Dr. Lloyd Jones has determined that between 17 and 45 years of age the specific gravity of the blood of women is lower than that of men. In old women the specific gravity rises above that of old men, and he suggests that their greater longevity is due to this.[92] No doubt the greater longevity of women is to be associated with the rise in specific gravity of their blood, but this rise in the specific gravity of women after 45 years is consequent upon their anabolic constitution. High specific gravity in general is associated with abundant and rich nutrition; it falls in women during pregnancy, lactation, and menstruation, and when these functions cease it is natural that the constructive metabolic tendency on which they are dependent should show itself in a heightened specific gravity of the blood (i.e., greater richness), and in consequence greater longevity. Some facts in the brain development of women point to the same conclusion. The growth of the brain is relatively more rapid in women than in men before the twentieth year. Between 15 and 20 it has reached its maximum, and from that time there is a gradual decline in weight until about the fiftieth year, when there is an acceleration of growth, followed by a renewed diminution after the sixtieth year. The maximum of brain weight is almost reached by men at 20 years, but there is a slow increase until 30 or 35 years. There is then a diminution until the fiftieth year, followed by an acceleration, and at 60 years again a rapid diminution in weight; but the acceleration is more marked and the final diminution less marked in woman than in man.[93] A table prepared by Topinard shows that woman from 20 to 60 years of age has from 126 to 164 grams less brain weight than man, while her deficit from 60 to 90 years is from 123 to 158 grams.[94] The only explanation at hand of this relative superiority of brain weight in old women is that with the close of the period of reproduction (the anabolic surplus being no longer consumed in the processes associated with reproduction) the constructive tendency still asserts itself, and a slight access of growth and vitality results to the organism. * * * * *