The Fallopian tubes receive their external covering from the peritoneum, which becomes connected at their open extremity with the membrane which lines them. Between the external and internal membrane is the proper tissue of the tubes, and which, except in very muscular subjects, seldom display the fibrous structure; still, nevertheless, two layers of fibres have been observed--an outer or longitudinal, and an inner or circular layer. The Fallopian tubes are lined with mucous membrane, forming numerous longitudinal rugæ. The canal is not pervious during the early months of foetal life, the abdominal extremity being closed and rounded; this appears to open about the fourth month. The canal is relatively larger, the younger the embryo is, and may, therefore, be easily demonstrated at this time. At the period of impregnation, the Fallopian tubes implant themselves by means of their fimbriated extremity upon that part of the ovary where the Graafian vesicle is about to burst; they become remarkably engorged with blood, assuming a deep purple colour, and are now much thicker; the canal enlarges, so that a tolerably-sized probe can be introduced, whereas, at other periods it will scarcely admit a large bristle. The uterine extremity of the tube is closed by a continuation of that pulpy coagulable lymph-like secretion which now lines the cavity of the uterus, forming the membrana decidua of Hunter, and which, especially on the side where the corpus luteum is found, extends into the tube to nearly the distance of an inch. The tubes are now observed to be in a state of distinct peristaltic motion, "like writhing worms," as Mr. Cruickshank has well expressed it; "the fimbriæ were also black and embraced the ovaria (like fingers laying hold of an object) so closely and so firmly, as to require some force and even slight laceration to disengage them."[6] From the great degree of vascularity which is observed in the Fallopian tubes at this period, some anatomists have been induced to consider that their proper tissue was vascular, analogous to the corpora cavernosa penis. Besides the peristaltic motion already mentioned, other movements called ciliary have been observed in the Fallopian tubes at this period, consisting of minute portions of mucous membrane moving briskly and whirling round their axis, apparently for the purpose of propelling the ovum.[7]