to the outside of
the system. They eventually die and are reabsorbed by the
body. A man becomes sterile by this procedure, but fluids
are still produced and are ejaculated during intercourse.
The male libido is not affected, erections and ejaculations
still occur, but there are no sperm in the semen. In addition,
because testosterone is still being produced, all the
secondary male sex characteristics are maintained.
The next part of the tube is the ejaculatory duct.
Pos-terior to the urinary bladder, each ductus deferens joins
its ejaculatory duct (see Figure 19-1). Each duct is about 1
inch long. The ducts eject spermatozoa into the urethra.
The urethra is the terminal duct of the system. It serves as a
common passageway for both spermatozoa coming from
the testes and urine coming from the bladder.