The blood discharged during menstruation has certain peculiar properties. It is always fluid, and rarely contains fibrinous clots, it is always mixed with a larger or smaller quantity of mucus, which gives it a sticky character; the reaction is alkaline, the smell characteristic. Only when the bleeding is very profuse are coagulated masses evacuated. On microscopical examination of menstrual blood, we detect erythrocytes and leucocytes, the proportional number of the latter being greater than in pure blood; there is an admixture also of epithelium from the genital mucous membranes, cylindrical cells from the uterus, flattened cells from the superficial layers of the stratified scaly epithelium of the vagina, also various micro-organisms and granular detritus. At the beginning of each menstruation, the admixture of mucus is greatest, so that the discharge sometimes has the appearance of blood-stained mucus; but during the height of the discharge the consistency is almost that of pure blood. The quantity of blood lost at each period is said to vary from 90 to 240 grammes (about 3 to 8 fluid ounces); but in tropical climates the average is said to be 600 grammes (20 ounces). According to the accurate analysis of _Denis_, menstrual fluid contains in a thousand parts: Total solid constituents 175.00 Comprising Fat 3.90 Blood-corpuscles 64.40 Albumin 48.30 Extractives 1.10 Salts 12.00 Mucus 45.30 ————— Water 825.00 —————— Both the quality and the quantity of the blood are subject to great variations. Thus, for instance, _Bouchardat_ estimates the solid constituents at 99.20 per mille, _Vogel_ at 161 per mille, and _Simon_ at 215 per mille. The amount of blood discharged during menstruation depends upon the temperament, the constitution, and the occupation, of the woman concerned. It is greater in vivacious brunettes than in phlegmatic blondes, greater in southern women than in those dwelling in the north, greater in town dwellers than in women living in the open plains, greater in those whose mode of life is sedentary than in those engaged in some active occupation. Similar considerations apply with regard to the duration of each period. The mean duration is in the great majority of cases from four to five days, being generally the same in successive periods in the same individual; in exceptional cases the flow may last a week or more. Menstruation lasting more than eight days must be regarded as abnormal.