tutes the glans already mentioned. The two
others are situated symmetrically on the dorsal part of the penis. All
three consist of caverns or diverticula formed by blood-vessels, which
are empty when the penis is flaccid. By a complex nervous mechanism
based on vascular paralysis due to nervous phenomena called inhibition
and dynamogeny, the nervous irritations cause an accumulation of blood
in the spaces of the cavernous bodies which become so gorged with
blood as to form stiff and hard rods. The size of the penis is thereby
increased considerably and its stiffness allows it to penetrate the
vagina of the female. At the same time and by the same mechanism the
verumontanum swells so as to close the ureter from the bladder, while
the seminal ducts open toward the urethral orifice. In this way the
copulatory organ is ready for its function.

Repeated irritations are however necessary to provoke the ejaculation
of semen. This is finally produced by excitation of a special muscle
which compresses the seminal vesicles in a spasmodic manner and
ejaculates the semen by the urethra. After ejaculation, the
accumulation of blood in the cavernous bodies gradually diminishes and
the penis again becomes flaccid.

This apparatus is thus very complicated and is put in action by
several nervous irritations which may be disturbed in many ways in
affections of the nervous system. We may observe here that the nervous
centers of erection and ejaculation may be put in action directly by
the brain, or indirectly by peripheral irritation of the glans.

Those peripheral nerves which provoke sexual excitation are especially
the nerves of the glans. This possesses a skin or mucous membrane
which is extremely delicate and is protected against external
irritation by a fold of skin called the _prepuce_, or foreskin. The
prepuce is often too narrow so that it cannot be withdrawn behind the
glans. It then forms a pocket in which sebaceous matter, semen, urine,
etc., accumulate and decompose. This anomaly, called _phimosis_, does
not exist among the Jews owing to circumcision, or the removal of the
prepuce in the newly born, which forms part of their religious rites.
Hygienic considerations sometimes oblige us to perform this operation
in others. The bad habit of masturbation, so common in boys, is often
provoked by phimosis, and shows that simple mechanical irritation of
the glans, due here to secretions contained in the prepuce, may lead
to ejacu