4: The Abdomen, Pelvis, and Perineum

Chapter 4imageThe Abdomen, Pelvis, and Perineum

1 Skeleton and Subdivisions

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.

The abdominopelvic cavity extends from the floor of the pelvis inferiorly and rises in a dome superiorly to the level of the fifth intercostal space within the thoracic cage.

SKELETON OF THE ABDOMEN

The skeleton of the abdomen consists of a thoracic component, a vertebral component, and a pelvic component (Figure 4-1).

Pelvic Component

Os Coxae (Hip Bone)

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 ischium is represented mainly by the ischial tuberosity, which bears the weight of the body when a person is in an upright, seated position.

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.

DIVISIONS OF THE ABDOMEN

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.

The other system divides the abdomen into nine regions based on two sagittal and two transverse planes through the abdomen.

The four planes divide the abdomen into nine regions.

2 The Abdominal Walls

The abdominal cavity is enclosed by four walls: (1) an anterolateral wall, (2) a posterior wall, (3) a superior wall (thoracic diaphragm), and (4) an inferior wall (pelvic diaphragm).

ANTEROLATERAL ABDOMINAL WALL

Surface Features

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.

Layers

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).

Muscles and Aponeuroses

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.

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.

Inguinal Region

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.

Spermatic Cord

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.

Blood and Nerve Supply

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.

CLINICAL NOTES

SUPERIOR ABDOMINAL WALL

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.

Origins of the Thoracic Diaphragm

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.

INFERIOR ABDOMINAL WALL

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 Perineum

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.

3 Peritoneum and the Peritoneal Cavity

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.

DEVELOPMENT OF THE GUT AND PERITONEUM

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.

As the individual develops and the gut elongates and differentiates, some sections of the gut rotate and their mesenteries become adherent to the body wall, rendering them no longer mobile.

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).

4 Blood and Nerve Supply to the Abdomen

The arterial supply and nerve supply to the abdominal structures arise from the thorax and pass through the diaphragm. The thoracic components of these structures are described in Chapter 4.

ARTERIAL SUPPLY: THE ABDOMINAL AORTA

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.

VENOUS RETURN: THE INFERIOR VENA CAVA

The external iliac veins, which drain the lower limbs, and the internal iliac veins, which drain the pelvis, unite within the pelvis to form the right and left common iliac veins (Figure 4-14).

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.

Tributaries of the Gastrointestinal Tract and Associated Glands

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.

The blood supply and return of the gut therefore passes through two sets of capillaries: the first in the gut itself and the second in the liver. This is referred to as a portal system.

Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 4: The Abdomen, Pelvis, and Perineum
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