It is noticeable that a slighter disturbance of normal conditions is needed to render digestion painful than to cause painful ovulation, that is, pain preceding the menstrual flow. Pain in menstruation, which is much more frequent, is dependent upon other conditions than the activity of the ovaries, and lasts a very much shorter time than does either the function of ovulation, or even than the uterine congestion secondary to it. Outside of actual uterine disease, the pain at this moment is most often dependent on uterine cramp, itself excited by a spasmodic contraction of blood-vessels that interfere with its circulation. As these remarks are addressed to non-medical readers, a word of explanation is here necessary. It has been shown by experiment that the sudden arrest of the circulation in muscular fibre is sufficient to induce in the latter violent contractions. Thus, the cramps of the legs in cholera patients are due to the stagnation of blood in their muscles. These cramps are even more easily induced in the muscular fibre of the viscera--the unstriped, involuntary muscles--such as exist in the intestine, bladder, and uterus. Anything that will cause a sudden contraction of the blood-vessels in the uterus will, therefore, by cutting off the supply of blood, cause the muscular fibre of the uterus to 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.