These facts already indicate a radical difference between the nature of the cerebral actions involved in emotion and in thought. From them also we should infer in all cases where vaso-motor paralysis was apparently traceable to excess of cerebral activity, either _that exhaustion had already occurred, or that the activity was not intellectual but emotional_. In the first case, we should be immediately brought to our fourth possible condition for uterine haemorrhage, dependent on modifications of the cerebro-spinal system. It is admitted, as the result of many experiments and pathological observations that need not here be quoted, that exhaustion of certain parts of the brain and spinal cord may induce vaso-motor paralysis, and that, if a cause for haemorrhage is already in operation, a passive flow of blood may be indefinitely increased. Such a course is the menstrual crisis, without which even the vaso-motor paralysis is usually unable to determine uterine haemorrhage.[40] In connection with it, physical exercise, pushed to the point of exhausting the spinal cord, and the peculiar centre in its lumbar portion, or mental effort so excessive and prolonged as to exhaust the brain, and the general vaso-motor centre, might become causes of menorrhagia. It is evident, however, that if such exhaustion had been produced previous to the menstrual epoch, the effect would be precisely the same as if the morbific causes operated only at the time of menstruation. From this point of view the precaution suggested by Dr. Clarke, of intermitting intellectual effort during the menstrual period, would be inadequate whenever it was not superfluous. But in Dr. Clarke's theory this period has a peculiar influence in rendering morbific conditions that at other times are innocuous. This, in virtue of the law already quoted, that the evolution of force at one centre of the nervous system is incompatible with an evolution of equal intensity at another, since it diminishes the sum of resources distributed to the nervous system as a whole. Hence, relatively to the amount of power left in the brain, the same exertion becomes very much more fatiguing, and may easily lead to exhaustion with all its consequences.