THE VAGINA. The vagina is a membranous canal, extending from the vulva to the uterus obliquely through the pelvic cavity, between the bladder and rectum, having about the same direction as the axis of the pelvis. It is described as being five or six inches in length and about two inches in diameter, but it would be more correct to say that it is capable of being distended to these or greater dimensions, for in its common state the os uteri is seldom found to be more than three inches from the external orifice, and the vagina is contracted as well as shortened. In great part the walls of the vagina are composed of spongy erectile tissue, and their vascularity is a cause of considerable hemorrhage consequent on their rupture. Three layers combine to form the walls; one external or cellulo-fibrous, a middle or muscular one, and the internal or mucous one. The latter is of a pale red hue, which becomes violet during menstruation and especially during pregnancy. The mucous coat is disposed in the form of rugæ or folds anteriorly and posteriorly, which are better developed in young virgins and aged females; during advanced pregnancy, and for a short time after delivery, they are entirely effaced. [Illustration: Fig. 12—Section of the Uterus, &c. Fig. 12—Uterus, ovaries and Fallopian tubes. Section of the uterus, etc. a, Fundus of the uterus; b, cavity of the womb; c, cavity of the neck of the uterus; d, d, the cavity of the Fallopian tubes; e, fimbriated extremity; f, f, the ovaries; g, the vagina; h, h, the round ligaments; i, i, the ligaments of the ovaries. ] The upper part of the vagina is connected to the circumference of the os uteri but not in a straight line, for the former stretches beyond the latter, and being joined to the cervix, its mucous membrane is reflected over the os uteri, which by this mode of union is suspended with protuberant lips in the vagina, and permitted to change its position in various ways and directions. THE EXTERNAL ORGANS. The situation of the external organs of generation are indicated in the accompanying diagram (Fig. 11.) It is not deemed necessary here to describe these, but in regard to the hymen (the membrane that in infancy nearly closes the orifice of the vagina), we may remark that it is not a perfect test of virginity. There are, however, examples recorded in works on midwifery where a slight surgical operation was necessary after marriage, because this membrane was uncommonly strong. CHAPTER III. PHYSIOLOGY OF THE UTERUS AND OVARIES. Menstruation is a periodical flow of blood having its source in the walls of the uterus. But menstruation is excited by and dependent upon ovulation, and the effective cooperation of both the uterus and ovaries is necessary to both menstruation and conception. We shall consider these functions separately.