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.--Foetal 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 foetus, are called the _foetal_ membranes.[17] Although at first in a semi-liquid state, it soon becomes firmer and more compact, assuming the character of a membrane: it appears to be nothing else than an effusion of coagulable lymph on the internal surface of the uterus, having "scarcely a more firm consistence than curd of milk or coagulum of blood." (Hunter, _op. cit._ p. 54.) Hence, although much thicker than the other membranes, it is weaker; it is also much less transparent.