Regional anatomy
The abdomen is the part of the trunk inferior to the thorax . Its musculomembranous walls surround a large cavity (the abdominal cavity), which is bounded superiorly by the diaphragm and inferiorly by the pelvic inlet. The abdominal cavity may extend superiorly as high as the fourth intercostal space, and is continuous inferiorly with the pelvic cavity. It contains the peritoneal cavity and the abdominal viscera.
SURFACE TOPOGRAPHY
Topographical divisions of the abdomen are used to describe the location of abdominal organs and the pain associated with abdominal problems. The two schemes
most often used are:
- a four-quadrant pattern, and
- a nine-region pattern.
Four-quadrant pattern
A horizontal transumbilical plane passing through the umbilicus and the intervertebral disc between vertebrae LIII and LIV and intersecting with the vertical median plane divides the abdomen into four quadrants—the right upper, left upper, right lower, and left lower quadrants X and passes posteriorly through the body of vertebra LIII. (Note, however, that sometimes the transpyloric plane, halfway between the jugular notch and the symphysis pubis or halfway between the umbilicus and the inferior end of the body of the sternum, passing posteriorly through the lower border of vertebrae LI
and intersecting with the costal margin at the ends of the ninth costal cartilages, is used instead.)
-The inferior horizontal plane (the intertubercular plane) connects the tubercles of the iliac crests, which are palpable structures 5cm posterior to the anterior superior iliac spines, and passes through the upper part of the body of vertebra LV.
- The vertical planes pass from the midpoint of the clavicles inferiorly to a point midway between the
anterior superior iliac spine and pubic symphysis.
These four planes establish the topographical divisions in the nine-region organization. The following designations are used for each region: superiorly the right hypochondrium,
the epigastric region, and the left hypochondrium; inferiorly the right groin (inguinal region), pubic region, and left groin (inguinal region); and in the middle the right flank (lateral region), the umbilical region, and the left flank (lateral region)
Each of these five muscles has specific actions, but together the muscles are critical:
- for the maintenance of many normal physiological functions,
-to keep the abdominal viscera within the abdominal cavity,
- to protect the viscera from injury, and
- to help maintain the position of the viscera in the erect posture against the action of gravity.
Contraction of these muscles assists in both quiet and forced expiration by pushing the viscera upward (which helps push the relaxed diaphragm farther into the thoracic
cavity) and in coughing and vomiting. All these muscles are also involved in any action that
increases intra-abdominal pressure, including parturition (childbirth), micturition (urination), and defecation (expulsion of feces from the rectum).
Flat muscles
External oblique
The most superficial of the three flat muscles in the anterolateral group of abdominal wall muscles is the external oblique, which is immediately deep to the superficial
fascia . Its laterally placed muscle fibers pass in an inferomedial direction, while its large aponeurotic
component covers the anterior part of the abdominal wall to the midline. Approaching the midline, the aponeuroses are entwined, forming the linea alba, which extendsmfrom the xiphoid process to the pubic symphysis.
Associated ligaments
The lower border of the external oblique aponeurosis forms the inguinal ligament on each side (Fig. 4.9). This thickened reinforced free edge of the external oblique aponeurosis passes between the anterior superior iliac spine laterally and the pubic tubercle medially . It folds under itself forming a trough, which plays an important role in the formation of the inguinal canal.
Several other ligaments are also formed from extensions of the fibers at the medial end of the inguinal ligament:
- The lacunar ligament is a crescent-shaped extension of fibers at the medial end of the inguinal ligament.
Internal oblique
Deep to the external oblique muscle is the internal oblique muscle, which is the second of the three flat muscles . This muscle is smaller and thinner than the external oblique, with most of its muscle
fibers passing in a superomedial direction. Its lateral muscular components end anteriorly as an aponeurosis that blends into the linea alba at the midline.
Transversus abdominis
Deep to the internal oblique muscle is the transversus abdominis muscle , so named
because of the direction of most of its muscle fibers. It ends in an anterior aponeurosis, which blends with the linea alba at the midline.
Transversalis fascia
Each of the three flat muscles is covered on its anterior and posterior surfaces by a layer of deep (or investing) fascia. In general, these layers are unremarkable except for the layer deep to the transversus abdominis muscle (the transversalis fascia), which is better developed.
The transversalis fascia is a continuous layer of deep fascia that lines the abdominal cavity and continues into the pelvic cavity. It crosses the midline anteriorly, associating with the transversalis fascia of the opposite side, and is continuous with the fascia on the inferior surface of the
diaphragm. It is continuous posteriorly with the deep fascia covering the muscles of the posterior abdominal wall and attaches to the thoracolumbar fascia.
After attaching to the crest of the ilium, the transversalis fascia blends with the fascia covering the muscles associated with the upper regions of the pelvic bones and with
similar fascia covering the muscles of the pelvic cavity. At this point, it is referred to as the parietal pelvic (or endopelvic) fascia.
There is therefore a continuous layer of deep fascia surrounding the abdominal cavity that is thick in some areas, thin in others, attached or free, and participates in the
formation of specialized structures.
Vertical muscles
The two vertical muscles in the anterolateral group of abdominal wall muscles are the large rectus abdominis and the small pyramidalis .
Rectus abdominis
The rectus abdominis is a long, flat muscle and extends the length of the anterior abdominal wall. It is a paired muscle, separated in the midline by the linea alba, and it widens and thins as it ascends from the pubic symphysis to the costal margin. Along its course, it is intersected by three or four transverse fibrous bands or tendinous intersections. These are easily visible on individuals with a well-developed rectus abdominis.
Pyramidalis
The second vertical muscle is the pyramidalis. This small, triangular muscle, which may be absent, is anterior to the rectus abdominis, has its base on the pubis, and its apex is attached superiorly and medially to the linea alba.
Peritoneum
Deep to the extraperitoneal fascia is the peritoneum . This thin serous membrane lines the walls of the
abdominal cavity and, at various points, reflects onto the abdominal viscera, providing either a complete or a partial covering. The peritoneum lining the walls is the parietal
peritoneum; the peritoneum covering the viscera is the visceral peritoneum.
The continuous lining of the abdominal walls by the parietal peritoneum forms a sac. This sac is closed in men, but has two openings in women where the uterine tubes
provide a passage to the outside. The closed sac in men and the semiclosed sac in women is called the peritoneal cavity.
Innervation
The skin, muscles, and parietal peritoneum of the anterolateral abdominal wall are supplied by T7 to T12 and L1 spinal nerves. The anterior rami of these spinal nerves pass around the body, from posterior to anterior, in an inferomedial direction . As they proceed, they give off a lateral cutaneous branch and end as an anterior cutaneous branch.
The intercostal nerves (T7 to T11) leave their intercostal spaces, passing deep to the costal cartilages, and continue onto the anterolateral abdominal wall between the internal oblique and transversus abdominis muscles . Reaching the lateral edge of the rectus sheath, they enter the rectus sheath and pass posterior to the lateral aspect of the rectus abdominis muscle. Approaching the midline, an anterior cutaneous branch passes through the rectus abdominis muscle and the anterior wall of the rectus sheath to supply the skin.
Spinal nerve T12 (the subcostal nerve) follows a similar course as the intercostals. Branches of L1 (the iliohypogastric nerve and ilio-inguinal nerve), which originate from the lumbar plexus, follow similar courses initially, but deviate from this pattern near their final destination.
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