t these organs have in addition another specific function,
that of internal secretion. Whilst the spermatogonia become transformed
into spermatozoa, other cellular structures of the testicle, more
especially the interstitial cells, produce, it is assumed, the internal
secretion of the gland. The constituents of this internal secretion,
having been poured into the general circulation, are supposed to give
rise to the specific masculine sexual development, and, in particular,
to lead to the appearance of the secondary sexual characters. This
matter will subsequently be discussed in detail, and here I shall merely
add that perhaps none of the proper constituents of the internal
secretion find their way into the external secretion of the testicle.

This external secretion of the testicles does, however, receive the
admixture of a number of other secretions, to constitute the semen as
actually discharged, viz., the secretion of the prostate gland, that of
the seminal vesicles, Cowper's glands, and the glands of the vasa
deferentia, and perhaps also that of the glands of Littre. The term
semen is, indeed, often applied to the secretion of the testicles alone;
but to avoid misunderstanding, Fuerbringer[8] recommends that only the
mixed secretion, as actually discharged, should be spoken of as the
semen, and that this term should never be employed to denote the
testicular secretion alone.

In what has gone before, I have not only described the structure of the
male sexual organs, but have alluded also in passing to their functions.
These latter must, however, be described more fully. Let us begin with
_erection_, which, as we saw, is due to distension of the penis with
blood. How is this distension brought about? It results from stimulation
of the erection centre. Until recently, it was supposed that this centre
was situated in the lumbar enlargement of the spinal cord; but now,
owing to the researches of L. R. Mueller, it is believed to form part of
the sympathetic plexuses of the pelvis. Stimulation of the centre leads
to distension of the penis with blood, and thus to erection of that
organ. The stimulation of the centre can be effected in either of two
ways.

In the first place, by psychical processes. Thus, in a man, the sight of
a woman exercises such a stimulus, the stimulation proceeding from the
brain along the spinal cord to reach the centre. The psychical stimulus
may also consist of reminiscences. In this way the