The second or destructive stage is marked by destructive changes which give rise to the usual phenomena of the menstrual period; there is a discharge of blood, mucus, and disintegrated mucous membrane.  The actively growing cells of the uterine lining membrane undergo rapid destructive changes, the fabric of the half-formed decidua tumbles to pieces, the turgid capillaries burst and pour out the menstrual flow, which sweeps away all the useless debris.  The irritation sets up reflex uterine contractions, and so the blood is squeezed out of the distended capillaries and washes away the degenerated cells.

The third or reparative stage, as its name indicates, is one of repair, in which by constructive changes the epithelial lining which was thrown off is replaced by new, which is formed in from three to four days.

The fourth or quiescent stage includes the remaining twelve or fourteen days of the menstrual cycle, and represents the quiescent period prior to the initiative changes which mark the beginning of the next period.

Average Duration of the Menstrual Flow.—­ The average duration of the menstrual flow is five days, although the variations are considerable in healthy women.  A flow lasting any place from two to six days is perfectly consistent with health; but a flow continuing less than two or more than six days generally indicates local or general disease.

Character of the Menstrual Flow.—­ For the first few hours, or perhaps for the first day, the flow is usually slight in quantity and light in color; on the second and third days the flow reaches its height, and is profuse and dark, but it should never be clotted; after this it gradually ceases.  The amount of the flow varies from five to ten ounces.  If less than five or six or more than eighteen napkins are pretty well saturated through, the amount may be considered abnormal.

Relation of Ovulation to Menstruation.—­ It has not yet been decided just in what relation the processes of ovulation and menstruation stand to each other.  It is supposed that the transit of the ovum to the uterus occupies at least one week.  It has been thought that the decidua of a particular menstrual period is related, not to the ovum discharged at that period, but to the ovum discharged at the preceding period.

The menstrual wave, or the wave of “supplementary nutrition,"[*] upon which the menstrual process ultimately depends, was first established by Dr. Mary Putnam Jacobi in the Boylston prize essay for 1876; showing that menstrual life is associated with a wave of well-marked vital energy, which manifests itself in a monthly fluctuation of the tempera ture of the body, in the daily amount of the excretion of urea and of carbonic acid, and of the rate and tension of the pulse.  The wave attains its maximum during the week preceding menstruation, and slowly falls to its minimum, which is reached the week after menstruation.

* Dr. Goodman and Dr. Stephenson have since written on this subject, and the “wave” is often known as the Stephenson wave.