_Hymen._ The lining membrane of the vagina is of a reddish-gray colour, interspersed here and there, especially at its upper part, with livid spots like extravasation. At the os externum it forms a fold or duplicature called _hymen_, running across the sides of the posterior part of the opening, and usually of a crescentic figure, the cavity looking upwards. The duplicatures of membrane are united by cellular tissue. In some instances, the hymen arises from the whole circumference of the os externum, having a small orifice in the centre for the escape of the menses and vaginal secretions: in some rare cases it is cribriform; and in others it completely closes the vaginal entrance. When torn in the act of sexual intercourse, it generally forms three or four little triangular appendages, called _carunculæ myrtiformes_, arising from the posterior and lateral portions of the os externum. From the identity of its fibrous coat with that of the uterus, the vagina possesses considerable powers of contraction, when excited by the presence of any body which distends it; hence it is a valuable assistance to the uterus during labour: it also stands in the same relation to the abdominal muscles that the rectum does, so that as soon as it is distended by the head, &c. it calls them into the strong involuntary action, which characterizes the bearing down pains of the second stage of labour. The orifice of the vagina (os externum) is surrounded by a thin layer of muscular fibres, which arise from the anterior edge of the sphincter ani; they enclose the outer margin of the vagina, cover its corpus cavernosum, and are inserted into the crura clitoridis at their union. It has been called the sphincter or constrictor vaginæ, and assists the corpus cavernosum still farther in contracting the os externum.