Chapter 4The Abdomen, Pelvis, and Perineum
The abdomen (including the pelvis) is a large body cavity containing the major portion of the digestive tract and the viscera of the genitourinary system. Enclosing the abdominal cavity are a muscular anterolateral abdominal wall, a muscular thoracic diaphragm superiorly, a muscular and bony posterior wall, and a muscular and membranous pelvic diaphragm below.
Because the abdominal cavity extends up into the thoracic cage, the thorax between the fifth intercostal space above to the costal margin below should be considered, along with the abdominal skeleton. The thoracic cage is discussed on pages 54 to 57.
The right and left os coxae comprise the lower limb girdle and take part in the hip joint (see Chapter 10). In addition, they join with the sacrum and the coccyx to form the pelvic cavity, which houses and protects several pelvic viscera. The os coxae is originally formed from three separate bones that fuse as one complete bone by about the sixteenth year. The bones are the ilium, ischium, and pubis, and they lend their names to the three regions of the single adult os coxae (Figure 4-2).
The ilium superiorly consists of a flared, flattened plate with a concave medial surface and ends superiorly as the iliac crest. The iliac tubercle is a small bony elevation on the superior lateral aspect of the iliac crest. The right and left iliac tubercles mark the widest points on an articulated pelvis. These points can be palpated and represent anthropological landmarks used to measure the width of the pelvis (bicristal diameter). The anterior aspect of the iliac crest exhibits two small elevations called the anterior inferior and the anterior superior iliac spines.
The pubis meets its fellow of the opposite side anteriorly at the pubic symphysis, a semimovable joint that loosens in women to expand the birth canal before childbirth. Lateral to the symphysis is the pubic tubercle. The inguinal ligament connects the pubic tubercle to the anterior superior iliac spine.
The pubis and ischium unite to form the obturator foramen, which is largely obliterated in life by a membrane. All three bony elements meet and contribute to the cup-shaped acetabulum on the lateral aspect, which receives the head of the femur. Projecting posteriorly from the ischium is the ischial spine, which divides the posterior aspect into the greater sciatic notch above (which transmits the sciatic nerve) and the lesser sciatic notch below.
The pelvic cavity is divided into an upper (false) pelvis, limited entirely by the iliac crests, and a lower (true) pelvis, surrounded by the right and left pubis, the ischium, and the sacrum, which extends into the lower pelvis as the promontory. The basic difference between typical male and female pelvises is that the diameter of the lower pelvis of a woman generally is larger to allow passage of a baby’s head during childbirth.
To facilitate descriptions of abdominal visceral locations, the abdomen is divided into regions (Figure 4-3). There are two systems in common use. One divides the abdomen into four quadrants based on the median sagittal plane in the abdomen intersecting with a transverse plane. This effectively divides the abdomen into upper left, upper right, lower left, and lower right quadrants.
Figure 4-3 Divisions of abdomen. A, Four quadrants based on two planes. UR, Upper right; UL, upper left; LR, lower right; LL, lower left. B, Nine regions based on four planes. Rt Hc, Right hypochondriac; Lt Hc, left hypochondriac; E, epigastric; Rt L, right lumbar; Lt L, left lumbar; U, umbilical; Rt I, right iliac; Lt I, left iliac; Hg, hypogastric.
The iliac crest is a convenient site for harvesting bone to aid surgical procedures that require transplants for bone reconstruction. A relatively noninvasive technique uses a power-driven trephine (a cylindrical saw) to remove a core of bone from the anterior iliac crest for use in maxillofacial (oral) surgery. The grafts are used to fill in bony defects (cleft palates) and to augment alveolar bone for eventual dental implants.
Palpable abdominal landmarks are the costal margins and the xiphoid process, the iliac crests, the superior and inferior anterior iliac spines, and the pubic tubercles (Figure 4-4). In thin, muscular individuals a linear depression runs in the midline of the abdomen from the xiphoid process to the symphysis pubis. It represents the underlying linea alba, which is the union of right and left muscular aponeuroses. The linea semilunaris is a curved line lateral to the midline. It represents the lateral limits of the rectus abdominis muscle. Transverse bands running from the linea semilunaris and the midline represent underlying tendinous insertions of the rectus abdominis muscle. The umbilicus (belly button) is the scarred result of the postnatal closure of the umbilical cord. A slight crease runs from the anterior superior iliac spine toward the pubic tubercle. It represents the position of the inguinal ligament. Just above and medial to the pubic tubercle is the site of the superficial inguinal ring, the site of indirect inguinal hernias.
From outward within, the layers that compose the anterolateral abdominal wall below the skin are the (1) superficial fascia, (2) deep fascia, (3) muscles and aponeurosis, (4) transversalis fascia, (5) extraperitoneal layer, and (6) peritoneum (Figure 4-5).
Four pairs of bilateral muscles and their flattened tendons, or aponeuroses, contribute to the anterolateral abdominal wall (Table 4-1 and Figure 4-6). There are three pairs of flat muscles (external oblique muscle, internal oblique muscle, and transverse abdominis muscle) and one pair of straplike muscles (rectus abdominis muscle). The three flat muscles are layered in sheets, but the fiber directions of each muscle run in different directions, resulting in a strong laminated muscular unit. Each muscle is fleshy laterally and forms membranous aponeuroses medially. As the aponeuroses of the three flat muscles approach the midline, they form a membranous sheath that wraps around the rectus abdominis muscle.
Figure 4-6 Muscles of anterior abdominal wall. A, External oblique muscle is shown on left and deeper internal oblique muscle on right. B, Transversus abdominis muscle and rectus abdominis muscle are shown on left. Rectus abdominis muscle has been cut superiorly and inferiorly to show bed of rectus sheath and epigastric arteries on right.
The external oblique muscle is the outermost of the group and runs from the lower ribs downward and medially. As the muscle fibers approach the midline, they give way to a membranous aponeurosis, which forms a portion of the anterior sheath of the rectus abdominis muscle. It then meets its counterpart of the opposite side in the midline linea alba. The inferior fibers attach on the iliac crest and form a tendinous free border running from the anterior superior iliac spine to the pubic tubercle. This tendinous free border folds inward on itself and is called the inguinal ligament.
The internal oblique muscle originates from the iliac crest and runs upward and medially to insert into the costal margin, the linea alba, and the pubis, along with the underlying transverse abdominis muscle, as the conjoint tendon. The internal oblique muscle becomes aponeurotic as it approaches the midline and takes part in the formation of the rectus sheath.
The transversus abdominis muscle is the innermost of the three flat muscles and originates from the lumbodorsal fascia, iliac crest, inguinal ligament, and lower costal cartilages. It runs in a transverse direction medially to insert as an aponeurosis, along with its opposite fellow, into the midline linea alba. It also forms a part of the membranous sheath for the rectus abdominis muscle. The lower fibers, along with the lower fibers of the internal oblique muscle, insert into the pubis as the conjoint tendon.
The rectus abdominis muscle runs inferiorly from the costal margin and lower thoracic cage to the pubis. The muscle is enclosed in a membranous sheath formed by the aponeuroses of the three flat muscles. The muscle inserts via three tendinous insertions into the anterior wall of the membranous sheath, in addition to its inferior attachment to the pubis.
The formation of the rectus sheath is rather complicated, as shown in Figure 4-5. Two representative cuts through the muscle layer are shown. At level A the internal oblique apilioneurosis splits to encircle the rectus abdominis muscle. At level B all of the aponeuroses are anterior to the muscle, leaving a deficiency in the sheath posteriorly. The arcuate line marks the limit of the aponeurotic contribution to the posterior wall of the bed of the rectus sheath. Below this arcuate line, only the transversalis fascia separates the rectus abdominis muscle from the underlying peritoneum.
The muscles of the anterior wall function in two ways: (1) flexion and rotation of the trunk, and (2) compression of the anterior wall. First, the right and left oblique muscles, acting together with the rectus abdominis muscle, flex the trunk. Unilateral contractions of the oblique fibers, however, result in a rotation or twisting of the trunk.
Second, the abdominal muscles, acting in concert when the back is stabilized, tense the anterolateral abdominal wall to protect underlying viscera. They contract to aid in raising intra-abdominal pressure during forced expiration, coughing, sneezing, defecation, micturition, and childbirth.
The extraperitoneal layer, a fatty, connective tissue layer, is interposed between the transversalis fascia and the deeper peritoneum. Abdominal organs form in this layer during embryological development.
The deep, fatty, extraperitoneal layer beco-mes membranous and continuous with the more organized connective tissue of the peritoneal layer below. Deeper still is a smooth, glistening layer of mesothelial cells, which line the peritoneal cavity. Peritoneum and the contents of the abdomen are discussed on page 99.
The inguinal region is superior to the medial portion of the inguinal ligament. Here the lower fibers of the internal oblique and transversus abdominis muscles do not insert into the inguinal ligament but rather insert as the conjoint tendon into the pubis, leaving a free inferior gap (see Figure 4-6). The external oblique aponeurosis covers this gap incompletely, its inferior medial fibers diverging as a triangular opening as it approaches the pubis. The opening is the superficial inguinal ring, which serves as the external opening of the inguinal canal. The canal is formed by the deficiency of the transversus abdominis and internal oblique muscles and their conjoint tendon posteriorly and the external oblique aponeurosis anteriorly. Issuing from this canal in males is the spermatic cord, from which the testis is suspended within the scrotum. In females the round ligament of the uterus passes through the canal to attach to the labium majus.
The gonads develop within the extraperitoneal layer of the abdomen (Figure 4-7). The gubernaculum extends from this site down to the developing scrotum in males and to the labia majora in females. It marks the path for the descent of the testes in males.
During the third prenatal month in the male fetus, a portion of peritoneum (processus vaginalis) begins to pouch outward through the transversalis fascia, through the inguinal canal, and down into the scrotum. It is guided by the previous descent of the gubernaculum. As the processus vaginalis passes through each layer from deep to superficial, it drags with it portions of each layer. These layers eventually form the coverings of the spermatic cord.
The stage is now set for the descent of the testes. These glands develop within the extraperitoneal layer and in the seventh intrauterine month follow the processus vaginalis down to the scrotum. They drag with them the ductus deferens and the testicular blood supply from the abdominal aorta. The resulting deficiency in the transversalis fascia is the deep inguinal ring.
In the female fetus the ovaries descend slightly but remain with the pelvis. The gubernaculum becomes fibrous and persists as the round ligament of the uterus running from the lateral aspects of the uterus through the inguinal canal to the skin of the labia majora.
For the last layer, the superior portion of the processus obliterates. If it remains patent, an inguinal hernia can result. The inferior portion wraps around the testes to form a fluid-filled space termed the tunica vaginalis. The space affords the testes some movement in the scrotum.
The spermatic cord contains four structures that are associated with the testes: (1) the ductus (vas) deferens, which carries sperm back into the abdomen to the seminal vesicle; (2) the testicular artery, which is a branch of the abdominal aorta; (3) the pampiniform plexus of veins that form up as the testicular vein, which empties into the inferior vena cava of the abdomen; and (4) the vessels and nerves of the ductus deferens, which supply the ductus.
The anterolateral abdominal wall is supplied by the lower six intercostal nerves of the thorax, which stream downward and medially to the abdomen. Inferiorly, the abdominal wall is supplied by two branches of the first lumbar anterior ramus. These are named specifically as the iliohypogastric and ilioinguinal nerves, and they supply the skin and musculature of the lower aspect of the anterolateral abdominal wall.
The arterial supply is from two sources. Posteriorly segmented branches of the aorta follow the spinal nerves. Anteriorly segmented branches arise from the superior and inferior epigastric arteries running in the bed of the rectus sheath (see Figure 4-6). The superior epigastric artery arises superiorly as a continuation of the internal thoracic artery; the inferior epigastric artery arises inferiorly from the external iliac artery. The two epigastric arteries anastomose above the umbilicus and supply the contents of the rectus sheath and the sheath itself. Collateral branches from the epigastric arteries anastomose with the aortic collateral vessels posteriorly.
Vasectomy is an elective surgical procedure in men for the purpose of birth control. The ductus (vas) deferens is exposed bilaterally through relatively small openings in the anterosuperior wall of the scrotum. The ducts are sectioned and ligated, resulting in sterility but not impotency. The prostate gland and seminal vesicles still contribute secretions to the ejaculate.
The testes normally descend into the scrotum during the eighth or ninth month in utero. Occasionally one or both testes fail to descend and remain in the abdomen. The empty scrotum after birth is a clue. Without surgical intervention the testes fail to produce sperm in the higher temperatures of the abdominal cavity, resulting in infertility.
Intra-abdominal pressures are relatively high and become higher still with expulsive contractions of the abdominal muscles. Consequently, weak spots in the abdominal wall may collapse and allow passage of a section of bowel. The most common site of an abdominal hernia is the inguinal region. Inguinal hernias are either indirect or direct.
An indirect hernia is more common in young boys and results when the processus vaginalis persists and allows a loop of bowel to penetrate the deep inguinal ring and enter the inguinal canal along with the spermatic cord and pass through the external inguinal ring down into the scrotum. In girls the herniated bowel and abdominal wall coverings pass into the labium majus.
Direct hernias occur in older men and do not pass through the internal inguinal ring. Instead, they push through a weakened abdominal wall just medial to the inguinal canal and do not assume the coverings of the spermatic cord. However, the gut ultimately bulges through the external ring, making the determination of whether it is direct or indirect difficult.
Varicocele results when the pampiniform plexus of veins draining the testis becomes tortuous and dilated, assuming the feeling of a “bag of worms.” It usually occurs on the left side and may be a reflection of the drainage of the left testicular vein into the left renal vein. The condition is not serious but is thought to lead to a diminished sperm output on the affected side because of the increase in temperature.
The diaphragm is a fibromuscular partition that separates the abdominal cavity below from the thoracic cavity above. It does not lie in a flat plane but rather domes upward; therefore abdominal contents just below the diaphragm are found to be within the lower confines of the thoracic rib cage, yet still technically within the abdominal cavity.
The muscular slips of the diaphragm originate from three sites of attachment (Figure 4-8). Sternal slips arise from the posterior aspect of the xiphoid process. Costal slips originate from the internal surfaces of the lower six costal cartilages and the twelfth rib. Lumbar attachments arise as a right crus from the vertebral bodies and discs of L1, L2, and L3, and a left crus arising from the vertebral bodies and discs of L1 and L2. The two crura cross each other to form the median arcuate ligament through which enters the abdominal aorta. Tendons arching from the crura to the transverse processes of L1 form medial arcuate ligaments. Tendons arching from the transverse processes of L1 to the midpoints of the twelfth ribs form lateral arcuate ligaments.
The main function of the diaphragm is respiration. The contracting muscle fibers pull down on the central tendon during thoracic inspiration. The mechanisms involved with inspiration and expiration are discussed in Chapter 3, page 67.
The diaphragm originates early in development from the cervical region, and as it descends during development it drags its cervical nerve and blood supply along with it. The phrenic nerve is the motor and sensory supply to the diaphragm. It arises in the neck from anterior rami of spinal nerves C3, C4, and C5. The phrenic nerve descends through the thoracic inlet and travels inferiorly on either side of the middle mediastinum to reach the diaphragm (see Chapter 3, page 84).
The posterior abdominal wall consists of skin and fascia, bone (including the lumbar and thoracic vertebrae, the lower ribs, and the os coxae), and a fleshy component consisting of three muscles (see Figure 4-8).
The quadratus lumborum muscle is flat and runs from the twelfth rib and all the lumbar transverse processes down to the iliac crest. Unilaterally the muscle helps bend the vertebral column to the same side. Its nerve supply is from anterior rami of upper lumbar nerves.
The inferior abdominal wall is funnel-shaped. The cup portion of the funnel is represented by the levator ani muscles; the stem of the funnel is represented by the rectum passing inferiorly through the pelvic outlet.
The levator ani muscles originate along the internal aspects of the os coxae (Figure 4-9 and Table 4-2). The fibers run medially and inferiorly toward the rectum to blend with the longitudinal smooth muscle of the rectum. A portion of each levator ani muscle runs from the ischial tuberosity to the coccyx and is given a special name, the coccygeus muscle.
The levator ani muscles help draw the rectum superiorly during defecation. The muscle is deficient anteriorly to allow passage of the urethra in men and the urethra and the vagina in women (see also Figure 4-37).
The levator ani muscles form not only the floor of the abdominopelvic cavity but also the roof of the most inferior aspect of the trunk, the perineum. Superficially the perineum is that area bounded by the thighs and the buttocks. On a deeper plane, the perineum is bounded by the ischiopubic rami converging on the pubic symphysis anteriorly and the sacrotuberous ligaments converging on the coccyx posteriorly.
These boundaries roughly define a diamond-shaped area. If a line is drawn joining the right and left ischial tuberosities, the perineum is further divided into a posterior anal triangle and an anterior urogenital triangle.
The anal triangle is the posterior half of the perineum. A fat-filled space between the inferior surface of the levator ani muscles and the medial aspect of the pelvic walls is termed the ischiorectal fossa. This space allows the rectum to expand during defecation.
The urogenital triangle is the anterior half of the perineum. Stretched across, between the pubic rami, is the urogenital diaphragm, which presents a free border posteriorly. Contained within this diaphragm is a thick fibrous perineal membrane inferiorly; the sphincter urethrae muscle above; and another, more delicate, fibrous membrane superiorly. The superior membrane and the inferior perineal membrane sandwich the muscle fiber of the sphincter urethrae.
In men the muscle fibers surround the urethra. In women these fibers surround the vagina, with a few fibers surrounding the more anteriorly placed urethra. Further descriptions of the male and female perineum are presented later in this chapter.
Peritoneum is a lining tissue consisting of a single inner layer of squamous cell mesothelium and a thin, supporting, and nutritive outer layer of connective tissue. Peritoneum lines the primitive gut cavity rather simply, but as abdominal structures develop in the extraperitoneal layer and push the overlying peritoneum into the abdominal cavity, complex folds of peritoneum result.
To better understand the complexities of the peritoneal linings, it is advantageous to consider the development of the gut during embryonic life. Figure 4-11 shows the primitive gut, which is essentially a tube suspended from the posterior body wall by a dorsal mesentery. Running to the suspended gut through the dorsal mesentery are three branches of the abdominal aorta: (1) the celiac trunk, which supplies the foregut; (2) the superior mesenteric artery, which supplies the midgut; and (3) the inferior mesenteric artery, which supplies the hindgut.
Two glands develop as outgrowths of the gut. The liver develops anteriorly in the primitive ventral mesentery, which extends superiorly from the umbilicus. The pancreas develops posteriorly in the dorsal mesentery. Both glands drag along their respective blood supplies from the gut as they develop. The spleen also develops in the dorsal mesentery.
Figure 4-12, A, shows a transverse section through the developing abdomen. As indicated, the stomach and spleen move to the left, and the liver moves to the right. Figure 4-12, B, represents a later stage in development, with the liver now occupying most of the upper right quadrant and the stomach and spleen occupying the upper left quadrant. In addition, the mesentery of the stomach becomes adherent to the posterior body wall and is lost. In like manner, the mesenteries of the duodenum, ascending colon, descending colon, and rectum are fused to the posterior body wall, and these sections of gut become “retroperitoneal.” With blunt dissection, these obliterated mesenteries can be restored in the laboratory and occasionally in actual abdominal surgery (as needed).
As a result, only the following structures retain a mesentery; that is, they are suspended from the posterior body wall by a double-layered fold of peritoneum: the jejunum, ileum, transverse colon, and sigmoid, or pelvic, colon. Although the mesentery of the stomach is lost during development, the stomach remains suspended by the greater and lesser omenta. The remaining structures do not possess a mesentery in the adult and are said to be retroperitoneal.
The liver does not have a mesentery, nor is it considered retroperitoneal. Rather, it is suspended from the diaphragm and posterior body wall by a broad-based peritoneal attachment, called the coronary ligament, which is described along with the liver (on page 115).
The descending aorta of the thorax passes through the diaphragm to become the abdominal aorta (Figure 4-13). It descends on the posterior body wall to the pelvis, and at vertebral level L4 it divides into two terminal branches: the right and left common iliac arteries. These, in turn, divide into the right and left external iliac arteries, which descend to supply the lower limb, and the right and left internal iliac arteries, which supply pelvic structures. The abdominal aorta gives off a number of branches within the abdomen before terminating as the common iliac arteries at vertebral level L4.
Although there are five lumbar arteries, only the first four pairs arise from the aorta. The fifth pair arises from the internal iliac arteries, which then turn laterally to supply the lower abdominal wall. Anteriorly the segmented lumbar arteries anastomose with collateral branches of the superior and inferior epigastric arteries.
Three unpaired branches arise from the abdominal aorta to supply the gut and its associated glands, the liver, the pancreas, and the spleen. Each of the three branches supplies the derivatives of the embryonic foregut, midgut, and hindgut. These branches are described in detail later in this chapter along with the descriptions of the abdominal viscera they supply.
The celiac trunk arises as a short stem at vertebral level T12, L1, just below the diaphragm, and immediately breaks up into three main branches. These three branches of the celiac trunk supply foregut derivatives within the abdomen (i.e., abdominal esophagus, stomach, the duodenum [proximal to the entrance of the bile duct], the liver and gallbladder, part of the pancreas, and the spleen).
The superior mesenteric artery arises immediately below the celiac trunk and supplies derivatives of the midgut (i.e., the distal half of the duodenum, jejunum, ileum, cecum and appendix, ascending colon, and transverse colon). Its territory ends at the left colic flexure. It also supplies the distal half of the duodenum and the inferior portion of the pancreas.
The right and left common iliac veins unite at vertebral level L5 to form the inferior vena cava. As the inferior vena cava passes upward through the abdomen, it picks up several tributaries. Superiorly the inferior vena cava passes through the diaphragm to enter the thorax and pour its venous flow into the right atrium.
Five pairs of lumbar veins drain the body walls. Only the upper four lumbar veins drain to the inferior vena cava. The fifth drains directly or indirectly to the common iliac vein. Paralleling the inferior vena cava on either side are ascending lumbar veins, which arise from the common iliac veins and ascend on the posterior abdominal wall through the diaphragm to the thorax, where they become the azygos and hemiazygos veins. As they ascend in the abdomen, they communicate with the lumbar veins and therefore provide an alternative route for venous flow from the body wall.
The renal veins receive venous blood from the right and left kidneys. The suprarenal veins receive venous blood from the right and left suprarenal glands. The testicular/ovarian veins receive venous blood from the male testicles or the female ovaries.
Venous return from the gastrointestinal tract does not return directly to the inferior vena cava (see Figure 4-27). Instead, veins returning from the gut join to form the portal vein. The portal vein enters the liver, carrying nutrients absorbed from the gut. Within the liver the portal vein ultimately ends as a capillary bed; at this level, nutrients are exchanged for processing and storage within the liver. The portal capillary beds are drained by hepatic veins, which leave the liver to enter the inferior vena cava as several hepatic veins.
Figure 4-27 Transverse sections through abdomen at two levels to demonstrate structures passing to and from liver through lesser omentum. Section 1 passes through the lower aspect of the liver. Section 2 passes through the epiploic foramen.
The anterior rami of the lower six thoracic spinal nerves and the first lumbar nerve supply the various layers of the anterolateral abdominal walls. Two branches of the anterior rami of L1, the iliohypogastric and ilioinguinal nerves, supply the lower portion of the abdominal wall (see Figure 10-20).
The anterior rami of lumbar nerves L1 to L4 unite and divide within the substance of the psoas muscle to form the lumbar plexus. Branches of the lumbar plexus supply a portion of the lower limb and consequently are considered in Chapter 10 with the lower limb.
The scheme of the sympathetic nervous system is outlined in Chapter 1 (pages 27 to 30) and is illustrated in Figure 1-26.