arious
mammals, such as the congested conjunctiva of the rabbit's eye
and the drooping ears of the pig. Many monkeys exhibit congestion
of the face and nipples, as well as of the buttocks, thighs, and
neighboring parts; sometimes they are congested to a very marked
extent, and in some species a swelling, occasionally prodigious,
of the soft tissues round the anal and generative openings, which
is also at the time brilliantly congested, indicates the progress
of the pro-estrum.... The growth of the stroma-tissue [in the
uterus of monkeys during the pro-estrum] is rapidly followed by
an increase in the number and size of the vessels of the stroma;
the whole becomes richly supplied with blood, and the surface is
flushed and highly vascular. This process goes on until the whole
of the internal stroma becomes tense and brilliantly injected
with blood.... In all essential points the menstruation or
pro-estrum of the human female is identical with that of
monkeys.... Estrus is possible only after the changes due to
pro-estrum have taken place in the uterus. A wave of disturbance,
at first evident in the external generative organs, extends to
the uterus, and after the various phases of pro-estrum have been
gone through in that organ, and the excitement there is
subsiding, it would seem as if the external organs gain renewed
stimulus, and it is then that estrus takes place.... In all
animals which have been investigated coition is not allowed by
the female until some time after the swelling and congestion of
the vulva and surrounding tissue are first demonstrated, and in
those animals which suffer from a considerable discharge of blood
the main portion of that discharge, if not the whole of it, will
be evacuated before sexual intercourse is allowed." (W. Heape,
"The 'Sexual Season' of Mammals," _Quarterly Journal of
Microscopical Science_, vol. xliv, Part I, 1900. Estrus has since
been fully discussed in Marshall's _Physiology of Reproduction_.)
This description clearly brings out the fundamentally vascular
character of the process I have termed "tumescence"; it must be
added, however, that in man the nervous elements in the process
tend to become more conspicuous, and more or less obliterate
these primitive limitations of sexual desire. (See "Sexual
Periodicity" in the first volume