ained from outside, as in birds, or by both methods
combined or succeeding each other, as in cats.

In many animals the male contributes to the raising of the young; a
point to which we shall return. Here, we indicate these complicated
details simply to show that sexual union only contributes one link in
the long chain of reproduction. Let us study its mechanism in man.

=The Copulatory Organ of Man. The Testicles. The Seminal
Vesicles.=--Nature is often very sparing even in the highest
organizations. It has thus combined in the male the urethra with the
copulatory organ, and the sexual germinal glands, or _testicles_, with
an accessory gland, the _epididymis_. Hundreds of thousands of
spermatozoa are contained in the glandular tubes of these organs,
which, when they are mature can always produce new ones by cell
division. The spermatozoa accumulate at the extremity of the duct of
the gland in a reservoir called the _seminal vesicle_, where they
float in the mucus, thus constituting the seminal fluid or _sperm_.
This liquid has a special odor. The two seminal vesicles are situated
in the abdominal cavity underneath the urinary bladder, each having a
duct which meets that of the other side and opens by the side of it in
the deep part of the urethra. Here the secretion of several other
glands, especially of the _prostate_, is added to the sperm and mixes
with it. The point where the two seminal ducts open into the urethra
forms a small elevation, the _verumontanum_. From this point the male
urethra emerges from the abdominal cavity and is continued along the
special prolongation which forms the penis, or virile member of
copulation. In the ordinary way the penis only serves for the emission
of urine. It hangs flaccid and terminates in a rounded swelling called
the glans, at the end of which opens the urethra (Fig. 18). This
opening serves also for the emission of the sperm.

=Erection. The Corpus Cavernosum.=--The most curious part of this
apparatus is the mechanism of _erection_, or the power possessed by
the penis of swelling under the influence of certain nervous
irritations, increasing in length and diameter as well as becoming
rigid. This phenomenon is produced by three organs called the
_cavernous bodies_ which form the principal bulk of the penis. One of
them, situated in the middle and underneath and formed by two bodies
united into one, surrounds the urethra and terminates in front in a
dilatation which consti