ays."[86] Among the naked women of Tierra del Fuego, it
is said that there is often no physical sign of the menses for six months
at a time. These observations are noteworthy, though they clearly
indicate, on the whole, that primitiveness in race is a very powerless
factor without a cold climate. On the other hand, again, there is some
reason to suppose that in Europe there is a latent tendency in some women
for the menstrual cycle to split up further into two cycles, by the
appearance of a latent minor climax in the middle of the monthly interval.
I allude to the phenomenon usually called _Mittelschmerz_, middle period,
or intermenstrual pain.

Since the investigations of Goodman, Stephenson, Van Ott, Reinl,
Jacobi, and others, it has been generally recognized that
menstruation is a continuous process, the flow being merely the
climax of a menstrual cycle, a physiological wave which is in
constant flux or reflux. This cycle manifests itself in all a
woman's activities, in metabolism, respiration, temperature,
etc., as well as on the nervous and psychic side. The healthier
the woman is, the less conscious is the cyclic return of her
life, but the cycle may be traced (as Hegar has found) even
before puberty takes place, while Salerni has found that even in
amenorrhoea the menstrual cycle still manifests itself in the
temperature and respiration. (_Rivista Sperimentale di
Freniatria_, XXX, fasc. 2-3.)

For a summary of the phenomena of the menstrual cycle, see
Havelock Ellis, _Man and Woman_, fourth ed., revised and
enlarged, Ch. XI; "The Functional Periodicity of Women." Cf.
Keller, _Archives Generales de Medecine_, May, 1897; Hegar,
_Allgemeine Zeitschrift fuer Psychiatrie_, 1901, Heft 2 and 3;
Helen MacMurchy, _Lancet_, Oct. 5. 1901; A.E. Giles,
_Transactions Obstetrical Society London_, vol. xxxix, p. 115,
etc.

_Mittelschmerz_ is a condition of pain occurring about the middle
of the intermenstrual period, either alone or accompanied by a
slight sanguineous discharge, or, more frequently, a
non-sanguineous discharge. (In a case described by Van Voornveld,
the manifestation was confined to a regularly occurring rise of
temperature.) The phenomenon varies, but seems usually to occur
about the fourteenth day, and to last two or three days. Laycock,
in 1840 (_Nervous Diseases of Women_, p. 46), gave inst