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surface of the hymen is directly continuous with the vaginal wall, and on it are to be seen slight ridges continuous with the vaginal rugæ.
Developmentally the hymen appears to be a portion of the vagina.
On each side of the vaginal opening, and close against the medial side of the attached margin of the labium minus, is the minute opening of the duct of the glandula vestibularis major (O.T. Bartholin's gland). This is usually just large enough to be visible to the unaided eye.
Numerous minute mucous glands, the glandulæ vestibulares minores, open on the surface of the mucous membrane of the vestibule, between the urethral and vaginal orifices. The opening of the ductus para-urethralis at the side of the urethral orifice has been already noted, p. 1285.
Clitoris. The clitoris is the morphological equivalent of the penis, and is composed of a body and two crura. Upon the summit of the body is a minute glans. Unlike the penis, the clitoris is not traversed by the urethra.
The corpus clitoridis is composed for the most part of erectile tissue resembling that of the penis in the male. It is about an inch or an inch and a half in length, and is bent upon itself, forming an angle open downwards. The body of the clitoris tapers towards its distal end, which is covered by the glans clitoridis. The organ is enclosed in a dense fibrous coat, and is divided by an incomplete septum, the septum corporum cavernosorum, into two symmetrical and somewhat cylindrical portions, the corpora cavernosa clitoridis. These represent the corpora cavernosa penis of the male, and diverge from one another at the root of the clitoris to form the crura clitoridis. A ligamentum suspensorium clitoridis passes from the fibrous coat of the body of the clitoris to the symphysis pubis (Fig. 1036). The glans clitoridis is a small mass of erectile tissue which is fitted over the pointed end of the body. It possesses, like the glans penis, which it represents, a very sensitive epithelium. The præputium, or fold of skin which covers it, and the frenulum clitoridis, which is attached to it inferiorly, are continuous with the labia minora (Fig. 1036).
The crura clitoridis diverge from the body posteriorly, and are attached to the sides of the pubic arch. Each is continuous with one of the corpora cavernosa, and has a firm fibrous sheath, which is covered by the corresponding ischio-cavernosus or erector clitoridis muscle. In structure the crura and body of the clitoris resemble the corpora cavernosa penis, while the glans more closely resembles the bulbus vestibuli, with which it is continuous through a structure known as the pars intermedia.
In the seal and some other animals, a bone, which represents the os penis of the male, is developed in the septum of the clitoris. This bone receives the name os clitoridis.
Arteries and Nerves of the Clitoris.-Each crus receives a branch, the arteria profunda clitoridis, from the internal pudendal, while the glans is supplied by branches of the arteria dorsalis clitoridis.
The nerve-supply of the clitoris is derived partly from the hypogastric sympathetic plexus and partly from the dorsal nerves of the clitoris, which are branches of the pudendal nerves.
Bulbus Vestibuli. The bulbus vestibuli is a mass of erectile tissue, in the female, which corresponds developmentally to the corpus cavernosum urethræ of the male. In the female the fusion of the two halves of this structure is not nearly so complete as in the male, for the vagina and urethra separate the bulbus vestibuli into a right and a left portion which are only slightly connected in front by a narrow median part called the pars intermedia. Each half of the bulb is thick and massive posteriorly, and more pointed in front, where it joins the pers intermedia. It rests against the lateral wall of the vagina, and upon the superficial aspect of the fascia inferior of the urogenital diaphragm. It represents onehalf of the corpus cavernosum urethræ of the male. Superficially it is covered by the bulbo-cavernosus muscle. The pars intermedia lies above the opening of the urethra, and becomes continuous with the tissue of the glans clitoridis.
The bulbus vestibuli is for the most part composed of minute convoluted bloodvessels, held together by a very small amount of connective tissue. These vessels frequently anastomose with one another, and those of each half communicate with the vessels of the pars intermedia and the glans clitoridis.
The blood-supply of the bulb is derived, on each side, from the arteria bulbi vestibuli, branch of the internal pudendal.
GLANDULE VESTIBULARES MAJORES.
The greater vestibular glands (O.T. glands of Bartholin) are placed one on each side of the lower part of the vagina, and represent the bulbo-urethral glands in
FIG. 1036.-DISSECTION OF FEMALE PERINEUM TO SHOW THE CLITORIS, THE BULB OF THE VESTIBULE, AND THE LARGER VESTIBULAR GLANDS (D. J. Cunningham).
the male. They are often overlapped by the posterior ends of the bulbus vestibuli, and are covered by the bulbo-cavernosus muscle. Each is about the size and shape of a small bean, and possesses a long slender duct which opens into the rima pudendi in the angle between the attached border of the labium minus and the vaginal opening.
DEVELOPMENT OF THE URO-GENITAL ORGANS.
THE URO-GENITAL PASSAGES.
General Account. In tracing the developmental history of the uro-genital passages we may for convenience begin with an embryo of fifteen days old. About this time a duct, which runs in a longitudinal direction, and occupies a position on the lateral side of the protovertebral somites, begins to develop on each side of the body. With the exception of the anterior portion of the cloaca and the proximal part of the allantois, this duct, which has received the name of primary excretory or Wolffian duct, is the earliest formed structure from which, or in connexion with which, the parts of the adult urino-genital system arise.
The Wolffian duct serves as the canal, or duct, for the primitive secretory organs-the pronephros and the mesonephros of the embryo. With the atrophy of these the duct suffers modification, yet both sexes in the adult possess structures which have their embryonic origin from the Wolffian duct. In the male the duct of the epididymis, the ductus deferens, and the ejaculatory duct, are to be looked upon as directly developed from the Wolffian duct of the embryo; while in the female the longitudinal duct of the ep-oöphoron and the appendices vesiculosi are rudimentary structures having a like origin. Further, the ureter and its pelvis arise in both sexes as an outgrowth from the Wolffian duct (Fig. 1037). In the male the vesicula seminalis is developed as a diverticulum of the Wolffian duct.
The primitive secretory organs, the pronephros and the mesonephros, develop in connexion with the anterior part of the Wolffian duct (p. 48), and, during the early life of the embryo, the latter of these is a most important structure. Even in the embryo
the pronephros is a vestigial organ, and its development in all higher vertebrates is very incomplete. It disappears almost as soon as it is formed, and it is replaced by the far more important mesonephros. With the development of the permanent kidney the mesonephros atrophies, yet some of its tubules persist in the adult. The ductuli
efferentes, the ductuli aberrantes, and the rudimentary paradidymis (organ of Giraldés) in the male, and the rudimentary tubules of the ep-oöphoron and of the par-oöphoron in the female, are structures which owe their origin to the tubules of the mesonephros.
Soon after the formation of the Wolffian ducts two other longitudinally disposed canals, called the Müllerian ducts, are developed. These open at ther cephalic ends into the body cavity, and at their caudal ends, unlike the Wolffian ducts, they unite with one another in the median plane. From them are formed. in the female-the uterine tubes, the uterus, and the vagina; and in the male—the appendices of the testis and the utriculus prostaticus.
The Wolffian and Müllerian ducts open at their caudal ends into the ventral or urogenital part of the cloaca, which in the course of development becomes transformed into the bladder and the uro-genital canal of the embryo. The developing ureter at first arises as a diverticulum from the Wolffian duct, at a short distance from the point where the latter joins the cloaca. Soon, however, the ureters acquire inde pendent openings into the cloaca, which become gradually shifted further from one another and from those of the Wolffian ducts. The ureters are now found to open into the anterior portion of the cloaca which lies nearer to the head of the embryo than the part with which the Wolffian ducts are connected. This cephalic portion of the anterior subdivision of the cloaca which receives the ureters becomes the bladder and the upper part of the urethra. The caudal part, lying below the level of the entrance of the Wolffian ducts. is called the uro-genital canal, and is represented in the adult male by the lower part of the prostatic and by the membranous portions of the urethra; in the female by the lower part of the urethra and the part of the uro-genital fissure which immediately surrounds the openings of the urethra and vagina (Figs. 10441045). The united Müllerian ducts open into the lower part of the cloaca or uro-genital canal between the The structures developed from the cloaca Wolffian ducts of opposite sides. In the male the are indicated in blue, those from the position of this opening, which is represented in the Wolffian duct in red, and the ectoderm adult by the orifice of the utriculus prostaticus, remains almost unchanged; in the female, on the other hand, bladder become separated and acquire a downgrowth from the fused Müllerian ducts gives openings into the ectodermal cloacal origin to a new passage, the vagina, which establishes fossa is shown in II. and III. (A. H. an opening behind that of the urethra in the uroYoung and A. Robinson). genital fissure of the adult.
FIG. 1037.-DIAGRAM TO ILLUSTRATE THE
MANNER IN WHICH THE URETER, THE
BLADDER ARISE IN THE EMBRYO.
The manner in which the rectum and
CM. Ectoderm of
Ur. Uro-genital canal.
VD. Ductus deferens. cloacal fossa. VS. Seminal vesicle. K. Pelvis of kidney. WD. Wolffian duct.
After the complete separation of the cloaca into anterior or uro-genital and posterior or rectal subdivisions, the rectum establishes a communication with the exterior in the floor of the shallow depression known as the ectodermal cloacal fossa. At a little later time the uro-genital canal also joins this fossa at a point in front of the opening of the rectum. The ectodermal cloacal fossa lies in front of the vestigial tail, and extendforwards as far as a tubercle known as the cloacal tubercle, which later gives rise to the genital eminence and a pair of elevations called the labio-scrotal folds. The genital eminence becomes converted into the clitoris or penis according to the sex. The labioscrotal folds extending backwards on each side form the labia majora of the female, and.
fusing posteriorly, give rise to the scrotum in the male. In the female the slit-like opening of the uro-genital canal retains its position, and its margins becoming elongated, form the labia minora. The vaginal opening arises, as we have seen, by a downgrowth of the fused Müllerian ducts which lies in front of the rectum and behind the primitive uro-genital canal. The latter becomes the urethra in the female. In the male the slitlike opening of the uro-genital canal is prolonged anteriorly by an active growth at the base of the genital eminence, and its margins uniting, give rise to the penile portion of the urethra.
THE WOLFFIAN DUCT AND EMBRYONIC SECRETORY ORGAN.
The Wolffian Duct.-The Wolffian duct arises in the mesoderm, about the fifteenth day, as a solid cord of cells occupying a position immediately to the lateral side of the protovertebral somites and to the medial side of the body cavity (Fig. 1038). When first recognised the duct lies immediately beneath the ectoderm, and as it grows backwards to reach the cloaca it is often found to be intimately connected with the ectoderm. This close connexion of the duct with the ectoderm, in the early stages, is by some authorities supposed to indicate a primitive ectodermal origin of the canal, but by others, and apparently with more reason, to be a trace of the opening of ducts on the surface of the body, such as exists in connexion with the excretory organs of lower animals. During the third week the cellular cord which represents the Wolffian duct acquires a lumen, and about the end
of the same week the duct FIG. 1038.-TRANSVERSE SECTION THROUGH THE TRUNK OF A HUMAN EMBRYO OF ABOUT 5 MM. LENGTH.
in its growth reaches the
cloaca. As soon as the
cloaca has become divided into dorsal and ventral subdivisions, the Wolffian duct is seen to end in the caudal part of the ventral subdivision, which becomes the bladder and uro-genital canal (Fig. 1040).
The mesonephros or Wolffian body is developed in the mesoderm of the "intermediate cell mass," immediately adjoining the Wolffian duct, and consists of a number of transversely arranged canals or tubules, each of which opens by one end into the Wolffian duct, while its other extremity ends blindly. These transverse tubules, like the canal into which they open, are at first solid cellular structures, and only later acquire a distinct lumen. Increasing rapidly in size and number, the tubules become twisted and tortuous, and the blind end of each dilates to form a capsule invaginated upon itself and containing a bunch of capillary blood-vessels similar to the glomeruli of the adult kidney. It would appear that primitively one tubule is developed in the portion of the intermediate cell mass (nephrotome) corresponding to each mesodermic somite, but, in higher vertebrates at all events, such a correspondence between the number of somites and the number of tubules cannot be demonstrated. In the posterior part of the mesonephros the number of tubules is very numerous, and greater than the number of segments in this region. The tubules of the mesonephros arise in all segments from the sixth cervical to the third lumbar. The tubules in the anterior part atrophy and disappear at a very early time, even while others are being formed towards the caudal end of the embryo. When at its greatest development (fifth to eighth week) the mesonephros forms a relatively large glandular mass, composed of tubules resembling in a general way those of the adult kidney, which projects into the dorsal part of the body cavity, and extends from the region of the liver to the caudal end of the body cavity. Along its lateral aspect lies the Wolffian duct.
In anamniate vertebrates, fishes, and amphibia, the mesonephros persists as the secretory organ of the adult.
Pronephros. From what is known regarding the development of lower animals, it seems certain that the Wolffian duct originally served as the duct of the still earlier secretory organ
FIG. 1041.-TRANSVERSE SECTION THROUGH THE LOWER PART OF THE TRUNK OF A HUMAN EMBRYO F ABOUT 7 WEEKS. (Specimen in the possession of Prof. J. Symington.)
the Pronephros. In man the pronephros arises nearer to the head end of the embryo than the later formed mesonephros, and its tubules can only with difficulty be distinguished