(From mensis,a month,) catamenia, menstrua, emmenia,gynaecia, periodical discharges of blood from the uterus, vagina, or both, from about the age of fourteen to about fifty. In warm climates they appear at about eight or nine years of age; in temperate ones at thirteen to fourteen, and in the arctic regions not till nineteen or twenty. The quantity discharged is from four to ten ounces; but in this there is much variety, and the discharge continues from two to eight or ten days. In some relaxed constitutions there is occasionally not more than a week's interval, and in general the more lax the constitution, the larger is the discharge, and the longer its continuance. The indolent, the sanguine, and the luxurious, have generally a large periodical evacuation. Usually, the earlier the period when they first appear, the sooner they disappear. In this country they disappear about the forty-fifth year, though, from accidental circumstances, the cessation may happen in the thirty-sixth or be protracted to the fiftieth year. We have known instances of their continuing to the fifty-second, when they have not appeared at a late period. The tales so frequently detailed of their returning at the ages of sixty, and even of eighty, do not merit any particular attention, though often well founded: for in these cases, the discharge is truly haemorrhagic, generally temporary, and often critical.

The menses flow chiefly from the uterus, and occasionally from the vagina alone, as happens sometimes during pregnancy. When the natural discharge is stopped, a vicarious bleeding takes place from the nose, the lungs, the nipple, the haemorrhoidal veins, the stomach, the bowels, and even the gums, without any particular inconvenience.

Before that peculiar state of irritability which disposes to an irregular balance of the circulation, and consequently to topical congestion, had its full weight in our physiological and pathological inquiries, a discharge of blood implied, in the opinion of pathologists, plethora. That a general fulness was the cause of the menstrual discharge was scarcely doubted by the soundest physiologists, for the fancies of the lunar influence and of fermentation were soon rejected. This opinion had undoubtedly many observations to support it. The access of the catamenia was marked by general load and oppression; the breasts swelled; the stomach was often disordered; and their suppression was followed by other sanguine discharges. Yet the acuteness of modern philosophers soon discovered that these views would not explain all the various phenomena. They saw that the catamenia continued to recur notwithstanding the system was exhausted, that the fullest habits had not, invariably, the most copious or frequent discharges; for, on the contrary, these were usually observed in the weak and irritable. They perceived also, that a copious general bleeding would not stop their appearance, and the most copious discharge would not always relieve any internal inflammation. If also this view was correct, why did not the catamenia occur at other ages, when the vessels were distended ? why not in the intermediate periods, if the arterial system was unusually full ? The partial congestions, suggested by the writers of the Stahlian school, came therefore to their aid. The topical load, in a system so irritable, and so generally sympathizing, as that of the uterus, would produce equal uneasiness; from the peculiar sympathy between the uterus and the breasts, the mammae would swell; and, when any the most purely topical discharge was suppressed, other irregular determinations were known in other instances to come on. Nothing appeared, therefore, to be inconsistent with topical plethora; and this satisfactorily explained all the difficulties of the former system. The idea had loosely floated in the minds of many physiologists before the time of Dr. Cullen; but to him we are indebted for its expansion into a system at once elegant and correct.

In different parts of this work, and particularly in the article of Haemorrhoids, q. v. we have explained the gradual development of the different parts of the body from the distention of the arterial system, ultimately depending on the progressive changes of the relative degree of resistance in the coats of the arteries and veins. The genital systems of either sex experience this change about the same time, and as the vessels of the uterus easily admit of considerable dilatation, congestion is the consequence, which is relieved by the exhaling arteries yielding to the impulse. No rupture of the veins or arteries takes place, for the discharge is steady, regular, and seldom considerable in a given time. After it has continued for even a short period, every inconvenience is removed, the previous load is no longer felt, and the fulness of the mammae subsides. The continuance of the discharge is different in different constitutions, but it usually continues from three to five days, when it ceases, sometimes leaving a serous discharge for a day or two, sometimes a mucous one, which, if it continues, constitutes the disease called leucorrhaea, or fluor atbus.

The recurrence of the catamenia is with more difficulty explained. Women, from the sedentary life which they lead, and from a looser contexture of vessels, are more subject to plethoric congestions than men, and the uterus is, from its structure, more likely to receive these accumulated fluids. By degrees, these topical congestions become habitual, and recur independently of any real general plethora. This explanation appears to be supported by the irregular returns of the catamenia in the earlier periods, and the irregular continuance of the discharge before the habit is established. Why the accumulation should require a lunar month before it is equal to produce the effect, it is impossible to ascertain, as why the period of fourteen days should be most commonly required to produce the crisis of fever, or why the seventh and the fourteenth year should be marked by striking changes in the constitution. Such is the determination of Him"in whom we live, and move, and have our being."