=The vagina= is an elastic, muscular sheath or tube, about four inches long, lying behind the bladder and urethra and in front of the rectum. It leads interiorly up and backward from the vulva to the cervix, which it encases for about half an inch. The space between the outer surface of the cervix that extends into the vagina, and the surrounding vaginal walls, is called the _fornix_. For convenience of description, this is divided into four sections or fornices: the anterior, posterior and lateral fornices. Between the posterior fornix and the rectum a fold of the peritoneum drops down and forms a blind pouch known as _Douglas’ cul-de-sac_. At this point the delicate peritoneum is separated from the vagina by only a thin, easily punctured, muscular wall. This is a fact of grave surgical significance, for unless instruments and nozzles introduced into the vagina are very gently and skillfully directed, they may easily pierce this thin partition. Septic material may thus gain entrance to the peritoneal cavity and peritonitis result. The bore of the vaginal canal ordinarily permits of the introduction of one or two fingers. It is somewhat flattened from before backward, and on cross section resembles the letter H. During labor this canal becomes enormously dilated, being then four or five inches in diameter, and permits the passage of the full term child. The vagina is lined with a thick, heavy, mucous membrane which normally lies in transverse folds or corrugations called _rugæ_. These folds are obliterated and the lining stretched into a smooth surface as the canal dilates during labor. Attention must be drawn to the fact that the vagina, cervix, uterus and tubes form a continuous canal from the vulva to the easily infected peritoneum, a fact which makes absolute surgical cleanliness in obstetrics virtually a matter of life or death to the patient. This muscular tube is lined throughout its entire length with mucous membrane, which, though continuous, changes somewhat in character along its course. The epithelial cells of the lining of the tubes and body of the uterus have hair-like projections, cilia, which maintain a constant waving motion from above downward. The effect of this sweeping current is to carry down toward the outlet any object or secretion which may be upon the surface of the lining of the tubes or uterine cavity. The unfertilized ovum is thus swept down to meet the germ cell of the male and become fertilized. Along this variously constructed canal, at different periods in the life of the individual, pass the matured ovum, the menstrual flow, the uterine secretions, the fetus, the placenta and lochia, (the discharge which occurs during the puerperium). Although the bladder and rectum are not organs of reproduction, they are contained in the pelvic cavity and lie in such close proximity to the internal genitalia that at least a passing word must be devoted to their description.