Part 55

=Structure.=—The greater part of the surface of the ovary has a covering of peritoneum. The peritoneal investment is absent at the attached border where the vessels and nerves enter; this area is termed the =hilus=. The ovulation fossa is covered by a layer of columnar cells, a remnant of the primitive =germinal epithelium=. The =stroma= of the ovary (Stroma ovarii) is a network of connective tissue and unstriped muscular fibers. In the deeper or medullary portion of the stroma (Zona vasculosa) the meshes are largely occupied by the numerous blood-vessels. In the peripheral or cortical portion (Zona parenchymatosa) are numerous =ova= (Ovula) in various stages of development. The immature ova are surrounded by follicle cells; those more advanced in development are inclosed also by a condensation of the stroma termed the =theca folliculi=; within the theca is a quantity of fluid, the =liquor folliculi=. These constitute the =Graafian follicles= (Folliculi oöphori), which enlarge as they mature, becoming visible to the naked eye as vesicles with a diameter of a centimeter or more. When fully developed, the follicles are superficially situated, and often project slightly from the surface of the ovary. At intervals follicles rupture and their contents escape. This process, which sets free the ovum, is termed =ovulation=; it takes place in the mare only at the ovulation fossa, and occurs during the periods of œstrum.

[Illustration:

FIG. 409.—SCHEMATIC REPRESENTATION OF CHANGES IN OVARY OF MARE FROM FŒTAL TO ADULT STATE. (After Born.)

The changes affect chiefly the free border (upper in figure) and the extent and form of the area covered by germinal epithelium (_4_). _1_, Peritoneum (broad ligament); _2_, ligament of ovary; _3_, fimbria of Fallopian tube; _4_, germinal epithelium; _5_, vessels, which reach the ovary at the attached border (_6_). The latter represents the hilus, while the deep depression at the free border is the ovulation fossa. (Ellenberger, in Leisering’s Atlas.) ]

After rupture of a follicle its cavity is partly occupied by a blood-clot, constituting what may be termed a =corpus rubrum=. By proliferation and enlargement and fatty changes, the follicle cells are transformed into lutein cells, forming a yellow mass known as a =corpus luteum=. If impregnation takes place, the accompanying increase in vascularity of the organs may cause the corpus luteum to reach a large size; if impregnation does not occur, it is much smaller, and is sooner replaced by scar tissue, the =corpus albicans=.

In the young foal the ovaries are large and oval in form. The free border is convex and is covered by germinal epithelium, which extends over part of the surfaces also. This area is distinguishable by its dull gray appearance from the peritoneal surface, which has the usual smooth glistening character. As growth proceeds the ovary gradually becomes bent until it assumes its definitive curved shape. The germinal epithelium is then limited to the ovulation fossa. The ovary migrates somewhat during development from its primitive position, which is the same as that of the testicle.

In old animals the ovaries commonly consist largely of fibrous tissue, in which there are often cysts of various sizes. The ova, present in enormous number at birth, have then been extruded, or destroyed by phagocytic action or degeneration.

=Vessels and Nerves.=—The =arteries= of the ovary are derived from the =ovarian artery=. The artery is relatively large and is flexuous; it reaches the attached border of the ovary by passing between the layers of the mesovarium. The veins are large and numerous. They form a plexus somewhat like that of the spermatic cord. The =lymph vessels= pass to the lumbar glands. The =nerves= are derived from the sympathetic system through the renal and aortic plexuses. They accompany the arterial branches.

THE UTERINE OR FALLOPIAN TUBES

The =uterine= or =Fallopian tubes= (Tubæ uterinæ) act as excretory ducts of the ovaries, since they convey the ova from the reproductive glands to the uterus. They are not, however, in direct continuity with the glands, but rather partly in contiguity with, and partly attached to, them.[150] They are two flexuous tubes, about ten to twelve inches (ca. 25 to 30 cm.) long, which extend from the extremities of the uterine cornua to the ovaries. Each is inclosed in a peritoneal fold, derived from the external layer of the broad ligament, and termed the =mesosalpinx=. This largely covers the outer aspect of the ovary, and forms with it and the broad ligament a pouch called the =bursa ovarica=. The =uterine extremity= of the tube communicates with the cavity of the cornu by a minute orifice, the =ostium uterinum=. The =ovarian= or =fimbriated extremity= is expanded into a funnel-shaped =infundibulum=. The margin of the latter is slit into irregular processes or =fimbriæ=, some of which, the =fimbriæ ovaricæ=, are attached in the ovulation fossa. In the center of the fimbriated extremity is a small opening, the =ostium abdominale=, by which the tube communicates with the peritoneal cavity. The tube appears normally to be applied to the ovary, so that the extruded ova pass into it and are conveyed to the uterus.[151]

Pedunculated cysts, the hydatids of =Morgagni=, are often found on one or more of the fimbriæ. In the mesosalpinx are blind flexuous tubules, which constitute the =paroöphoron=, a remnant of the Wolffian body. They are most evident in the young adult, and tend to disappear with increasing age. Not uncommonly they give rise to cysts.

=Structure.=—The tube is covered externally by a =serous coat= formed by the mesosalpinx. The serous membrane is continued on the fimbriæ, and meets the mucous lining on them. The =fibrous coat= or adventitia is continuous with the fibrous lamina of the broad ligament. The =muscular coat= consists chiefly of circular fibers, although longitudinal fibers occur externally. The =mucous coat= is thin and is much plicated. The folds (Plicæ tubariæ) are chiefly longitudinal, but in the wide part of the tube (Ampullæ tubæ) they are very complex, so that on cross-sections the spaces between the folds may be mistaken for branched tubular glands. The folds are continued on to the fimbriæ. The =epithelium= is a single layer of columnar ciliated cells, the cilia producing a current directed toward the uterus. At the fimbriated extremity this epithelium passes gradually into the squamous type of the serous coat.

=Vessels and Nerves.=—The =arteries= are derived from the =utero-ovarian artery=. The =veins= are satellites of the arteries. The =lymphatics= pass with the ovarian vessels to the lumbar glands. The nerves have a similar origin to those of the ovary.

THE UTERUS

The =uterus= is a hollow muscular organ, which receives the Fallopian tubes anteriorly and opens into the vagina posteriorly. It is situated chiefly in the abdominal cavity, but extends a short distance into the pelvic cavity. It is attached to the lumbar wall of the abdomen and the lateral walls of the pelvic cavity by two folds of peritoneum termed the broad ligaments. It consists of two cornua or horns, the body, and the neck.

The =cornua= or =horns= of the uterus (Cornua uteri) are situated entirely in the abdomen. They appear to vary considerably in position; commonly they are pressed up against the sublumbar muscles by the intestine (left portions of large colon, small colon, and small intestine). They are cylindrical, and are about ten inches (ca. 25 cm.) in length. The anterior extremity of each forms a blunt point which receives the Fallopian tube. Posteriorly they increase somewhat in caliber, converge, and unite with the body. The =dorsal= (or attached) =border= is slightly concave and is attached to the broad ligament. The =ventral border= is convex and free.

[Illustration:

FIG. 410.—GENITAL ORGANS OF MARE, DORSAL VIEW.

The left cornu and adjacent part of the body of the uterus and the vagina and vulva are opened up. _1_, Ovary; _2_, Fallopian tube, with _2′_, its fimbriated end; _3_, cavity of corpus uteri; _4_, cavity of left cornu; _5_, right cornu, with _5′_, its communication with the body; _6_, portio vaginalis uteri; _6′_, os uteri; _7_, broad ligament of uterus; _8_, ligament of ovary; _9_, vagina; _10_, vulva; _10′_, glandulæ vestibulares majores; _10″_, glandulæ vestibulares minores; _11_, transverse fold; _12_, external urethral opening; _13_, _13′_, labia vulvæ; _14_, glans clitoridis; _14′_, junction of crura to form corpus clitoridis indicated by dotted lines; _15_, urinary bladder; _a_, utero-ovarian artery with ovarian (_a′_) and uterine (_a″_) branches; _b_, uterine artery. (After Ellenberger, in Leisering’s Atlas.) ]

When a soft uterus is distended, the cornua are strongly curved, but this does not represent the natural form. When fixed _in situ_ the horns are either slightly curved or nearly straight. The two horns are commonly unsymmetrical in length and diameter in mares which have borne young.

The =body= of the uterus (Corpus uteri) is situated partly in the abdomen, partly in the pelvic cavity. It is cylindrical, but considerably flattened dorso-ventrally, so that in cross-section it is elliptical. Its average length is about six inches (15 cm.), and its diameter, when moderately distended, about four inches (10 cm.). Its =dorsal surface= (Facies rectalis) is related to the small colon and rectum. Its =ventral surface= (Facies vesicalis) is in contact with the bladder, and has inconstant relations with various parts of the intestine. The term =fundus uteri= is applied to the anterior wide part from which the cornua diverge.

The position of the body of the uterus is variable, especially in regard to its anterior part. It is often pressed up against the rectum, and may be deflected to either side—most frequently to the left—by the pelvic flexure of the colon or coils of the small colon.

The =cervix= or =neck= of the uterus (Cervix uteri) is the constricted posterior part which joins the vagina. Part of it (Portio vaginalis uteri) projects into the cavity of the vagina; it is therefore not visible externally, but may be felt through the vaginal wall. It is about two inches (5 cm.) in length, and an inch and a half (3.5 to 4 cm.) in diameter.

=Attachments.=—The body and cornua are attached to the abdominal and pelvic walls by two extensive peritoneal folds, the =broad ligaments= (Ligamenta lata). These extend on either side from the sublumbar region and the lateral pelvic walls to the dorsal border of the cornua and the lateral margins of the body of the uterus. They contain the vessels and nerves of the uterus and ovaries, connective tissue, and unstriped muscular fibers. The ureters are situated along their parietal margins. The outer layer of each forms a fold, the =round ligament= of the uterus (Ligamentum teres uteri), which blends with the parietal peritoneum over the internal inguinal ring; its anterior extremity is situated above the extremity of the cornu and is rounded and free. It contains muscular tissue, vessels, and nerves, and is the homologue of the gubernaculum testis. The anterior part of the neck is continuous with the vagina, and thus has a more fixed position than the rest of the organ.

=The cavity= of the uterus (Cavum uteri) is largely obliterated in the non-pregnant state by the contraction of the wall and by folds of the mucous lining. At the extremity of each cornu it communicates by a minute opening with the Fallopian tube. The cavity of the neck is termed the =cervical canal= (Canalis cervicis); it is closed ordinarily by mucous folds and a plug of mucus. It opens into the vagina by the =os uteri externum=, and into the body by the =os uteri internum=.

=Structure.=—The wall of the uterus consists of three coats—an external serous, a middle muscular, and an internal mucous. The =serous coat= (Perimetrium) is, for the most part, closely adherent to the muscular coat. It is continuous with the broad ligaments. The =muscular coat= consists essentially of two layers, a thin external stratum of longitudinal fibers, and a thick layer of circular fibers. Between these is a very vascular layer (Stratum vasculare) of connective tissue with circular and oblique muscular fibers in it. The circular coat is very thick in the neck, where it forms a =sphincter= half an inch or more (ca. 1.5 cm.) in thickness. The =mucous membrane= rests directly on the muscular coat. It is thick, and is covered by columnar epithelium. It contains numerous long tubular =uterine glands= (Glandulæ uterinæ). It is brownish-red in color, except in the neck, where it is pale.

=Vessels and Nerves.=—The chief =arteries= are the =uterine= and the =uterine branch= of the =utero-ovarian=, which have a flexuous course in the broad ligaments; there is also a branch from the =internal pudic= artery. The =veins= form pampiniform plexuses and accompany the arteries. The =lymphatics= are numerous and go to the internal iliac and lumbar glands. The =nerves= are derived from the sympathetic through the uterine and pelvic plexuses.

The foregoing statements refer to the non-gravid uterus. In the pregnant state it undergoes important changes in size, position, and structure. The increase in size affects chiefly the gravid cornu (except in the case of twins) and the body. The cornu attains a length of about two and a half to three feet (ca. 80 to 90 cm.), and a corresponding diameter; in this process it extends much beyond the ovary, and the broad ligament. The gravid uterus is entirely abdominal in position, and extends along the ventral wall, chiefly to the left of the median plane. It weighs about nine pounds (ca. 4 kg.), according to Ellenberger. The broad ligaments increase greatly in size and contain more muscular tissue. The vessels are greatly enlarged and form new branches. The muscular coat, in spite of the increase in size and number of the fibers, is somewhat thinner, except in the neck. The mucous membrane is thicker and more vascular; the epithelium becomes non-ciliated and stratified.

THE VAGINA

The =vagina= is the passage which extends horizontally through the pelvic cavity from the neck of the uterus to the vulva. It is tubular, is about six to eight inches (ca. 15 to 20 cm.) in length, and, when slightly distended, about four or five inches (ca. 10 to 12 cm.) in diameter. Its dilatability appears to be limited only by the pelvic wall. There is no external line of demarcation between the vagina and the uterus or the vulva.

It is related dorsally to the rectum, ventrally to the bladder and urethra, and laterally to the pelvic wall. The recto-genital pouch of the peritoneum extends between the vagina and rectum for a distance of about two inches (5 cm.), and ventrally the vesico-genital pouch passes backward somewhat less between the vagina and bladder. Thus most of the vagina is retroperitoneal and is surrounded by a quantity of loose connective tissue, a venous plexus, and a variable amount of fat.

=Structure.=—With the exception of the short peritoneal part, as indicated above, the proper wall of the vagina is composed of muscular and mucous coats. The =muscular coat= is composed of a thin layer of longitudinal fibers, and a thicker layer of circular fibers; it contains also many elastic fibers. The =mucous coat= is highly elastic, and is covered with a stratified epithelium. It has no glands.

Under usual conditions the cavity is practically obliterated by apposition of the walls, so that the lumen is a transverse slit; this condition is pronounced when the ampulla of the rectum is full. The anterior end of the vagina is largely occupied by the intravaginal part of the neck of the uterus, so that the cavity is here reduced to an annular recess termed the =fornix vaginæ=. The posterior part is directly continuous with the vulva without any line of demarcation except the valvular fold, which covers the external urethral orifice; in very young subjects this fold is continued on either side, forming the =hymen=, which narrows the entrance to the vagina (Introitus vaginæ).[152]

=Vessels and Nerves.=—The blood-supply is derived from the branches of the =internal pudic arteries=. The =veins= form a plexus which is drained by the =internal pudic veins=. The =lymphatics= pass to the internal iliac lymph glands. The =nerves= are derived from the sympathetic through the pelvic plexus.

THE VULVA

The =vulva= or =urogenital sinus= is the terminal part of the genital tract.[153] It is continuous in front with the vagina, and opens externally at the vulvar cleft below the anus. There is no external line of demarcation between the vagina and vulva. The tube is about five inches (ca. 12 cm.) in length, measured from the external urethral orifice to the inferior commissure; dorsally it is considerably shorter. It is related dorsally to the rectum and anus, ventrally to the pelvic floor, and laterally to the sacro-sciatic ligament, the semimembranosus muscle, and the internal pudic artery. The external orifice or =vulvar cleft= (Rima vulvæ) has the form of a vertical slit, about four to six inches (ca. 10 to 15 cm.) high, and is margined by two prominent rounded folds, the labia. The =labia= of the vulva (Labia vulvæ) meet above at an acute angle, forming the =superior commissure= (Commissura dorsalis), which is about two inches (ca. 5 cm.) below the anus. Below they unite to form the thick rounded =inferior commissure= (Commissura ventralis), which lies about two inches (ca. 5 cm.) behind and below the ischial arch. When the labia are drawn apart, a rounded body, about an inch (ca. 2.5 cm.) wide, is seen in the inferior commissure; this is the =glans clitoridis=, the homologue of the glans penis. Overlying it is a thin fold, the =prepuce= of the clitoris (Præputium clitoridis). At the anterior extremity of the ventral wall of the vulva, _i. e._, about four or five inches (ca. 10 to 12 cm.) from the inferior commissure, is the =external urethral orifice= or meatus urinarius (Orificium urethræ externum). It readily admits the finger and is very dilatable. It is covered by a fold of mucous membrane, the free edge of which is directed backward.

=Structure.=—The =labia= are covered by thin, pigmented, smooth skin, which is richly supplied with sebaceous and sweat glands. This is continuous at a distance of about half an inch (ca. 1 to 1.5 cm.) from the free edge with a thin glandless mucous membrane. Between these is a layer of striped muscle, the =constrictor vulvæ=; this fuses above with the sphincter ani, and embraces the clitoris below, spreading out laterally at the inferior commissure. It constricts the vulvar orifice and elevates the clitoris. The =constrictor vestibuli= muscle embraces the vulva in front of the preceding; it is joined on either side by a band of unstriped muscle which arises from the first and second coccygeal vertebræ, and is homologous with the proximal part of the retractor penis. It constricts the vulva. Under this muscle in the lateral wall is a flattened, oval body, the =bulbus vestibuli= (Fig. 453); this is an erectile structure, homologous with the corpus spongiosum urethræ of the male. It is about two and a half to three inches (ca. 6 to 8 cm.) long, and an inch or more (ca. 3 cm.) wide. It consists of a venous network inclosed in a fibrous capsule, and is supplied with blood by a large branch of the internal pudic artery. The =mucous membrane= of the vulva is reddish in color, and forms longitudinal and transverse folds. It presents ventrally two linear series of small papillæ which converge toward the inferior commissure; these mark the orifices of the ducts of the =glandulæ vestibulares minores=. On either side of the dorsal wall is a group of eight to ten larger prominences on which the ducts of the =glandulæ vestibulares majores= (of Bartholin) open.

Quite exceptionally there may be found on either side of the urethral orifice the opening of the canals of Gartner (Ductus epoöphori longitudinales).

The =clitoris= is the homologue of the penis. It arises from the ischial arch by two crura, which unite to form a =body= as large as one’s little finger. This projects backward into the inferior commissure of the vulva, capped by a small rounded =glans=. The organ is composed of erectile tissue similar to the corpus cavernosum penis. The =ischio-cavernosus= (or erector clitoridis) is a very feeble muscle which depresses the clitoris. The veins of the clitoris communicate by an intermediate plexus on either side with the bulbus vestibuli.

THE FEMALE URETHRA

The female =urethra= (Urethra feminina) represents only that part of the canal of the male which lies between the internal urethral orifice and the openings of the ejaculatory ducts. Its length is about two inches (5 cm.), and its lumen is sufficient to admit of the introduction of the finger; it is, however, capable of remarkable dilatation if sufficient care and patience are exercised in the process. It lies centrally on the pelvic floor, and is related and attached dorsally to the vagina.

=Structure.=—The intrinsic =muscular coat= consists of longitudinal and circular layers of unstriped fibers. Between these is a layer of areolar tissue which contains a rich =venous plexus=. The =mucous membrane= is thrown into longitudinal folds when the canal is closed; it is highly elastic, and is covered with stratified epithelium.

The =urethral muscle= (M. compressor urethræ) embraces the urethra and is continued backward for some distance under the constrictor vestibuli. It is covered by a fibro-elastic membrane.

THE MAMMARY GLANDS

The =mammary glands= (Glandulæ lactiferæ) are modified cutaneous glands which are so closely associated functionally with the genital organs as to be considered accessory to them.

In the mare they are two in number, and are placed on either side of the median plane in the inguinal region. Each gland has the form of a short cone, much compressed transversely, and having a flat inner surface. It consists of the glandular mass or body of the gland (Corpus mammæ) and the =teat= or =nipple= (Papilla mammæ). The base is related to the abdominal wall, to which it is attached by areolar tissue, which contains a venous plexus, the superficial inguinal lymph glands, and a variable amount of fat. The apex is constituted by the teat, which is also flattened transversely and varies in length from one to two inches (ca. 2.5 to 5 cm.). Between the bases of the teats is the intermammary groove. On the apex of each teat two or three small orifices are placed close together; these are the openings of the lactiferous ducts.

=Structure.=—The =skin= over the glands is thin, pigmented, chiefly hairless, and supplied with numerous large sebaceous and sweat glands. Under this are two layers of =fascia= except on the teats. The superficial fascia presents no special features. The deep fascia consists of two laminæ detached from the abdominal tunic which descend on either side of the median plane, forming a =septum= between the two glands, and constituting their ligamentum suspensorium.

These laminæ are separated by a layer of areolar tissue, so that it is possible to remove one gland if diseased by careful dissection between the layers of the septum.

The =gland substance= or =parenchyma= is pinkish gray in color, and of firmer consistence than the fat which is found around and within the gland. It is inclosed by a fibro-elastic capsule which sends inward numerous trabeculæ; these form the =interstitial tissue=, and divide the gland into =lobes= and =lobules=. In the latter are the secretory =tubules= and =alveoli=, which unite to form the larger =ducts=. Each lobe has a duct, which opens at the base of the teat into a space called the =lactiferous sinus= (Sinus lactiferus); this is lined with mucous membrane, and from it two (or three) =lactiferous ducts= (Ductus lactiferi) pass through the extremity of the nipple. These ducts are lined with a non-glandular mucous membrane, which is covered with stratified squamous epithelium. They are surrounded by unstriped muscular tissue, the bulk of the fibers being arranged in a circular manner to form a =sphincter=.