As the result of long-continued investigation, Johnstone has come to the conclusion that the lining membrane of the uterus belongs to that class of organs whose function it is to replace organic waste. "Menstruation is a periodic wasting away of those corpuscles that are too old to make a placenta." He has further found that, as compared with the uteri of very many of the lower animals, the human uterus is very scantily supplied with lymphatics, and the only way to rid the uterus of the overripe, and therefore consequently useless, tissue is to wash it out through the vagina by a blood-stream. The tough wall of the human uterus and the increased blood-pressure caused by the erect position cause the difference between menstruation in the human female and rut in the lower animals. The strong light of recent investigations has necessitated the laying aside of many time-honored theories; and as the close of the nineteenth century has seen the emancipation of the uterus from the thralldom of the ovary, so we may believe that the twentieth century will find women of such fine physique as to prove the error of the popular fallacy that the cause of woman's weakness lies in the performance of her functions. The Function of the Uterus.-- The function of the uterus is to provide a favorable place for the reception of the product of conception, where it may be protected and nourished during the period of its development. The purpose of menstruation is to keep the uterus in suitable condition for the reception of this product of conception at any time. It is now known that the menstrual flow is not the whole of menstruation, and that the changes going on in the uterus are almost as continuous as the process of digestion. The whole of the reproductive life of woman has been divided into cycles of twenty-eight days each; these cycles have been divided into four stages. Stages of the Menstrual Cycle.-- The first or constructive stage is one of preparation for the reception of the ovum. During this stage the preparing of a decidua takes place, or building a nest for the expected egg; there is a swelling of the mucous membrane, an enlargement of the uterine glands, and an increase in the connective tissue. It is thought that this stage lasts for one week; when pregnancy does not occur, it is followed by degenerative changes. The second or destructive stage is marked by destructive changes which give rise to the usual phenomena of the menstrual period; there is a discharge of blood, mucus, and disintegrated mucous membrane. The actively growing cells of the uterine lining membrane undergo rapid destructive changes, the fabric of the half-formed decidua tumbles to pieces, the turgid capillaries burst and pour out the menstrual flow, which sweeps away all the useless debris. The irritation sets up reflex uterine contractions, and so the blood is squeezed out of the distended capillaries and washes away the degenerated cells.