o contract in painful
cramps. The small blood-vessels are themselves provided with circular
muscular fibres, whose contraction necessarily draws the walls of the
vessels together, obliterates their canal, and shuts out the blood. This
contraction is effected by stimulation of the fine nerves, called
vaso-motor, that are distributed to these muscular fibres, and which are
derived from the sympathetic ganglia, that form part of that same
ganglionic system from which the nerves of the ovaries and other viscera
are supplied. The utero-ovarian blood-vessels derive their nerves from
the hypogastric plexus, which, formed by branches from both sympathetic
ganglia and spinal cord, is the exclusive source of the innervation of
the uterus and ovaries. The ganglionic nervous excitement coincident
with the maturation of the ovule and the congestion of the uterus, is
easily communicated to the vaso-motor nerves of the latter organ. At the
very moment, therefore, that the uterine blood-vessels are dilated, and
blood is being exhaled into the uterine cavity, an excessive stimulation
of the vaso-motor nerves may cause the blood-vessels to contract; the
flow is then temporarily arrested, the circulation in the uterus
disturbed, and its muscular fibres thrown into cramps.

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 excess