Close behind the clitoris, and a little below it, is the orifice of the urethra, lying between the two nymphæ: it is surrounded by several lacunæ or follicles of considerable depth, secreting a viscid mucus; its lower or posterior edge is, like the lower portion of the urethra, covered by a thick layer of cellular tissue, and a plexus of veins, which occasionally become dilated and produce much inconvenience; it is this which gives the urethra the feel of a soft cylindrical roll at the upper part of the vagina; and in employing the catheter, by tracing the finger along it, the orifice will be easily found.
Labia. The labia extend from the pubes to within an inch of the anus, the space between the vulva and anus receiving the name of perineum.
The opening between the labia is called the fossa magna: it increases a little in size and depth, as it descends, forming a scaphoid or boat-like cavity, viz. the fossa navicularis.
The labia are thicker above, becoming thinner below, and terminate in a transverse fold of skin, called the frænulum perinei, or fourchette, the edge of which is almost always slightly lacerated in first labours. They are composed of skin cushioned out by cellular and fatty substance, and lined by a very vascular membrane, which is thin, tender, and red, like the inside of the lips; they are also provided with numerous sebaceous follicles, by which the parts are kept smooth and moist.
CHAPTER III.
DEVELOPMENT OF THE OVUM.
Membrana decidua.—Chorion.—Amnion.—Placenta.—Umbilical cord.—Embryo.—Fœtal circulation.
Membrana decidua. The earliest trace of impregnation which is to be observed in the cavity of the uterus, and even before the ovum has reached it, is the presence of a soft humid paste-like secretion, with which the cavity of the uterus is covered, and which is furnished by the secreting vessels of its lining membrane. This is the membrana decidua of Hunter: properly speaking, it should be called the maternal membrane, in contra-distinction to the chorion and amnion, which, as belonging peculiarly to the fœtus, are called the fœtal membranes.[17]