Anterior Abdominal Wall

• The boundaries of the abdominal cavity are the diaphragm superiorly, the pelvic floor inferiorly, and anterior and posterior abdominal muscles.
• The diaphragm arches upwards into the ribcage, so part of the abdominal cavity, and hence some abdominal organs, will be behind and subsequently protected by the ribcage. An example of such an organ is the stomach.
• The abdominal cavity is enclosed by muscles laterally, but these lateral muscles are referred to as anterior muscles because these flat muscles have tendons that run anteriorly.
• The pelvic floor is a muscular sheet that sags down towards the pelvic outlet. This provides a physical barrier to the inferior abdominal cavity.
• The abdomen is surrounded by these muscles:

  • rectus abdominis anteriorly
  • External oblique abdominis, internal oblique abdominis, and transversus abdominis (these three muscles are layered on top of each other, and they are the lateral muscles but are considered anterior muscles because of their tendons).
  • the posterior muscles: Quadratus lumborum and psoas major
• The bony boundaries of the abdominal cavity are:

  • the 11th and 12th ribs. These are considered boundaries because the diaphragm attaches to them
  • the costal margin
  • the 5 lumbar vertebrae
  • the iliac fossa (the large plates of the pelvis. Also sometimes referred to as the false pelvis).
  • the inguinal ligament
• The inguinal ligament is the most inferior boundary of the abdominal cavity because it lies below the lower limb. The inguinal ligament attaches to the anterior superior iliac spine (ASIS) laterally and the pubic tubercle medially.
• The anterior bony boundaries of the costal margin and the inguinal ligament can be felt on the body surface.
• Surface markings on the abdomen are the linea alba and the linea semilunaris.
• The linea alba is a white line located at the midline and this marks the place where all the aponeurosis tendons of the anterior muscles meet. The linea alba is also the gap between the two rectus abdominis muscles. The linea alba is avascular (hence its white colour), subsequently making it a good place for an emergency abdominal incision.
• The linea semilunaris are slightly curved lines more lateral to the linea alba. These lines are formed by the lateral boundaries of the rectus abdominis muscles.
• The abdomen can be divided up into sections. This is useful for locating internal organs, and for mapping out regions of abdominal pain in diseases.
• The abdomen is split into 9 segments:

  • right hypochondrium (top right)
  • epigastrium (top middle)
  • left hypochondrium (top left)
  • right lumbar (middle right)
  • umbilical region (middle)
  • left lumbar (middle left)
  • right inguinal (bottom right)
  • suprapubic region (bottom middle)
  • left inguinal (bottom left)
• The abdomen is split into 9 segments by 2 vertical lines and 2 horizontal lines.

  The 2 vertical lines are mid-clavicular/mid-inguinal lines (named as such because these lines roughly pass through the middle of the clavicle and the inguinal ligament). On the abdomen, these vertical lines are called the lateral lines of the abdomen.

  The upper horizontal line is the transpyloric plane and the lower horizontal line is the trans-tubercular plane (named because this plane passes between the tubercles of the iliac crest).
• The layers of the abdomen from superficial to deep:

  • skin
  • Camper fascia
  • Scarpa fascia
  • external oblique abdominis
  • internal oblique abdominis
  • transversus abdominis
  • endo-abdominal/transversalis fascia
  • extraperitoneal fat
  • parietal peritoneum

  Rectus abdominis is at the same depth as external oblique, internal oblique, and the transversus abdominis
• Camper and Scarpa fascia are the 2 layers that form the thick layer of superficial fascia of the (lower) abdomen.

  Camper fascia contains fat, superficial veins, and cutaneous nerves. It's also continuous with the superficial fascia of the upper limb.

  Scarpa fascia (which is deep to Camper fascia) has a membranous appearance and it binds to the fascia of the thigh below the inguinal ligament. Scarpa fascia is important in preventing the spread of infection, and limiting the spread of fluid in the lower abdominal wall.
• There is hardly any deep fascia underneath the Scarpa fascia in the abdomen. This is because deep fascia is inextensible, and so would hinder the movements of the abdomen during breathing, coughing, and defaecation. These process require the wall of the abdomen to adjust, and that can't happen if deep fascia is present.
• There are two rectus abdominis muscles in the abdomen. Each rectus abdominis is a multi-bellied muscle with tendonous intersections. These muscles attach to the ribcage superiorly and to the pubic bone inferiorly. Rectus abdominis run straight down across the anterior wall.

  Rectus abdominis is a flexor of the trunk and a depressor of the ribcage (acts as an accessory muscle for expiration).

  The region between the 2 rectus abdominis muscles is the linea alba, and the lateral boundary of each rectus abdominis muscle forms the linea semilunaris on each side.
• There are 3 layers of muscle that run laterally, but are still considered anterior muscles: external oblique abdominis, internal oblique abdominis, and transversus abdominis.
• The fibres of external oblique abdominis run forward and downwards. This muscle attaches to the lower ribs, usually to ribs 5-12, and the posterior part of the muscle attaches to the anterior part of the iliac crest.
• The tendon of external oblique abdominis is an aponeurosis that attaches to the linea alba medially, and to the pubic tubercle and pubic crest inferiorly. At the linea alba, the aponeurosis of each external oblique meets to form a raphe (a groove/ridge in an organ/tissue where 2 halves fused in an embryo). The lower part of this tendon folds back in on itself to form the inguinal ligament.
• When the right external oblique contracts, the right side of the trunk is pulled down to the left hip. When the left external oblique contracts, the left side of the trunk is pulled down to the right hip. The external oblique muscles are contralateral rotators.

  When both left and right external obliques contract, they flex the trunk.
• The fibres of the internal oblique abdominis run backwards and downwards. The internal oblique attaches to the lateral half of the inguinal ligament, the iliac crest, and the thoracolumbar fascia. It also attaches to ribs 9-12, and to the linea alba. The inferior part of the internal oblique inserts into the pubic crest.
• The internal oblique, through its connection to the thoracolumbar fascia stabilises the lumbar spine. They are also ipsilateral rotators of the trunk.

  When the left internal oblique contracts with the right external oblique, the trunk is turned to the left, and when the right internal oblique contracts with the left external oblique, the trunk is turned to the right.

  When both pairs of external and internal oblique muscles contract, the trunk flexes.
• The fibres of the transversus abdominis run almost horizontally.

  The transversus abdominis originates from:

  • the internal surfaces of the bone and cartilages that form the thoracic outlet and iliac crest.
  • the lateral third of the inguinal ligament
  • the thoracolumbar fascia.
• Transversus abdominis attaches medially to the linea alba.

  The aponeurotic tendon of the lower part of transversus abdominis fuses with the internal oblique abdominis to form a conjoint tendon, and this conjoint tendon is inserted into the pubic crest.
• The transversus abdominis is involved in:

  • abdominal compression during forced exhalation
  • stabilising the back via the tension given to the thoracolumbar fascia
  • may be involved in defaecation, micturition (urination), and parturition (child birth).
• The rectus sheath is formed from the aponeurotic tendons of the anterior muscles and the transversalis fascia. A rectus sheath is formed because in order for the tendons to meet at the linea alba, they need to by-bass rectus abdominis. And the arrangement of the rectus sheath differs above the umbilicus and below the umbilicus.
• The aponeuroses fuse at the lateral boarder of rectus abdominis. This forms the linea semilunaris.

  Above the umbilicus, the external oblique and half the internal oblique aponeuroses pass over rectus abdominis and form the anterior of the sheath, while the other half of the internal oblique aponeurosis and the transversus abdominis aponeurosis form the posterior of the rectus sheath.
• Below the umbilicus, the anterior of the rectus sheath is formed from the aponeuroses of all the tendons, because they all shifted anteriorly. This leaves only the transversalis fascia to be posterior to the rectus sheath.

  The shift in collagen fibres of the tendons forms a distinct line called the arcuate line. And if the shift is sudden enough, it can be seen on the surface.
• The arcuate line and the shifting of the aponeuroses exists to permit the passage of the inferior epigastric artery and its accompanying veins. With the arrangement of the rectus sheath below the umbilicus, these vessels will only have to pass through the transversalis fascia.
• The inferior epigastric artery and vein lie in front of the parietal peritoneum, and their presence creates a fold known as the lateral umbilical fold. Medial to this fold is the medial umbilical fold, created from the umbilical artery (only present in the foetus and was responsible for delivering blood from the pelvis to the placenta). The median umbilical fold is another fold at the midline, created by the medial umbilical ligament (a remnant of the urachus - a tube which connected the fetal bladder to the umbilicus. After birth the urachus fibroses to become a ligament).
• The peritoneal cavity is the space between the parietal peritoneum (a serous membrane deep to the transversalis fascia) and the visceral peritoneum.
• The peritoneal cavity contains some abdominal organs and between 5-20ml of peritoneal fluid (an excess of peritoneal fluid is known as ascites). This cavity and fluid facilitate the high mobility of the abdominal organs contained within.
• Not all abdominal organs are contained within the peritoneal cavity. These are organs that aren't as mobile and have no reason to be. An example of an organ not within the peritoneal cavity are the kidneys.
• The superficial veins of the abdomen lie within the Camper's fascia, originating from the umbilical region. These superficial veins can run upwards towards the chest if they are above the umbilicus. They connect to veins around the breast known as the lateral thoracic veins. These veins then drain into the axillary artery. These veins are known as thoraco-abdominal/thoraco-epigastric veins.

  The superficial veins below the umbilicus drain downwards to the femoral veins.
• The cutaneous nerve supply of the abdominal wall is segmented from T7-L1. At each level, the posterior and anterior rami of each spinal nerve gives rise to the posterior, and anterior & lateral cutaneous branches respectively. Together, these branches give rise to dermatomes that run obliquely over the abdominal wall.

  The umbilical region is supplied by T10.

  The groin region is supplied by L1.

  Nerves T7-T12 are known as thoraco-abdominal nerves. T12 is also called the subcostal nerve.
• The anterior abdominal walls are also supplied segmentally by nerves T7-12. These thoraco-abdominal nerves are sandwiched in between the internal oblique and transversus abdominis, and run around the trunk. The thoraco-abdominal nerves are also accompanied by blood vessels.
• The arteries that supply the abdominal walls are:

  • superior and inferior epigastric arteries
  • inferior phrenic arteries (these supply the diaphragm)
  • 4 pairs of lumbar arteries (these supply the posterior abdominal muscles).
  • thoraco-abdominal arteries.
• The superior epigastric artery is a continuation of the internal thoracic artery, the name changing when the artery as crossed the costal margin. The inferior epigastric artery ascended from the external iliac. The superior and inferior epigastric arteries anastomoses.