Part 160

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. 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.

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.

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).

=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.