The ventral body cavity is divided into two distinct subdivisions that are separated by the dome-shaped diaphragm (Fig. 3-1). The upper portion, superior to the diaphragm, is the thoracic cavity. The lower portion, inferior to the diaphragm, is the abdominopelvic cavity. For the sake of convenience, the large abdominopelvic cavity may be divided into an upper abdominal cavity and a lower pelvic cavity. This is an artificial division, because no partition separates the two cavities, and some structures may move from one region to the other.

FIG. 3-1 Divisions of the ventral body cavity.

TABLE 3-1

Horizontal and Vertical Abdominal Planes

Plane	Description	Vertebral Level
Transpyloric	Horizontal plane halfway between the xiphoid and the umbilicus	Indicates vertebral level L1
Subcostal	Horizontal plane through the most inferior point of the rib cage	Indicates vertebral level L3
Transumbilical	Horizontal plane through the umbilicus	Indicates the disc between L3 and L4
Interiliac	Horizontal plane between the highest points of the iliac crests	Indicates vertebral level L4
Transtubercular	Horizontal plane between the tubercles of the iliac crests	Indicates vertebral level L5
Midclavicular	Vertical plane from the midpoint of the clavicle to the midpoint of the inguinal ligament; right and left
Median	Vertical plane through the umbilicus; divides body into right and left halves

Vertical Planes.

The right midclavicular plane extends vertically from the midpoint of the right clavicle to the midpoint of a line joining the right anterior-superior iliac spine and symphysis pubis, or midinguinal point. The left midclavicular plane is in the same position as the right midclavicular plane except that it is on the left side. It extends from the midpoint of the left clavicle to the left midinguinal point. The midsagittal plane, or median plane, is a vertical plane through the umbilicus. It divides the body into right and left halves. These planes are illustrated in Fig. 3-2.

Horizontal Planes.

The transpyloric plane is the most superior of the horizontal planes. It is located about halfway between the jugular notch and the symphysis pubis or, more simply, midway between the xiphoid and umbilicus. This plane typically intersects the pyloric region of the stomach, which accounts for the name. Passing laterally to the right on this plane gives the location of the first part of the duodenum and the top of the head of the pancreas. Proceeding farther to the right, this plane intersects the ninth costal cartilage, which gives the location of the fundus of the gallbladder and then the upper portion of the hilus of the right kidney. Going to the left of the midline, the transpyloric plane gives the location of the neck of the pancreas and middle portion of the hilar region of the left kidney. Usually this plane marks the level of the first lumbar vertebra.

A line through the most inferior point of the rib cage gives the position of the subcostal plane, which marks the level of the third lumbar vertebra. The subcostal plane intersects the third part of the duodenum and lower border of the pancreatic head. There is no good method of locating the L2 vertebral level except that it is halfway between the transpyloric and subcostal planes.

The transumbilical plane passes horizontally through the umbilicus. In individuals with relatively normal abdominal contour, this marks the level of the intervertebral disc between the third and fourth lumbar vertebrae. The interiliac plane passes through the most superior point of the iliac crests. This plane marks the level of the fourth lumbar vertebra. The transtubercular plane passes through the tubercles of the iliac crests. The tubercles are small projections on the crests about 5 cm posterior to the anterior-superior iliac spine. This plane marks the level of the fifth lumbar vertebra. The horizontal planes are illustrated in Fig. 3-3.

FIG. 3-3 Horizontal planes of the abdominal cavity.

Abdominal Quadrants and Regions

Abdominal Quadrants.

The horizontal transumbilical plane and the vertical median plane divide the abdomen into four quadrants for descriptive purposes. For example, the pain of acute appendicitis usually localizes in the lower right quadrant (see Fig. 1-6 for an illustration of the four abdominopelvic quadrants.

Abdominal Regions.

Nine abdominal regions are described using the horizontal subcostal and transtubercular planes and the vertical right and left midclavicular planes. On the right and left sides the regions are, from superior to inferior, the hypochondriac region, the lumbar (or lateral) region, and the iliac (or inguinal) region. The three regions in the midline are, from superior to inferior, the epigastric region, the umbilical region, and the hypogastric region (see Fig. 1-7 for an illustration of the nine abdominopelvic regions).

QUICK CHECK

3.3. What horizontal plane marks the vertebral level L1?

3.4. What are the two planes that divide the abdomen into quadrants?

Osseous Components

The only osseous components of the abdomen are the lumbar vertebrae in the posterior wall. Some abdominal organs extend upward under the thoracic cage, but the components of the thoracic cage are not considered part of the abdomen.

Five large lumbar vertebrae with their intervertebral discs form the skeletal support for the posterior abdominal wall. A lumbar vertebra (Fig. 3-4) has a large body with short, thick, blunt spinous processes. The transverse processes are also thicker than in other vertebrae. The shape of the lumbar vertebrae and discs gives a normal lumbar curvature that is convex anteriorly. This curvature develops during the second year as a child begins to walk and puts increased weight on the lumbar region. An exaggeration of or increase in the convex curvature is called lordosis.

The spinal cord within the vertebral foramen ends at the level of the third lumbar vertebra at birth. Because of the different growth rates of the cord and the vertebral column, the spinal cord ends at the level of the second lumbar vertebra in the adult. Even though the spinal cord ends at L2, the meninges, subarachnoid space, and cerebrospinal fluid continue to the second sacral vertebra. This is clinically important when a spinal tap is performed to remove cerebrospinal fluid for laboratory examination. The needle is inserted between the third and fourth lumbar vertebrae, or sometimes between the fourth and fifth lumbar vertebrae, to remove the fluid from the subarachnoid space. This minimizes the possibility of damage to the spinal cord. Remember that the location of L4 is determined by the interiliac plane, through the superior points of the iliac crests.

QUICK CHECK

3.5. What bones form the skeleton of the abdomen?

3.6. At what vertebral level does the spinal cord end in the adult?

Muscular Components

The muscular components of the abdomen include the diaphragm, the muscles of the anterolateral wall, and the muscles of the posterior wall. The musculotendinous diaphragm forms a movable partition between the thoracic and abdominal cavities. The anterolateral abdominal wall is formed by four muscles and their aponeuroses. The posterior abdominal wall is formed primarily by two pairs of muscles and their attachments to the vertebrae, ribs, and ilium.

Diaphragm.

The musculotendinous diaphragm extends superiorly under the rib cage to the level of the fifth intercostal space when the individual is supine. Because of the large right lobe of the liver, the diaphragm usually rises to a slightly higher level on the right side than on the left. The central portion of the diaphragm consists of tendinous fibers that form a strong central tendon. All the muscular fibers of the diaphragm converge and insert on the central tendon. The muscular portion of the diaphragm is divided into three regions according to the origin of its fibers. A short and narrow sternal portion arises from the back of the xiphoid process. The extensive costal portion originates from the inner surface of the lower six costal cartilages. These costal muscular fibers form the two domes, or hemidiaphragms. The vertebral (or lumbar) portion arises from the upper lumbar vertebrae as a pair of muscular crura. Each crus is a thick, fleshy, muscular bundle that tapers inferiorly and becomes tendinous. Fibers from each crus spread out and ascend to attach to the central tendon. The right crus encircles the esophagus.

Because structures passing from the thoracic cavity into the abdominal cavity must penetrate the diaphragm, its continuity is interrupted by three large and several small apertures. Each opening is called a hiatus. At the level of the eighth thoracic vertebra, the wide caval hiatus for the inferior vena cava is located within the central tendon about 3 cm to the right of the median plane. Not only is the caval hiatus the most superior of the three openings, it is also the most anterior. In addition to the inferior vena cava, this opening transmits the right phrenic nerve and lymph vessels. Occasionally, the right hepatic vein passes through this opening before it enters the inferior vena cava.

The oval esophageal hiatus, at the level of the tenth thoracic vertebra, is an opening in the muscular diaphragm posterior to the central tendon. It is 2 or 3 cm to the left of the midline and is surrounded by the right crus of the diaphragm. In addition to the esophagus, the esophageal hiatus transmits the vagus nerve and esophageal branches of the left gastric blood vessels.

The long, oblique aortic hiatus is located between the right and left crura of the diaphragm and begins at the level of the twelfth thoracic vertebra. The aortic hiatus is the most posterior of the three large openings in the diaphragm. Technically, the aorta does not penetrate the diaphragm; rather, it passes between the crura slightly to the left of the midline. In addition to the aorta, this opening transmits the azygos vein and thoracic duct.

Muscles of the Anterolateral Abdominal Wall.

The anterior and lateral abdominal wall consists of four muscles and their aponeuroses with a covering of fascia and skin. The muscles, illustrated in Fig. 3-5 and by the computed tomography (CT) image in Fig. 3-6, are the rectus abdominis, the external oblique, the internal oblique, and the transverse abdominis. The skin and muscles of the anterior and lateral abdominal wall are innervated by intercostal nerves.

Anteriorly, on each side of the linea alba, the long, vertical rectus abdominis muscles extend the length of the abdominal wall from the symphysis pubis to the xiphoid process. The rectus abdominis is enclosed in a rectus sheath formed by the aponeuroses of the three lateral muscles.

The outermost layer of the lateral muscles is formed by the external oblique. Fibers of this muscle originate from the ribs and extend downward and medially. Most of the fibers terminate in a broad aponeurosis that inserts on the linea alba, iliac crest, and pubic tubercle. The inferior margin of this aponeurosis forms the inguinal ligament.

In contrast to the external oblique muscle, the fibers of the internal oblique muscle extend upward and medially, perpendicular to the external oblique, from the iliac crest to the inferior borders of the ribs. Medially the aponeurosis of the internal oblique muscle splits into two layers to enclose the rectus abdominis muscle.

The transverse abdominis muscle is the innermost of the three flat muscles. These fibers pass in a transverse or horizontal direction. This arrangement of the three flat muscles provides maximum support for the abdominal viscera and diminishes the risk of tearing of the muscles.

Muscles of the Posterior Abdominal Wall.

The paired psoas major and quadratus lumborum muscles, with their attachments to the vertebrae, ribs, and ilium form the musculature of the posterior abdominal wall. The long, thick psoas major muscles are lateral to the lumbar region of the vertebral column. Their fibers originate on the transverse processes, borders, and intervertebral discs of the lumbar vertebrae, pass along the brim of the pelvis, and enter the thigh to insert on the lesser trochanter of the femur. In transverse sections the psoas major muscles appear as large muscular masses adjacent to the lumbar vertebral bodies. The psoas major muscles are innervated by branches of the lumbar nerves.

The second pair of muscles associated with the posterior abdominal wall is the pair of quadratus lumborum muscles. This thick muscular sheet originates on the iliac crest and transverse processes of the lower lumbar vertebrae and ascends to insert on the transverse processes of the upper lumbar vertebrae and twelfth rib. They are innervated by branches of the lumbar nerves. In transverse sections these muscles appear lateral and posterior to the psoas major muscles. The arrangement of the posterior wall muscles is illustrated in Figs. 3-5 and 3-6 with the anterolateral muscles.

The iliacus muscle is a large, triangular sheet of muscle in the iliac fossa on the medial side of the alae, or wings, of the ilium. It originates in the iliac fossa and inserts with the psoas major on the lesser trochanter. This muscle is a part of the posterior abdominopelvic wall but does not appear in sections of the abdomen. It is located in the false pelvis and is evident in pelvic sections. The iliacus and psoas major muscles are closely associated, and together they often are referred to as the iliopsoas. The iliopsoas is the most powerful flexor of the thigh.

Examination of the posterior abdominal wall reveals a single longitudinal ridge and two oblique ridges. The longitudinal ridge line, sometimes called the longitudinal divide, is especially evident in transverse sections (see Figs. 3-5 and 3-6). It is formed by the lumbar vertebrae, the normal lumbar lordosis, the psoas major muscles, the inferior vena cava, and the aorta. On either side of the elevated longitudinal ridge is a paravertebral groove, or gutter. In superior regions of the abdomen, this groove is occupied by the liver on the right side and the spleen on the left side. The kidneys, ureters, and portions of the colon also are located in the paravertebral grooves. Whenever an organ crosses the midline, it is moved anteriorly because of the elevation of this longitudinal ridge. For example, the right lobe of the liver is rather posterior in position, whereas the left lobe is more anterior because it is moved forward by the longitudinal ridge. The pancreas offers another example of this anterior displacement. The tail of the pancreas is in a relatively posterior position, near the spleen. As the gland courses to the right, it is pushed forward by the longitudinal ridge so that the head and neck are more anteriorly situated.

Inferiorly, the longitudinal ridge divides to form two oblique ridges that mark the location of the pelvic inlet. These oblique ridges are formed by the bony pelvic inlet, the psoas major muscles, and the iliac blood vessels.

In addition to influencing organ position, the longitudinal and oblique ridges to some extent determine regions of fluid accumulation. Fluids tend to flow off the sides of the longitudinal ridges to accumulate in the paravertebral “valleys,” or grooves. The paravertebral grooves slope posteriorly from the oblique ridges so that fluids tend to flow down the abdominal portions of the oblique ridges and accumulate in the superior regions of the paravertebral grooves when the patient is supine.

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