In anatomy, the abdominal wall represents the boundaries of the abdominal cavity. The abdominal wall is split into the posterior (back), lateral (sides) and anterior (front) walls.
There is a common set of layers covering and forming all the walls: the deepest being the visceral peritoneum, which covers many of the abdominal organs (most of the large and small intestines, for example), and the parietal peritoneum- which covers the visceral peritoneum below it, the extraperitoneal fat, the transversalis fascia, the internal and external oblique and transversus abdominis aponeurosis, and a layer of fascia, which has different names according to what it covers (e.g., transversalis, psoas fascia).
In medical vernacular, the abdominal wall most commonly refers to the layers composing the anterior abdominal wall which, in addition to the layers mentioned above, includes the three layers of muscle: the transversus abdominis (transverse abdominal muscle), the internal (obliquus internus) and the external oblique (obliquus externus).
- Main part
The abdominal wall encloses the abdominal cavity, and can be divided into anterolateral and posterior sections. Its key functions include:
Forms a firm, flexible wall which keeps the abdominal viscera in the abdominal cavity.
Protects the abdominal viscera from injury.
Maintains the anatomical position of abdominal viscera against gravity.
Assists in forceful expiration by pushing the abdominal viscera upwards.
Involved in any action (coughing, vomiting) that increases intra-abdominal pressure.
The anterolateral abdominal wall consists of four main layers (external to internal); skin, superficial fascia, muscles and associated fascia, and parietal peritoneum. In this article, we shall look at the anatomy of the anterolateral abdominal wall – its musculature, surface anatomy and clinical correlations.
Superficial facsia
The superficial fascia consists of fatty connective tissue. The composition of this layer depends on its location:
Above the umbilicus – a single sheet of connective tissue. It is continuous with the superficial fascia in other regions of the body.
Below the umbilicus – divided into two layers; the fatty superficial layer (Camper’s fascia) and the membranous deep layer (Scarpa’s fascia).
The superficial vessels and nerves run between these two layers of fascia.
Muscles of the abdominal wall
The muscles of the anterolateral abdominal wall can be divided into two main groups:
Flat muscles – three flat muscles, situated laterally on either side of the abdomen.
Vertical muscles – two vertical muscles, situated near the mid-line of the body.
Flat Muscles
There are three flat muscles located laterally in the abdominal wall, stacked upon one another. Their fibres run in differing directions and cross each other – strengthening the wall, and decreasing the risk of herniation.
In the anteromedial aspect of the abdominal wall, each flat muscle forms an aponeurosis (a broad, flat tendon), which covers the vertical rectus abdominis muscle. The aponeuroses of all the flat muscles become entwined in the midline, forming the linea alba (a fibrous structure that extends from the xiphoid process of the sternum to the pubic symphysis).
External Oblique
The external oblique is the largest and most superficial flat muscle in the abdominal wall. Its fibres run inferomedially.
Attachments: Originates from ribs 5-12, and inserts into the iliac crest and pubic tubercle.
Functions: Contralateral rotation of the torso.
Innervation: Thoracoabdominal nerves (T7-T11) and subcostal nerve (T12).
Internal Oblique
The internal oblique lies deep to the external oblique. It is smaller and thinner in structure, with its fibres running superomedially (perpendicular to the fibres of the external oblique).
Attachments: Originates from the inguinal ligament, iliac crest and lumbodorsal fascia, and inserts into ribs 10-12.
Functions: Bilateral contraction compresses the abdomen, while unilateral contraction ipsilaterally rotates the torso.
Innervation: Thoracoabdominal nerves (T6-T11), subcostal nerve (T12) and branches of the lumbar plexus.
Transversus Abdominis
The transversus abdominis is the deepest of the flat muscles, with transversely running fibres. Deep to this muscle is a well-formed layer of fascia, known as the transversalis fascia.
Attachments: Originates from the inguinal ligament, costal cartilages 7-12, the iliac crest and thoracolumbar fascia. Inserts into the conjoint tendon, xiphoid process, linea alba and the pubic crest.
Functions: Compression of abdominal contents.
Innervation: Thoracoabdominal nerves (T6-T11), subcostal nerve (T12) and branches of the lumbar plexus.
Vertical Muscles
There are two vertical muscles located in the midline of the anterolateral abdominal wall – the rectus abdominis and pyramidalis.
Rectus Abdominis
The rectus abdominis is long, paired muscle, found either side of the midline in the abdominal wall. It is split into two by the linea alba. The lateral border of the two muscles create a surface marking, known as the linea semilunaris.
At several places, the muscle is intersected by fibrous strips, known as tendinous intersections. The tendinous intersections and the linea alba give rise to the ‘six pack’ seen in individuals with a well-developed rectus abdominis.
Attachments: Originates from the crest of the pubis, before inserting into the xiphoid process of the sternum and the costal cartilage of ribs 5-7.
Functions: As well as assisting the flat muscles in compressing the abdominal viscera, the rectus abdominis also stabilises the pelvis during walking, and depresses the ribs.
Innervation: Thoracoabdominal nerves (T7-T11).
Pyramidalis
This is a small triangular muscle, found superficially to the rectus abdominis. It is located inferiorly, with its base on the pubis bone, and the apex of the triangle attached to the linea alba.
Attachments: Originates from the pubic crest and pubic symphysis before inserting into the linea alba.
Functions: It acts to tense the linea alba.
Innervation: Subcostal nerve (T12).
Surface Anatomy
The abdominal cavity contains numerous organs – many of which can be palpated through the abdominal wall, or their position can be visualised by surface markings.
The umbilicus is the most visible structure of the abdominal wall, and is the scar of the site of attachment of the umbilical cord. It is usually located midway between the xiphoid process and the pubis symphysis.
The rectus abdominis muscle gives rise to abdominal markings. The lateral border of this muscle is indicated by the linea semilunaris, a curved line running from the 9th rib to the pubic tubercle. The linea alba is a fibrous line that splits the rectus abdominis into two. It is visible as a vertical groove extending inferiorly from the xiphoid process.
The abdomen is a large area, and so it split into nine regions – these are useful clinically for describing the location of pain, location of viscera and describing surgical procedures. The nine regions are formed by two horizontal and two vertical planes:
Horizontal planes:
Transpyloric plane – Horizontal line halfway between the xiphoid process and the umbilicus, passing through the pylorus of the stomach.
Intertubercular plane – Horizontal line that joins the iliac crests.
Vertical planes – run from the middle of the clavicle to the mid-inguinal point (halfway between the anterior superior iliac spine of the pelvis and the pubic symphysis). These planes are the mid-clavicular lines.
Clinical Relevance: Surgical Incisions in Abdominal Wall
Vertical Incisions
Median
An incision that is made through the linea alba. It can be extended the whole length of the abdomen, by curving around the umbilicus. The linea alba is poorly vascularised, so blood loss is minimal, and major nerves are avoided. All can be used in any procedure that requires access to the abdominal cavity.
Paramedian
Similar to the median incision, but is performed laterally to the linea alba, providing access to more lateral structures (kidney, spleen and adrenals). This method ligates the blood and nerve supply to muscles medial to the incision, resulting in their atrophy.
Transverse Incisions
Transverse
This incision is made just inferior and laterally to the umbilicus. This is a commonly used procedure, as it causes least damage to the nerve supply to the abdominal muscles, and heals well. The incised rectus abdominis heals producing a new tendinous intersection. It is used in operations on the colon, duodenum and pancreas.
Suprapubic (Pfannenstiel)
Suprapubic incisions are made 5cm superior to the pubis symphysis. They are used when access to the pelvic organs is needed. When performing this incision, care must be taken not the perforate the bladder (especially if it is not catheterised), as the fascia thins around the bladder area.
Subcostal
This incision starts inferior to the xiphoid process, and extends inferior parallel to the costal margin. It is mainly used on the right side to operate on the gall bladder, and on the left to operate on the spleen.
McBurney
This is a ‘grid iron’ incision, because it consists of two perpendicular lines, splitting the fibres of the muscles without cutting them – this allows for excellent healing. McBurney incision is performed at McBurney’s point (1/3 of the distance between the ASIS and the umbilicus). It is mostly used in appendectomies.
III. Conclusion
The anterior abdominal wall is highly distensible and is involved in various functions ranging from support of movements of the abdominal viscera to protection of the abdominal cavity. It is more flexible than the posterior abdominal wall, and supports lateral bending, flexion, extension or protrusion and twisting. It plays a role in the maintenance of posture, and increases in intra abdominal pressure to support defecation, parturition and micturition.
The anterior abdominal wall is also important because it is used in clinical surface anatomy for the localization of abdominal viscera. For these purposes, the abdominal wall is divisible into four quadrants by two imaginary lines; a midline (vertical) and a horizontal line which passes through the umbilicus. The quadrants are named as follows; the right upper quadrant, left upper quadrant, right lower quadrant and left lower quadrant. This form of dividing the abdomen into four quadrants is known as the four region scheme.
