In order to grasp it we have to break it up into its component parts.
Lloyd Morgan has pointed out that the components of an instinct may be
regarded as four: first, the internal messages giving rise to the impulse;
secondly, the external stimuli which co-operate with the impulse to affect
the nervous centers; thirdly, the active response due to the co-ordinate
outgoing discharges; and, fourthly, the message from the organs concerned
in the behavior by which the central nervous system is further
affected.[1]

In dealing with the sexual instinct the first two factors are those which
we have most fully to discuss. With the external stimuli we shall be
concerned in a future volume (IV). We may here confine ourselves mainly to
the first factor: the nature of the internal messages which prompt the
sexual act. We may, in other words, attempt to analyze the _sexual
impulse_.

The first definition of the sexual impulse we meet with is that which
regards it as an impulse of evacuation. The psychological element is thus
reduced to a minimum. It is true that, especially in early life, the
emotions caused by forced repression of the excretions are frequently
massive or acute in the highest degree, and the joy of relief
correspondingly great. But in adult life, on most occasions, these desires
can be largely pushed into the background of consciousness, partly by
training, partly by the fact that involuntary muscular activity is less
imperative in adult life; so that the ideal element in connection with the
ordinary excretions is almost a negligible quantity. The evacuation theory
of the sexual instinct is, however, that which has most popular vogue, and
the cynic delights to express it in crude language. It is the view that
appeals to the criminal mind, and in the slang of French criminals the
brothel is _le cloaque_. It was also the view implicitly accepted by