Pelvis and Perineum



Pelvis and Perineum



1 Introduction


The bowl-shaped pelvic cavity is continuous superiorly with the abdomen and bounded inferiorly by the perineum, the region between the thighs. The bones of the pelvic girdle demarcate the following two regions:



The pelvis contains the terminal gastrointestinal tract and urinary system and the internal reproductive organs. The perineum lies below the “pelvic diaphragm,” or muscles that form the pelvic floor, and contains the external genitalia. Our review of the pelvis and perineum focuses on the musculoskeletal structures that support the pelvis and then examines the viscera, blood supply, and innervation of these two regions.



2 Surface Anatomy


Key landmarks of the surface anatomy of the pelvis and perineum include the following (Fig. 5-1):




• Umbilicus: site that marks the T10 dermatome, that lies at the level of the intervertebral disc between L3 and L4; can lie slightly lower in infants or morbidly obese individuals and higher in late pregnancy.


• Iliac crest: rim of the ilium that lies at approximately the L4 level; also the approximate level of the bifurcation of the abdominal aorta into its two common iliac branches.


• Anterior superior iliac spine: superior attachment point for the inguinal ligament.


• Inguinal ligament: ligament formed by the aponeurosis of the external abdominal oblique muscle; forms a line of demarcation separating the lower abdominopelvic region from the thighs.


• Pubic tubercle: the inferior attachment point of the inguinal ligament.


• Posterior superior iliac spine: often seen as a “dimpling” of the skin just above the intergluteal (natal) cleft; often more obvious in females.


The surface anatomy of the perineum is reviewed later.



3 Musculoskeletal Elements


Bony Pelvic Girdle


The pelvic girdle is the attachment point of the lower limb to the body’s trunk. (The pectoral girdle is its counterpart for the attachment of the upper limb.) The bones of the pelvis include the following (Fig. 5-2):




The pelvis protects the pelvic viscera, supports the weight of the body, aids in ambulation by swinging side to side in a rotary movement at the lumbosacral articulation, provides for muscle attachments, and provides a bony support for the lower birth canal. The pelvic inlet is the circular opening where the lower abdominal cavity is continuous with the pelvic cavity; the promontory of the sacrum protrudes into this opening and is its posterior midline margin (Fig. 5-3). The pelvic outlet is diamond shaped and bounded by the pubic symphysis anteriorly, the pubic arches, the inferior pubic rami and ischial rami, the sacrotuberous ligament, and the coccyx. The perineum is enclosed by these boundaries and lies below the pelvic floor (see Fig. 5-17).



The pelvic girdle forms a stable articulation to support the transfer of weight from the trunk to the lower limb. Weight is transferred from the lumbar vertebral column to the sacrum, across the sacro-iliac joints to the coxal (pelvic or hip) bones, and then to the femur (thigh bone). The joints and ligaments reflect this stability (Fig. 5-3 and Table 5-1). The sacro-iliac ligaments are strong, and the posterior sacro-iliac ligament, posterior to the articular joints, is one of the strongest ligaments in the body and supports its entire weight (see Chapter 2).



Anatomical differences in the female bony pelvis reflect the adaptations for childbirth. The differences from the male pelvis include the following:



The female pelvis may assume variable shapes, as follows:



Various asymmetric shapes may also result from scoliosis, poliomyelitis, fractures, and other pathologies.




Muscles of the Pelvis


The muscles of the true pelvis line its lateral wall and form a floor over the pelvic outlet. (The pelvic inlet is demarcated by the pelvic brim.) Two muscles line the lateral wall (obturator internus and piriformis) and attach to the femur (see Table 6-5), and two muscles form the floor, or pelvic diaphragm (levator ani and coccygeus) (Fig. 5-4 and Table 5-2). The levator ani muscle consists of three muscle groups intermingled to form a single sheet of muscle (iliococcygeus, pubococcygeus, and puborectalis). The levator ani muscle is an important support structure for the pelvic viscera in bipeds (upright-walking humans) and helps maintain closure of the vagina and rectum. Bipedalism places greater pressure on the lower pelvic floor, and the coccygeus and levator ani muscles have been “co-opted” for a different use than originally intended in most land-dwelling quadruped mammals. Thus, the muscles once used to tuck the tail between the hind legs (coccygeus) and wag the tail (levator ani) now subserve a support function as we have evolved as bipeds and have lost our tail.





4 Viscera


Distal Gastrointestinal Tract


In both genders the distal gastrointestinal tract passes into the pelvis as the rectum and anal canal. The rectosigmoid junction superiorly lies at about the level of the S3 vertebra, and the rectum extends inferiorly to become the anal canal just below the coccyx (Fig. 5-5). As the rectum passes through the pelvic diaphragm, it bends posteriorly at the anorectal flexure and becomes the anal canal. The anorectal flexure helps maintain fecal continence through the muscle tone maintained by the puborectalis portion of the levator ani muscle. During defecation this flexure straightens, and fecal matter can then move into the anal canal. Superiorly, the rectum is covered on its anterolateral surface with peritoneum, which gradually covers only the anterior surface, while the distal portion of the rectum descends below the peritoneal cavity (subperitoneal) to the anorectal flexure. Features of the rectum and anal canal are summarized in Table 5-3.





Distal Urinary Tract


The distal elements of the urinary tract lie within the pelvis and include the following (Fig. 5-6):




• Distal ureters: pass retroperitoneally into the pelvic inlet and are crossed anteriorly by the uterine artery in females and the ductus deferens in males before terminating in the urinary bladder. The ureter enters the bladder and passes obliquely through the smooth muscle wall; this arrangement provides for a sphincter-like action.


• Urinary bladder: lies behind the pubic symphysis in a subperitoneal position; holds up to 800 mL of urine (less in women and even less during pregnancy), and contains a smooth triangular area internally between the openings of the two ureters and the single urethral opening inferiorly, the trigone of the bladder. The smooth muscle of the bladder wall is the detrusor muscle.


• Urethra: short in the female (3-4 cm) and contains two small para-urethral mucous glands (Skene’s glands) at its aperture; longer in the male (20 cm) and divided into the prostatic, membranous, and spongy portions. The prostatic portion (about 3 cm) traverses the prostate gland, the membranous portion (2 cm) traverses the external urethral sphincter (skeletal muscle), and the spongy portion (15 cm) traverses the corpus spongiosum on the ventral aspect of the penis.


Females have an external urethral sphincter composed of skeletal muscle under voluntary control and innervated by the somatic nerve fibers in the pudendal nerve (S2-S4). Males have the following urethral sphincters:



Micturition (urination/voiding) occurs by the following sequence of events:



• Normally, the sympathetic fibers relax the bladder wall and constrict the internal urethral sphincter (smooth muscle around the bladder neck, present only in males), thus inhibiting emptying.


• Micturition is initiated by the stimulation of stretch receptors (afferents enter the spinal cord via the pelvic splanchnic nerves, S2-S4) located in the detrusor (smooth) muscle of the bladder when it begins to fill.


• Parasympathetic efferents (pelvic splanchnics) induce a reflex contraction of the detrusor muscle and relaxation of the internal sphincter (males only), enhancing the urge to void.


• When appropriate (and sometimes not!), somatic efferents via the pudendal nerve (S2-S4) cause voluntary relaxation of the external urethral sphincter, and the bladder begins to empty.


• When complete, the external urethral sphincter contracts (in males the bulbospongiosus muscle contracts to expel the last few drops of urine from the spongy urethra), and the detrusor muscle relaxes under sympathetic control.





Female Pelvic Reproductive Viscera


The female pelvic reproductive viscera include the midline uterus and vagina and the adnexa (paired ovaries and uterine tubes).


The uterus is pear shaped, about 7 to 8 cm long, and exhibits a body (fundus and isthmus) and cervix. While the uterine cavity looks triangular in coronal section (see Fig. 5-8), in sagittal section it appears only as a thin slit (Fig. 5-6, top). The normal position of the uterus is in an anteflexed (anteverted) position and lies almost in the horizontal plane. A double sheet of peritoneum (actually a mesentery) called the broad ligament envelops the ovaries, uterine tubes, and uterus (Figs. 5-7 and 5-8). During embryonic development the ovaries are pulled into the pelvis by a fibromuscular band (homologue of the male gubernaculum). This ovarian ligament attaches the inferomedial pole of the ovary to the uterus, then reflects anterolaterally off the uterus as the round ligament of the uterus, enters the deep inguinal ring, courses down the inguinal canal, and ends in the labia majora of the perineum as a fibrofatty mass. Features of the female pelvic reproductive viscera are summarized in Table 5-4.



TABLE 5-4


Features of the Female Pelvic Viscera











































STRUCTURE CHARACTERISTICS
Urinary bladder Covered by peritoneum
Uterus Consists of a body (fundus and isthmus) and cervix; supported by pelvic diaphragm and ligaments; enveloped in broad ligament
Ovaries Suspended between suspensory ligament of ovary (contains ovarian vessels, nerves, and lymphatics) and ovarian ligament (tethered to uterus)
Uterine tubes (fallopian tubes) Courses in mesosalpinx of broad ligament and consists of fimbriated end (collects ovulated ova), infundibulum, ampulla, isthmus, and intrauterine portions
Vagina Fibromuscular tube that includes the fornix, a superior recess around protruding uterine cervix
Rectum Distal retroperitoneal portion of large intestine
Vesico-uterine pouch Peritoneal recess between bladder and uterus
Recto-uterine pouch (of Douglas) Peritoneal recess between rectum and uterus and lowest point in female pelvis
Broad ligament Peritoneal fold that suspends uterus and uterine tubes; includes mesovarium (enfolds ovary), mesosalpinx (enfolds uterine tube), and mesometrium (remainder of ligament)
Round ligament of uterus Reflects off uterus and keeps uterus anteverted and anteflexed; passes into inguinal canal and ends as fibrofatty mass in labia majora
Transverse cervical (cardinal or Mackenrodt’s) ligaments Fibrous condensations of subperitoneal pelvic fascia that support uterus
Uterosacral ligaments Extend from sides of cervix to sacrum, support uterus, and lie beneath peritoneum (form uterosacral fold)



The vagina, about 8 to 9 cm long, is a fibromuscular tube that surrounds the uterine cervix and passes inferiorly through the pelvic floor to open in the vestibule (area enclosed by the labia minora). Because the uterine cervix projects into the superoanterior aspect of the vagina, a continuous gutter surrounds the cervical opening, shallower anteriorly and deeper posteriorly, forming the anterior, lateral, and posterior fornices.


The ovaries are almond-shaped female gonads 3 to 4 cm long (but smaller in older women) attached to the broad ligament by its mesovarium portion. The ovary is suspended between two attachments: laterally to the pelvic wall by the suspensory ligament of the ovary (contains the ovarian vessels, lymphatics, and autonomic nerve fibers) and medially to the uterus by the ovarian ligament.


The uterine tubes (fallopian tubes), about 10 cm long, are suspended in the mesosalpinx portion of the broad ligament and are subdivided into four parts:













Jun 16, 2016 | Posted by in ANATOMY | Comments Off on Pelvis and Perineum
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