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Chapter 5. THE FEMALE PELVIS AND PERINEUM

5.1  THE PELVIC CANAL

The pelvis prolongs the abdomen within the bony ring formed by the sacrum and first few coccygeal vertebrae dorsally, the pelvic girdle laterally and ventrally. It conveys the terminal parts of the digestive, genital and urinary tracts to their anal and vulvar openings and also encloses a number of important vessels and nerves. Because of its extensive skeletal support the cavity cannot alter much in size but its dimensions are not absolutely fixed and a limited enlargement in some directions becomes possible at the time of parturition. The bony pelvis is composed of the ossa coxarum, sacrum and first three coccygeal vertebrae.

 

5.2 LIGAMENTS OF THE PELVIS

There are three, single or paired, pelvic ligaments that maintain the relationship of the pelvis to the (spinal cord.)
1) The dorsal and lateral sacroiliac ligaments: which are attached to the medial wing of the ilium and the lateral portion of the sacrum and the summits of the sacral spines. This articulation is very firm and rigid.
2) The sacrosciatic ligament: is an extensive quadrilateral ligamentous sheet that completes the lateral wall of the pelvic cavity; this runs between the lateral margin of the sacrum and the dorsal borders of the ilium and ischium, leaving open the greater and lesser sciatic foramen. In cattle the so-called lesser foramen is in fact by far the larger of the two.
3) The prepubic tendon is essentially the tendon of insertion of the recti abdomenis muscles. It is attached strongly to the cranial border of the pubic bones. It is of importance in fixing the sacroiliac articulation and maintaining the bony pelvis in its proper position.
 
5.2.1 Inguinal ligaments:
In the inguinal region, the aponeurosis of the external abdominal oblique muscle divides into two layers, especially evident in bovine; one of these layers curves dorsally and caudally to attach to the tuber coxae and the prepubic tendon. Between these two points the aponeurosis is greatly strengthened by additional collaginous fibers and is called the inguinal ligament.

 

5.3 GENERAL VISCERAL TOPOGRAPHY

The general topography of the pelvic viscera may be studied with the aid of the median section. The digestive, genital and urinary tracts are arranged one above the other and more or less symmetrically. All are intraperitoneal in the cranial part of the pelvis but shed their peritoneal coats more caudally where they are surrounded by considerable amounts of fat and loose connective tissue.
 
There are frequent movements of organs across the boundary between the abdomen and pelvis and no single disposition of the viscera can be described as the normal pattern. The amount of the reproductive tract that lies within the pelvis is especially variable, depending upon the age, the present status and the past history of the cow; but in the short term it is the bladder which is least constant since it extends forwards along the abdominal floor when distended and retires within the pelvis when voided of urine.

5.4 THE PERITONEUM

The pelvic peritoneum is continuous cranially with that of the abdomen. It lines the cavity for a variable distance caudally and is then reflected onto the viscera, and from one organ to another. The parietal and visceral parts of the peritoneum are also joined by various folds that pass between the pelvic walls and certain organs; these partially divide the cavity into a series of pockets or pouches. The primary division into dorsal (rectogenital) and ventral (genitopubic) pouches is achieved by a more or less horizontal partition that encloses the genital tract in its median part.
 
The lateral sections of this partition are furnished by the paired broad ligaments (ligamenta lata), peritoneal duplicatures that spring from the pelvic walls and adjacent parts of the flanks and attach medially to the ovaries and successive parts of the genital tract. The broad ligament shows regional and also functional differences for, unlike most serosal folds, it encloses considerable amounts of smooth muscle and connective tissue in addition to the vessels and nerves passing to and from the organs it supports. These tissues increase in amount as the animal approaches maturity and show further growth during pregnancy; they blend with the like tissues in the uterine wall, blurring the junction between this organ and its support. The connective tissues are especially well developed in the caudal parts of the ligament where the peritoneal layers are so widely separated that they continue smoothly onto the dorsal and ventral surfaces of the vagina.
 
The tissues within the broad ligament form special thickening in the region of the ovary. One of these, the proper ligament of the ovary, runs from the extremity of the uterine horn to the adjacent ovarian pole: this thickening is continued laterally as the round ligament of the uterus and is carried within a special fold of peritoneum: this ligament extends from the uterine horn to the internal opening of the inguinal canal. A more important complication of the broad ligament is supplied by what appears to be a side shoot that carries the uterine tube (salpinx): this, the mesosalpinx,runs parallel to the part of the thicker fold that carries the proper ligament. The pouch formed between the two is known as the ovarian bursa since it partially encloses this organ.
 
The subdivision of the pelvic cavity is completed by certain additional folds that suspend the rectum or attach to the bladder. The mesorectum incompletely divides the rectogenital pouch into right and left pararectal fossae. The lateral folds of the bladder carry the remains of the umbilical arteries in their free margins and caudally merge with the broad ligaments. The ventral fold of the bladder is much reduced in the adult and rarely reaches far towards the vertex. But in the fetus it is a prominent structure that extends to the umbilicus. The bladder folds help define vesicogenital and vesicopubic pouches. All these subdivisions of the pelvic peritoneal cavity communicate freely with the major peritoneal space.
 
5.4.1The Rectum & The Pelvic Diaphragm
The Rectum: Although the origin of the rectum is arbitrarily defined by the passage of the gut over the terminal line, its most caudal part is in fact distinguished from the colon by a wider calibre and thicker, more muscular wall. The rectum is generally distended with faces. The last part of the rectum is embedded in fat which, with the other masses within the ischiorectal fossae, provides the cushion that allows the gut to adjust to varying degrees of fullness.
 
The anal canal is held closed by the muscles which surround it and by the apposition of longitudinal mucosal folds. The anal opening is a short transverse slit through which the skin continues to provide the last stretch of the canal with a cutaneous epithelial covering. Most of the passage is lined by a softer mucosa moulded to form a series of interdigitating columns and depressions that seal the lumen.
 
The anus is guarded by two sphincters. The internal sphincter is merely a thickening of the circular muscle of the bowel but the external sphincter is striated and is under voluntary control. It forms a band about three centimeters wide directly below the skin. Many fascicles encircle the lumen; a few attach to the coccygeal vertebrae and a large portion continue into the constrictor muscle of the vulva.
 
5.4.2 The Pelvic Diaphragm:
The diaphragm receives its name from the human structure which forms a concave floor to the pelvic cavity, supporting the viscera that lie above it. In quadrupeds it consists of two parts that approach each other so obliquely that they are almost parallel. Each half consists of two striated muscles, Mm. coccygeus and levator ani, sandwiched between inner and outer fascial sheets.
 
These striated muscles share a common origin from the medial aspect of the sciatic spine and adjacent part of the sacrosciatic membrane but soon divide. The lateral muscle, the coccygeus, passes obliquely beside the rectum to attach to the transverse processes of the first three vertebrae of the tail and has no direct connection with the anus. The greater part of the levator ani lies caudal and ventral to the coccygeus and the majority of its fibres have a more horizontal course over the rectum. It is both thinner and wider than the coccygeus and spreads towards its insertion which consists of several more or less distinct parts that radiate from a fibrous partition within its substance. A large portion of the constrictor muscle of the vestibule takes origin from the ventral border of the levator and it may be that some levator fibres continue into the constrictor without break.
 
The fascia forms an essential part of the arrangement. The inner layer is a direct continuation of the parietal pelvic fascia of the rectum. The outer or perineal sheet diverges from the inner surface of the sacrosciatic membrane behind the origin of the muscles: it covers the lesser sciatic foramen and forms the medial wall of the ischiorectal fossa. The outer and inner sheets fuse together above and below the muscles of the diaphragm. The lower edge of the pelvic diaphragm is continuous with the urogenital diaphragm that completes the closure of the caudal pelvic aperture.
 
5.4.3 Clinical Consideration:
Although the muscles of the pelvic diaphragm undoubtedly help to retain the viscera within the pelvic, continuous activity is not required. When the animal is standing placidly the intrapelvic pressure is slightly subatmospheric and any tendency towards visceral displacement must be inward, away from the perineum. The muscles are activated when the intrapelvic pressure is raised i.e on elevation of the forequarters, in coughing, struggling and so forth; or when the perineum is touched or the animal fear such interference. The external anal sphincter behaves in a similar fashion and often there is synchronous activity in all three muscles. The coccygeus is employed alone for movement of the tail. The role of the diaphragmatic muscles in defecation is obscure: it plays no part in actual evacuation, but the levator is normally active in preparatory tensing of the rectum and following the passage of faeces.