1911 Encyclopædia Britannica/Reproductive System
REPRODUCTIVE SYSTEM, IN ANATOMY.—The reproductive system in some parts of its course shares structures in common with the urinary system (q.v.). In this article the following structures will be dealt with. In the male the testes, epididymis, vasa deferentia, vesiculae seminales, prostate, penis and urethra. In the female the ovaries, Fallopian tubes, uterus, vagina and vulva.
Male Reproductive Organs.
The testes or testicles are the glands in which the male reproductive cells are formed. They lie, one on each side, in the scrotum surrounded by the tunica vaginalis (see Coelom and Serous Membranes). Each is an oval gland about one and a half inches long with its long axis directed downward, backward and inward. There is a strong fibrous coat called the tunica albuginea, from which vertical and horizontal septa penetrate into the substance, thus dividing it into compartments or lobules in which the seminiferous tubes are coiled. It is estimated that the total length of these seminiferous tubes in the two glands is little short of a mile. (See fig. 1.)
At the posterior part of the testis the fibrous sheath is greatly thickened to form the mediastinum testis, and contains a plexus of tubules called the rete testis (see fig. 1), into which the seminiferous tubes open. In this way the secretion of the gland is carried to its upper and back part, whence from fifteen to twenty small tubes (vasa efferentia) pass to the epididymis. Each of these is convoluted before opening, and forms what is known as a conus vasculosus.
Under the microscope the seminiferous tubules are seen to consist of a basement membrane surrounding several layers of epithelial cells, some of which are constantly being transformed into spermatozoa or male sexual cells.
The epididymis (see fig. 1) is a soft body lying behind the testis; it is enlarged above to form the globus major or head, while below is a lesser swelling, the globus minor or tail. The whole epididymis is made up of a convoluted tube about 20 ft. long, from which one long diverticulum (vas aberrans) comes off. Between the globus major and the testis two small vesicles called the hydatids of Morgagni are often found.
The vas deferens is the continuation of the tube of the epididymis and starts at the globus minor; at first it is convoluted, but soon becomes straight, and runs up on the inner (mesial) side of the epididymis to the external abdominal ring in the abdominal wall. On its way up it is joined by several other structures, to form the spermatic cord; these are the artery (spermatic) and veins (pampiniform plexus) of the testis, the artery of the vas, the ilio-inguinal, genito-crural and sympathetic nerves, and the testicular lymphatics. After entering the external abdominal ring, these structures pass obliquely through the abdominal wall, lying in the inguinal canal for an inch and a half, until the internal abdominal ring is reached. Here they separate and the vas passes down the side of the pelvis and turns inward to meet its fellow at the back of the bladder, just above the prostate. The whole length of the vas is 12 to 18 in. and it is remarkable for the great thickness of its muscular walls, which gives it the feeling of a piece of whipcord when rolled between the finger and thumb.
A little above the globus major a few scattered tubules are found in children in front of the cord; these form the rudimentary structure known as the organ of Giraldès or paradidymis. As the vas deferens approaches the prostate it enlarges and becomes slightly sacculated to act as a reservoir for the secretion of the testis; this part is the ampulla (see fig. 2).
The vesiculae seminales are sac-like diverticula, one on each side, from the lower part of the ampullae of the vasa deferentia. They are about 2 in. long and run outward behind the bladder and parallel to the upper margin of the prostate for some little distance, but usually turn upward near their blind extremity. When carefully dissected and unravelled each is found to consist of a thick tube, about 5 in. long, which is sharply bent upon itself two or three times, and also has several short, sac-like pouches or diverticula. The vesiculae seminales are muscular sacs with a mucous lining which is thrown into a series of delicate net-like folds. The convolutions are held together by the pelvic cellular tissue, and by involuntary muscle continuous with that of the bladder. It is probable that these vesicles are not reservoirs, as was at one time thought, but form some special secretion which mixes with that of the testes. Where the vesiculae join the ampullae of the vasa deferentia the ejaculatory ducts are formed; these are narrow and thin-walled, and run, side by side, through the prostate to open into the floor of the prostatic urethra.
The prostate is partly a muscular and partly a glandular structure, situated just below the bladder and traversed by the urethra; it is of a somewhat conical form with the base upward in contact with the bladder. Both vertically and transversely it measures about an inch and a quarter, while antero-posteriorly it is only about three-quarters of an inch, though its size is liable to great variation. It is enclosed in a fibrous capsule from which it is separated by the prostatic plexus of veins anteriorly. It is often described as formed of three lobes two lateral and a median or posterior, but careful sections and recent research throw doubt on the existence of the last.
Microscopically the prostate consists of masses of long, slender, slightly branching glands, embedded in unstriped muscle and fibrous tissue; these glands open by delicate ducts (about twenty in number) into the prostatic urethra, which will be described later. In the anterior part of the gland are seen bundles of striped muscle fibres, which are of interest when the comparative anatomy of the gland is studied: they are better seen in young than in old prostates.
The male urethra begins at the bladder and runs through the prostate and perineum to the penis, which it traverses as far as the tip. It is divided into a prostatic, membranous and spongy part, and is altogether about 8 inches in length. The prostatic urethra runs downward through the prostate rather nearer the anterior than the posterior part. It is about an inch and a quarter long, and in the middle of the gland it bends forward forming an angle (see fig. 5); here it is from a third to half an inch wide, though at the base and apex of the prostate it is narrower. When it is slit open from in front a longitudinal ridge is seen in its posterior wall, which is called the verumontanum or crista urethra, and on each side of this is a longitudinal depression, the prostatic sinus, into which numerous ducts of the prostate open, though some of them open on to the antero-lateral surface. Near the lower part of the verumontanum is a little pouch, the utriculus masculinus, about one-eighth of an inch deep, the opening of which is guarded by a delicate membranous circular fold, the male hymen. Close to the opening of the utriculus the ejaculatory ducts, already mentioned, open into the urethra by very small apertures. The part of the urethra above the openings of these ducts really belongs to the urinary system only, though it is convenient to describe it here. After leaving the prostate the urethra runs more forward for about three-quarters of an inch, lying between the two layers of the triangular ligament, both of which it pierces. This is known as the membranous urethra, and is very narrow, being gripped by the compressor urethrae muscle.
|From C. S. Wallace's Prostatic Enlargement.|
Fig. 3.—Coronal Section through the Pelvis, showing the relations of the bladder above, prostate and bulb below.
The spongy urethra is that part which is enclosed in the penis after piercing the anterior layer of the triangular ligament. At first it lies in the substance of the bulb and, later, of the corpus spongiosum, while finally it passes through the glans. In the greater part of its course it is a transverse slit, but in traversing the glans it enlarges considerably to form the fossa navicularis, and here, in transverse section, it looks like an inverted T (⊥), then an inverted Y (⅄), and finally at its opening (external meatus) a vertical slit. Into the whole length of the urethra mucous glands (glands of Littré) open, and in the roof of the fossa navicularis the mouth of one of these is sometimes so large that it may engage the point of a small catheter and is known as the lacuna magna. As a rule the meatus is the narrowest part of the whole canal.
|From C. S. Wallace's Prostatic Enlargement.|
Fig. 4.—Transverse Section of a young Prostate, showing wavy striped muscle in front, urethra in the middle, and the two ejaculatory ducts behind.
Opening into the spongy urethra where it passes through the bulb are the ducts of two small glands known as Cowper's glands, which lie on each side of the membranous urethra and are best seen in childhood.
The penis is the intromittent organ of generation, and is made up of three cylinders of erectile tissue, covered by skin and subcutaneous tissue without fat. In a transverse section two of these cylinders (the corpora cavernosa) are placed above, side by side, while one, the corpus spongiosum, is below. Posteriorly, at what is known as the root of the penis, the two corpora cavernosa diverge, become more and more fibrous in structure, and are attached on each side to the rami of the ischium, while the corpus spongiosum becomes more vascular and enlarges to form the bulb. It has already been pointed out that the whole length of the corpus spongiosum is traversed by the urethra. The anterior part of the penis is formed by the glans, a bell-shaped structure, apparently continuous with the corpus spongiosum, and having the conical ends of the corpora cavernosa fitted into depressions on its posterior surface. On the dorsum of the penis the rim of the bell-shaped glans projects beyond the level of the corpora cavernosa, and is known as the corona glandis. The skin of the penis forms a fold which covers the glans and is known as the prepuce or foreskin; when this is drawn back a median fold, the frenulum praeputii, is seen running to just below the meatus. After forming the prepuce the skin is reflected over the glans and here looks like mucous membrane. The structure of the corpora cavernosa consists of a strong fibrous coat, the tunica albuginea, from the deep surface of which numerous fibrous trabeculae penetrate the interior and divide it into a number of spaces which are lined with endothelium and communicate with the veins. Between the two corpora cavernosa the sheath is not complete and, having a comb-like appearance, is known as the septum pectinatum. The structure of the corpus spongiosum and glans resembles that of the corpora cavernosa, but the trabeculae are finer and the network closer.
|From C. S. Wallace's Prostatic Enlargement.|
Fig. 5.—Sagittal Median Section of Bladder, Prostate and Rectum, showing one of the ejaculatory ducts.
Female Reproductive Organs.
The ovary is an organ which in shape and size somewhat resembles a large almond, though its appearance varies considerably in different individuals, and at different times of life. It lies in the side wall of the pelvis with its long axis nearly vertical and having its blunt end (tubal pole) upward. Its more pointed lower end is attached to the uterus by the ligament of the ovary, while its anterior border has a short reflection of peritoneum, known as the mesovarium, running forward to the broad ligament of the uterus. It is through this anterior border that the vessels and nerves enter and leave the gland.
Under the microscope the ovary is seen to be covered by a layer of cubical cells, which are continuous near the anterior border with the cells of the peritoneum. Deep to these is the ovarian stroma, composed of fibrous tissue, and embedded in it are numerous nests of epithelial cells, the Graafian follicles, in various stages of development. During the child-bearing period of life some of these will be nearing the ripe condition, and if one such be looked at it will be seen to contain one large cell, the ovum, surrounded by a mass of small cells forming the discus proligerus. At one point this is continuous with a layer of cells called the stratum granulosum which lines the outer wall of the follicle, but elsewhere the two layers are separated by fluid, the liquor folliculi. When follicle bursts, as it does in time, the ovum escapes on to surface of the ovary.
The Fallopian tubes receive the ova and carry them to uterus. That end of each which lies in front of the ovary is called the fimbriated extremity, and has a number of fringes (fimbriae) hanging from it; one of the largest of these is the ovarian fimbria and is attached to the upper or tubal pole of the ovary. The small opening among the fimbriae by which the tube communicates with the peritoneal cavity is known as the ostium abdominal, and from this the lumen of the tube runs from four to four and a half inches, until it opens into the cavity of the uterus by an extremely small opening. In the accompanying figure (fig. 6) the Fallopian tube and ovary are pulled out from the uterus; this, as has been explained, is not the position of the ovary in the living body, nor is it of the tube, the outer half of which lies folded on the front and inner surface of the ovary. The Fallopian tubes, like many other tubes in the body, are made chiefly of unstriped muscle, the outer layer of which is longitudinal and the inner circular; deep to this are the submucous and mucous coats, the latter being lined with ciliated epithelium (see Epithelial Tissues), and thrown into longitudinal pleats. Superficially the tube is covered by a serous coat of peritoneum. The calibre gradually contracts from the peritoneal to the uterine opening.
The uterus or womb is a pear-shaped, very thick-walled, muscular bag, lying in the pelvis between the bladder and rectum. In the non-pregnant condition it is about three inches long and two in its broadest part, which is above. The upper half or body of the uterus is somewhat triangular with its base upward, and has an anterior surface which is moderately flat, and a posterior convex. The lower half is the neck or cervix and is cylindrical; it projects into the anterior wall of the vagina, into the cavity of which it opens by the os uteri externum. This opening in a uterus which has never been pregnant is a narrow transverse slit, rarely a circular aperture, but in those uteri in which pregnancy has occurred the slit is much wider and its lips are thickened and gaping and often scarred. The interior of the body of the uterus shows a comparatively small triangular cavity (see fig. 6, B), the anterior and posterior walls of which are in contact. The base of the triangle is upward, and at each lateral angle one of the Fallopian tubes opens. The apex leads into the canal of the cervix, but between the two there is a slight constriction known as the os uteri internum. The canal of the cervix is about an inch long, and is spindle-shaped when looked at from in front; its anterior and posterior walls are in contact, and its lining mucous membrane is raised into a pattern which, from its likeness to a cypress twig, is called the arbor vitae. This arrangement is obliterated after the first pregnancy. On making a mesial vertical section of the uterus the cavity is seen as a mere slit which is bent about its middle to form an angle the opening of which is forward. A normal uterus is therefore bent forward on itself, or anteflexed. In addition to this, its long axis forms a marked angle with that of the vagina, so that the whole uterus is bent forward or anteverted. As a rule, in adults the uterus is more or less on one side of the mesial plane of the body. From each side of the uterus the peritoneum is reflected outward, as a two-layered sheet, to the side wall of the pelvis; this is the broad ligament, and between its layers lie several structures of importance. Above, there is the Fallopian tube, already described; below and in front is the round ligament; behind, the ovary projects backward, and just above this, when the broad ligament is stretched out as in fig. 6, are the epoöphoron and paroöphoron with the duct of Gärtner.
The round ligament is a cord of unstriped muscle which runs from the lateral angle of its own side of the uterus forward to the internal abdominal ring, and so through the inguinal canal to the upper part of the labium majus.
The epoöphoron or parovarium is a collection of short tubes which radiate from the upper border of the ovary when the broad ligament is pulled out as in fig. 6. It is best seen in very young children and represents the vasa efferentia in the male. Near the ovary the tubes are closed, but nearer the Fallopian tube they open into another tube which is nearly at right angles to them, and which runs toward the uterus, though in the human subject it is generally lost before reaching that organ. It is known as the duct of Gärtner, and is the homologue of the male epididymis and vas deferens. Some of the outermost tubules of the epoöphoron are sometimes distended to form hydatids. Nearer the uterus than the epoöphoron a few scattered tubules are occasionally found which are looked upon as the homologue of the organ of Giraldès in the male, and are known as the paroöphoron.
The vagina is a dilatable muscular passage, lined with mucous membrane, which leads from the uterus to the external generative organs; its direction is, from the uterus, downward and forward, and its anterior and posterior walls are in contact, so that in a horizontal section it appears as a transverse slit. As the orifice is neared the slit becomes H-shaped. Owing to the fact that the neck of the uterus enters the vagina from in front, the anterior wall of that tube is only about 2½ in., while the posterior is 3½. The mucous membrane is raised into a series of transverse folds or rugae, and between it and the muscular wall are plexuses of veins forming erectile tissue. The relation of the vagina to the peritoneum is noticed under Coelum and Serous Membranes.
The vulva or pudendum comprises all the female external generative organs, and consists of the mons Veneris, labia majora and minora, clitoris, urethral orifice, hymen, bulbs of the vestibule, and glands of Bartholin. The mons Veneris is the elevation in front of the pubic bones caused by a mass of fibro-fatty tissue; the skin over it is covered by hair in the adult. The labia majora are two folds of skin, also containing fibro-fatty tissue and covered on their outer surfaces by hair, running down from the mons Veneris to within an inch of the anus and touching one another by their internal surfaces. They are the homologies of the scrotum in the male. The labia minora are two folds of skin containing no fat, which are usually hidden by the labia majora and above enclose the clitoris, they are of a pinkish colour and look like mucous membrane.
The clitoris is the representative of the penis, and consists of two corpora cavernosa which posteriorly diverge to form the crura clitorides, and are attached to the ischium; the organ is about an inch and a half long, and ends anteriorly in a rudimentary glans which is covered by the junction of the labia minora; this junction forms the prepuce of the clitoris.
The orifice of the urethra is about an inch below the glans clitorides and is slightly puckered.
The hymen is a fold of mucous membrane which surrounds the orifice of the vagina and is usually only seen in the virgin. As has been pointed out above, it is represented in the male by the fold at the opening of the uterus masculinus. Occasionally the hymen is imperforate and then gives rise to trouble in menstruation.
The bulbs of the vestibule are two masses of erectile tissue situated one on each side of the vaginal orifice: above they are continued up to the clitoris; they represent the bulb and the corpus spongiosum of the male, split into two, and the fact that they are so divided accounts for the urethra failing to be enclosed in the clitoris as it is in the penis.
The glands of Bartholin are two oval bodies about half an inch long, lying on each side of the vagina close to its opening; they represent Cowper's glands in the male, and their ducts open by minute orifices between the hymen and the labia minora.
From the above description it will be seen that all the parts of the male external genital organs are represented in the female, though usually in a less developed condition, and that, owing to the orifice of the vagina, they retain their original bi-lateral form.
For further details see Quain's Anatomy (London: Longmans, Green & Co.); Gray's Anatomy (London: Longmans, Green & Co.); Cunningham's Text-Book of Anatomy (Edinburgh: Young J. Pentland), or Macalister's Anatomy (London: Griffin & Co.).
The development of the reproductive organs is so closely interwoven with that of the urinary that some reference from this article to that on the Urinary System is necessary. It will here be convenient to take up the development at the stage depicted in the accompanying figure (fig. 7), in which the genital ridge (a) is seen on each side of the attachment of the mesentery; external to this, and forming another slight ridge of its own, is the Wolffian duct, while a little later the Müllerian duct is formed and lies ventral to the Wolffian. The early history of these ducts is indicated in the article on the Urinary System. Until the fifth or sixth week the development of the genital ridge is very much the same in the two sexes, and consists of cords of cells growing from the epithelium-covered surface into the mesenchyme, which forms the interior of the ridge. In these cords are some large germ cells which are distinguishable at a very early stage of development. It must, of course, be understood that the germinal epithelium covering the ridge, and the mesenchyme inside it, are both derived from the mesoderm or middle layer of the embryo. About the fifth week of human embryonic life the tunica albuginea appears in the male, from which septa grow to divide the testis into lobules, while the epithelial cords form the seminiferous tubes, though these do not gain a lumen until just before puberty. From the adjacent mesonephros cords of cells grow into the attached part of the genital ridge, or testis, as it now is, and from these the rete testis is developed. Recent research, however, points to these cords of the rete testis et ovarii as being derived from the coelomic epithelium instead of from the mesonephros.
|From A. F. Dixon, Cunningham's Text-book of Anatomy.|
Fig. 7.—Transverse Section through a Rat Embryo.
a. shows position of germinal epithelium.
In the female the same growth of epithelial cords into the mesenchyme of the genital ridge takes place, but each one is distinguished by a bulging toward its middle, in which alone the large germ cells are found. Eventually this bulging part is broken up into a series of small portions, each of which contains one germ cell or ovum, and gives rise to a Graafian follicle. Mesonephric cords appear as in the male; they do not enter the ovary, however, but form a transitory network (rete ovarii) in the mesovarium. As each genital gland enlarges it remains attached to the rest of the intermediate cell mass by a constricted fold of the coelomic membrane, known as the mesorchium in the male, and the mesovarium in the female. Lying dorsal to the genital ridge in the intermediate cell mass is the mesonephros, consisting of numerous tubules which open into the Wolffian duct. This at first is an important excretory organ, but during development becomes used for other purposes. In the male, as has been shown, it may form the rete testis, and certainly forms the vasa efferentia and globus major of the epididymis; in addition to these, some of its separate tubes probably account for the vas aberrans and the organ of Giraldès (see fig. 8, E. and O.G.). In the female the tubules of the epoöphoron represent the main part, while the paroöphoron, like the organ of Giraldès in the male, is probably formed from some separate tubes (see fig. 8, Ep. O. and Par. O.).
The Wolffian duct, which, in the early embryo, carries the excretion of the mesonephros to the cloaca, forms eventually the body and tail of the epididymis, the vas deferens, and ejaculatory duct in the male, the vesicula seminalis being developed as a pouch in its course. In the female this duct is largely done away with, but remains as the collecting tube of the epoöphoron, and in some mammals as the duct of Gärtner, which runs down the side of the vagina to open into the vestibule.
The Müllerian duct, as it approaches the cloaca, joins its fellow of the opposite side, so that there is only one opening into the ventral cloacal wall. In the male the lower part only of it remains as the uterus masculinus (fig. 8, U.M.), but in the female the Fallopian tubes, uterus, and probably the vagina, are all formed from it (fig. 8, F.T. and U.). In both sexes a small hydatid or vesicle is liable to be formed at the beginning of both the Wolffian and Müllerian duct (fig. 8, P.H. and S.H.); in the male these are close together in front of the globus major of the epididymis, and are known as the sessile and pedunculated hydatids of Morgagni. In the female there is a hydatid among the fimbriae of the Fallopian tube which of course is Müllerian and corresponds to the sessile hydatid in the male, while another is often found at the beginning of the collecting tube of the epoöphoron and is probably formed by a blocked mesonephric tubule. This is the pedunculated hydatid of the male. The development of the vagina, as Berry Hart (Journ. Anat. and Phys. xxxv. 330) has pointed out, is peculiar. Instead of the two Müllerian ducts joining to form the lumen of its lower third, as they do in the case of the uterus and its upper two-thirds, they become obliterated, and their place is taken by two solid cords of cells, which Hart thinks are derived from the Wolffian ducts and are therefore probably of ectodermal origin, though this is open to doubt. These cords later become canalized and the septum between them is obliterated.
The common chamber, or cloaca, into which the alimentary, urinary and reproductive tubes open in the foetus, has the urinary bladder (the remains of the allantois) opening from its ventral wall (see Placenta and Urinary System).
During development the alimentary or anal part of the cloaca is separated from the urogenital, and in the article Alimentary System the hitherto accepted method of this separation is described. The question has, however, lately been reinvestigated by F. Wood Jones, who says that the anal part is completely shut off from the urogenital and ends in a blind pouch which grows toward the surface and meets a new ectodermal depression, the main point being that the permanent anus is not, according to him, any part of the original cloacal aperture, but a new perforation. This description is certainly more in harmony with the malformations occurring in this region than the old one, and only awaits confirmatory evidence to be generally accepted.
The external generative organs have at first the same appearance in the two sexes, and consist of a swelling, the genital eminence, in the ventral wall of the cloaca. This in the male becomes the penis and in the female the clitoris. Throughout the generative system the male organs depart most from the undifferentiated type, and in the case of the genital eminence two folds grow together and enclose the urogenital passage, thus making the urethra perforate the penis, while in the female these two folds remain separate as the labia minora or nymphae. Sometimes in the male the folds fail to unite completely, and then there is an opening into the urethra on the under surface of the penis—a condition known as hypospadias.
In the undifferentiated condition the integument surrounding the genital opening is raised into a horseshoe like swelling with its convexity over the pubic symphysis and its concavity toward the anus; the lateral parts of this remain separate in the female and form the labia majora, but in the male they unite to form the scrotum. The median part forms the mons Veneris or mons Jovis.
The Descent of the Testis.—It has been shown that the testis is formed in the loin region of the embryo close to the kidney, and it is only in the later months of foetal life that it changes this position for that of the scrotum. In the lower part of the genital ridge a fibro-muscular cord is formed which stretches from the lower part of the testis to the bottom of the scrotum; it is known as the gubernaculum testis, and by its means the testis is directed into the scrotum. Before the testis descends, a pouch of peritoneum called the processus vaginalis passes down in front of the gubernaculum through the opening in the abdominal wall, which afterwards becomes the inguinal canal, into the scrotum, and behind this the testis descends, carrying with it the mesonephros and mesonephric duct. These, as has already been pointed out, form the epididymis and vas deferens. At the sixth month the testis lies opposite the abdominal ring, and at the eighth reaches the bottom of the scrotum and invaginates the processus vaginalis from behind. Soon after birth the communication between that part of the processus vaginalis which now surrounds the testis and the general cavity of the peritoneum disappears, and the part which remains forms the tunica vaginalis. Sometimes the testis fails to pass beyond the inguinal canal, and the term “cryptorchism” is used for such cases.
In the female the ovary undergoes a descent like that of the testis, but it is less marked owing to the fact that the gubernaculum becomes attached to the Müllerian duct where that duct joins its fellow to form the uterus; hence the ovary does not descend lower than the level of the top of the uterus, and the part of the gubernaculum running between it and the uterus remains as the ligament of the ovary, while the part running from the uterus to the labium is the round ligament. In rare cases the ovary may be drawn into the labium just as the testis is drawn into the scrotum.
Comparative Anatomy.—In the Urochorda, the class to which Salpa, Pyrosoma and the sea squirts (Ascidians) belong, male and female generative glands (gonads) are present in the same individual; they are therefore hermaphrodite.
In the Acrania (Amphioxus) there are some twenty-six pairs of gonads arranged segmentally along the side of the pharynx and intestine and bulging into the atrium. Between them and the atrial wall, however, is a rudimentary remnant of the coelom, through which the spermatozoa or ova (for the sexes are distinct) burst into the atrial cavity. There are no genital ducts.
In the Cyclostomata (lampreys and hags) only one median gonad is found, and its contents (spermatozoa or ova) burst into the coelom and then pass through the genital pores into the urogenital sinus and so to the exterior. It is probable that the single gonad is accounted for by the fact that its fellow has been suppressed.
In the Elasmobranchs or cartilaginous fishes there are usually two testes or two ovaries, though in the dogfish one of the latter is suppressed. From each testis, which in fish is popularly known as the soft roe, vasa efferentia lead into the mesonephros, and the semen is conducted down the vas deferens or mesonephric duct into the urogenital sinus, into which also the ureters open. Sometimes one or more thin-walled diverticula—the sperm sacs—open close to the aperture of the vas deferens. In the female the ova are large, on account of the quantity of yolk, and they burst into the coelum, from which they pass into the large Müllerian ducts or oviducts. In the oviparous forms, such as the common doglish (Scyllium), there is an oviducal gland which secretes a horny case for the egg after it is fertilized, and these cases have various shapes in different species. Some of the Elasmobranchs, e.g., the spiny dogfish (Acanthias), are viviparous, and in these the lower part of the oviduct is enlarged and acts as a uterus. In male elasmobranchs the anterior part of the Müllerian duct persists. Paired intromittent organs (claspers) are developed on the pelvic fins of the males; these conduct the semen into the cloaca of the female.
In the teleostean and ganoid fishes (Teleostomi) the nephridial ducts are not always used as genital ducts, but special coelomic ducts are formed (see Coelom and Serous Membranes).
In the Dipnoi or mudfish long coiled Müllerian ducts are present, but the testes either pour their secretion directly into the coelom or, as in Protopterus, have ducts which are probably coelomic in origin.
In both the Teleostomi and Dipnoi the testes and ovaries are paired.
True hermaphroditism is known among fishes, the hag (Myxine) and the sea perch (Serranus) being examples. In many others it occurs as an abnormality.
In the Amphibia both ovaries and testes are symmetrical. In the snakelike forms which are found in the order Gymnophiona the testes are a series of separate lobules extending for a long distance, one behind the other, and joined by a connecting duct from which vasa efferentia pass into the Malpighian capsules of the kidneys, and so the sperm is conducted to the mesonephric duct, which acts both as vas deferens and ureter. The Müllerian ducts or oviducts are long and often coiled in Amphibia, and usually open separately into the cloaca. There is no penis, but in certain forms, especially the Gymnophiona, the cloaca is protrusile in the male and acts as an intromittent organ. Corpora adiposa or fat bodies are present in all Amphibians, and probably nourish the sexual cells during the hibernating period.
In Reptilia two testes and ovaries are developed, though they are often asymmetrical in position. In Lizards the vas deferens and ureter open into the cloaca by a common orifice; as they do in the human embryo. In these animals there are two penes, which can be protruded and retracted through the vent; but in the higher reptiles (Chelonia and Crocodilia) there is a single median penis rising from the ventral wall of the cloaca, composed of erectile tissue and deeply grooved on its dorsal surface for the passage of the sperm.
In birds the right ovary and oviduct degenerates, and the left alone is functional. In the male the ureter and vas deferens open separately into the cloaca, and in the Ratitae (ostriches) and Anseres (ducks and geese) a well-developed penis is present in the male. In the ostrich this is fibrous, and bifurcated at its base, suggesting the crura penis of higher forms.
Among the Mammalia the Monotremata (Ornithorhynchus and Echidna) have bird-like affinities. The left ovary is larger than the right, and the oviducts open separately into the cloaca and do not fuse to form a uterus. The testes retain their abdominal position; and the vasa deferentia open into the base of the penis, which lies in a separate sheath in the ventral wall of the cloaca, and shows an advance on that of the reptiles and birds in that the groove is now converted into a complete tunnel. In the female there is a well-developed clitoris, having the same relations as the penis.
In the marsupials the cloaca is very short, and the vagina and rectum open separately into it. The two uteri open separately and three vaginae are formed, two lateral and one median. The two lateral join together below to form a single median lower vagina, and it is by means of these that the spermatozoa pass up into the oviducts. The upper median vagina at first does not open into the lower one, but during parturition a communication is established which in some animals remains permanent (see J. P. Hill, Proc. Linn. Soc. N.S. Wales, 1899 and 1900). This tripartite arrangement of the upper part of the marsupial vagina is of especial interest in connexion with the views of the embryology of the canal detailed by Berry Hart and already referred to.
When, as in marsupials, the two uteri open separately into the vagina by two ora, the arrangement is spoken of as uterus duplex. When the two uteri join below and open by one os externum, it is known as uterus bipartitus. When the uterus bifurcates above and has two horns for the reception of the Fallopian tubes (oviducts), but is otherwise single, the term uterus bicornis is given to it, while the single uterus of man and other Primates is called uterus simplex. From the marsupials upward the ovarian end of the Fallopian tube has the characteristic fimbriated appearance noticed in human anatomy.
In some mammals, such as the sow and the cow, the Wolffian duct is persistent in the female and runs along the side of the vagina as the duct of Gärtner. It is possible that the lateral vaginae of the marsupials are of Wolffian origin.
In marsupials the testes descend into the scrotum, which lies in these animals in front of instead of behind the penis. In some mammals, such as the elephant, they never reach the scrotum at all; while in others, e.g. many rodents, they can be drawn up into the abdomen or lowered into the scrotum. The subject of the descent of the testicles has been very fully treated by H. Klaatsche, “Ueber den Descensus testiculorum,” Morph. Jahrb., Bd. xvi.
The prostate is met with in its most simple forms in marsupials, in which it is a mere thickening of the mucous membrane of the urethra; in the sheep it forms a bilateral elongated mass of gland tissue lying behind the urethra and surrounded by a well developed layer of striped muscle. In the sloth it is said to be altogether absent, while in many of the insectivores and rodents it consists of many lobes which usually show a bilateral arrangement. The vesiculae seminales are usually present in the Eutheria or higher mammals, and sometimes, as in the hedgehog, are very large, though they are absent in the Carnivora. Cowper's glands are usually present and functional throughout life. The uterus masculinus is also usually present, but there is grave doubt whether the large organ called by this name in the rabbit should not rather be regarded as homologous with part of the vesiculae seminales. The penis shows many diversities of arrangement; above the marsupials its two crura obtain an attachment to the ischium. In many mammals it is quite hidden by the skin in the flaccid condition, and its external orifice may range from the perineum in the marsupials to the middle of the ventral wall of the abdomen in the ruminants. In the Marsupialia, Rodentia, Chiroptera, Carnivora and some Primates an os penis is developed in connexion with the corpora cavernosa.
The clitoris is present in all mammals; sometimes, as in the female hyena, it is very large, and at others, as in the lemur, it is perforated by the urethra.
For further details and literature, see Oppel's Lehrbuch der vergleich. mikroskop. Anatomie der Wirbelthiere, Bd. iv. (Jena, 1904); also Gegenbaur's Vergleich. Anat. der Wirbelthiere, and Wiedersheim's Comparative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907).
(F. G. P.)
- Figs. 3, 4, 5 and 9 of this article are redrawn from Cuthbert S. Wallace's Prostatic Enlargement by permission of the managers of The Oxford Medical Publications.