But it is true that the stand-point in Dr. Clarke's book is somewhat different from this. He scarcely alludes to the presence of pain in menstruation, because this is presumed, when existing, to itself constitute a sufficient warning against over mental exertion, indeed, to render such exertion impossible. But the warning in question is directed against a more insidious accident, that may occur without pain, and which is more easily and imprudently defied. This imminent danger is haemorrhage, or an increase of the physiological flow to such an extent that the vitality of the patient is drained as from an open vein. The constant repetition of such haemorrhage may lead to uterine congestions, or even to amenorrhea, _i.e._, entire absence of menstruation. But it originates in functional disturbance, in exhaustion of the nervous system by intellectual exertion. On account of the imminence of this danger, the period of real incapacity for mental effort lasts much longer than conscious discomfort is likely to do--lasts, indeed, as long as the physiological afflux of blood to the uterus--which, by the means described, may at any moment become excessive. Dr. Clarke alleges but one kind of proof of this assertion. He relates a certain number of cases, interesting in themselves, but whose histories are lacking in many important details, where healthy girls, whose menstruation was at first perfectly normal, became, after two or three years' study at school, liable to monthly haemorrhages, so excessive that their health was completely undermined. No organic cause for such disorder could be discovered. By interruption of study, rest, amusement, travel, the haemorrhages were diminished, the health restored. In several of these cases, however, resumption of study on the old plan was followed by the immediate return of all the previous accidents, and often the constitution was entirely ruined. We think that this argument might be exactly paralleled by the following, which should prove whisky drinking to be an efficient[37] cause of yellow fever. A physician might select twenty cases of men, personally known to him, who had lived twenty and thirty years in New York or Boston, and never had yellow fever. During this time they had taken little or no whisky, but afterwards, removing to New Orleans, they fell into the habit of drinking, and, at varying intervals from that date, caught the fever, and in many instances, died. Therefore, fever was due, at least in these cases, to the newly contracted habit of drinking whisky. -A and -B = -C -A +B = C. Therefore, C = A.