This may be in the form of a headache, or a nasal hæmorrhage, or diarrhoea, or abundant discharge of uric acid, or some other unusual occurrence. I think," he adds, "this is much more common than is ordinarily supposed, and a careful examination or inquiry will generally, if not invariably, establish the existence of a periodicity of the character referred to." Dr. Harry Campbell, in his book on _Differences in the Nervous Organization of Men and Women_, deals fully with the monthly rhythm (pp. 270 et seq.), and devotes a short chapter to the question, "Is the Menstrual Rhythm peculiar to the Female Sex?" He brings forward a few pathological cases indicating such a rhythm, but although he had written a letter to the _Lancet_, asking medical men to supply him with evidence bearing on this question, it can scarcely be said that he has brought forward much evidence of a convincing kind, and such as he has brought forward is purely pathological. He believes, however, that we may accept a monthly cycle in men. "We may," he concludes, "regard the human being--both male and female--as the subject of a monthly pulsation which begins with the beginning of life, and continues till death," menstruation being regarded as a function accidentally ingrafted upon this primordial rhythm. It is not unreasonable to argue that the possibility of such a menstrual cycle is increased, if we can believe that in women, also, the menstrual cycle persists even when its outward manifestations no longer occur. Aëtius said that menstrual changes take place during gestation; in more modern times, Buffon was of the same opinion. Laycock also maintained that menstrual changes take place during pregnancy (_Nervous Diseases of Women_, p. 47). Fliess considers that it is certainly incorrect to assert that the menstrual process is arrested during pregnancy, and he refers to the frequency of monthly epistaxis and other nasal symptoms throughout this period (W. Fliess, _Beziehungen zwischen Nase und Geschlechts-Organen_, pp. 44 et seq.). Beard, who attaches importance to the persistence of a cyclical period in gestation, calls it the muffled striking of the clock. Harry Campbell (_Causation of Disease_, p. 54) has found post-climacteric menstrual rhythm in a fair sprinkling of cases up to the age of sixty.