This form of disease is rare--so rare that observers having a large field of observation may never meet with over half a dozen cases. In regard to many points very diverse views are held, and the limits of a practical work do not permit even a statement of all of them. The nature of the membrane is one of these points too important to pass over. When thrown off entire, its internal surface is smooth and marked by the openings of the utricular glands; its external or uterine face is rough and villous. It presents the exact shape of the interior of the uterus, with openings corresponding to the Fallopian tubes and the os. It is impossible to escape the conviction that this membrane is the lining membrane of the uterus, thrown off as a whole, instead of by gradual melting down of its superficial layers, as in normal menstruation. The microscope sustains this view, and this is the generally received opinion; yet that the membrane is not always such is testified by competent observers from observations with the same instrument. It seems probable that this disputed point will be settled, as have been so many others in medicine, in favor of both parties. Siredey suggests the possibility of different kinds of membrane in these cases, while Barnes boldly states this as a fact. Various theories have been advanced to account for the formation of the membrane. An abnormal course of conception, a changed ovarian influence, a peculiar endometritis, have been from time to time favorite terms in which to express our ignorance. Only in regard to the first has unanimity been obtained. That the membrane is always a product of conception is not now maintained by any respectable authority. It is a well-established fact of the utmost importance that such membranes may be expelled when there has never been sexual intercourse. The membrane of dysmenorrhoea is to be distinguished from fibrinous masses, the remains of blood-clots from which the corpuscles have been squeezed; from mucus coagulated into shreds by astringent injections; and from the products of membranous vaginitis. Neither of these will present much difficulty with the aid of the microscope. The case is very different, however, when the membrane is to be distinguished from the decidua of an early pregnancy. From a single specimen or a single attack a diagnosis cannot be made. Thomas gives an instance of disagreement as to the nature of the same membrane by two of the highest microscopical authorities. The recurrence of the attacks at the regular menstrual periods will establish the diagnosis.