The penis is made up of two
cylinder-shaped vascular bodies called the corpora cavernosa.
The urethra is the tube that is used for both ejaculation and
urination.
The urethra is surrounded by erectile tissue as well as arteries,
veins and nerves.
The long part of the penis is called the shaft, at the end
is the head or glans penis. The meatus is the opening.
The process of erection is now more fully understood:
- An erection starts in the brain. Stimulation - either psychological
or physical causes the nerves to send messages to the vascular
system and these in turn cause an increase in blood flow to the
penis. The arteries in the penis supply blood to the erectile
tissue and the corpora cavernosa. These expand in response to
the increase in blood flow and pressure.
- Blood flow must stay in the penis in order to maintain its
rigidity. The erectile tissue is surrounded by elastic sheathes
called the tunicae. These prevent blood from leaving the penis
during erection.
- After ejaculation, or a reduction in stimulus, the pressure
in the penis decreases, the blood is released and the penis returns
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