Since, as everyone admits, delivery occurs with conspicuous regularity about the end of the fortieth week of pregnancy, and pregnancy corresponds, therefore, to ten menstrual cycles, some have been led to believe that labor and menstruation have a common basis. The truth of this supposition, however, must be doubtful until we know the cause of menstruation. Yet it is a matter of common observation that the uterus becomes unusually irritable about the time when the tenth menstrual period would be due. Strong purgatives administered with other drugs on or after the calculated date frequently bring about delivery, whereas previous attempts of this kind prove unsuccessful. To account for this peculiar irritability of the uterus about the fortieth-week of pregnancy, microscopical changes in its tissues have been suggested but sought in vain. Nor will the distention of the organ explain it. A great many theories have been offered to explain the causation of labor, but they have now only an historical interest. To-day we are just beginning to learn the correct methods of studying the problem. The experience of ages has firmly established the fact that the fetus is expelled when ready to enter the world, or as we say, when it has become mature. But how does the fetus assert its maturity? There is the kernel of the matter; that is the real problem, a problem for the solution of which, happily, we possess better facilities than have heretofore existed. One solution that has been suggested assumes that the fetus loses ultimately its power to assimilate the nourishment provided through the mother's blood. In consequence, it is argued, the material which previously enabled the fetus to grow now collects-- in the maternal circulation, stimulating the womb to contract. A part of this explanation, namely, that the material which stimulates the muscle fibers, whatever it may be, is a chemical substance and that it circulates in the mother's blood, is almost certainly true. There are, however, very weighty reasons for believing that this substance has not the character of food. A more plausible supposition is that the fetus produces this material in the course of its natural living processes, and the substance would accordingly be a waste-product. THE COURSE OF LABOR.--The current view that labor begins in the early evening and generally ends during the night is incorrect. This impression has grown out of the fact that the whole process frequently consumes twelve hours and must in such an event include some part of the night. Statistical evidence indicates that almost as many births occur at one hour of the twenty-four as another; to be precise, only five per cent. more children are born between 6 P.M. and 6 A.M. than between 6 A.M. and 6 P.M.