Or the opposite event may occur. As the stimulation of the vaso-motor nerves causes contraction of the blood-vessels, so their exhaustion or paralysis causes relaxation of these same vessels, consequently, over-distension with blood; and, if the door to haemorrhage be once opened by the existence of the menstrual nisus, an excessive flow of blood.[39] Such vaso-motor paralysis may depend on one of three circumstances: 1st. The original stimulus may be excessive, and hence necessarily followed by reaction. 2nd. Schiff has shown that galvanization of a cerebro-spinal nerve causes a dilatation of the blood-vessels in the vicinity, as if the vaso-motor force were overpowered by the excessive stimulation of the controlling nerves. If excessive action of the brain or spinal cord be analogous in its effects to galvanism of a spinal nerve, it might be supposed to cause vaso-motor paralysis and haemorrhage. 3d. In general exhaustion of the nervous system, both of its ganglionic and cerebro-spinal apparatus, the vaso-motor nerves suffer with the rest, and the blood-vessels lose their tone in consequence. It is to such exhaustion that Dr. Clarke especially attributes excessive uterine haemorrhage in young girls, and, as already said, he refers the exhaustion to a single cause, namely, to the attempt to impose on the nervous system two actions of equal intensity, contrary to the fundamental law that an intense evolution of nerve-force in one part of the organism necessitates repose in the remainder. Independently of the three conditions where excessive menstruation is connected with vaso-motor paralysis, a fourth may be found directly in the excitement of the ovarian plexus of nerves. This evolution of nerve-force which accompanies the maturation of the ovule, is the immediate cause of the afflux of blood to the utero-ovarian vessels. The effect upon the latter is probably due to the spinal nerve-fibres contained in the plexus, and upon which the ganglionic excitement acts like the galvanism in Schiff's experiment, already described. Direct stimulation of the vaso-motor nerves, alone, as has been said, contracts the blood-vessels. Stimulation of the spinal fibres associated with them exercises the contrary effect. An excessive stimulation of those fibres which enter into the ganglionic masses, would have an effect similar to that of excessive stimulation directly addressed to the cerebro-spinal system, and the blood-vessels would be not only dilated, but paralyzed.