The Abdominal Wall

In this article, we shall look at the layers of this wall, its surface anatomy and common surgical incisions that can be made to access the abdominal cavity.


Functions of the Abdominal Wall

Before we consider the anatomy of the abdominal wall, it is useful to have an overview of its functions. We can then consider the relationship of structure and function.

The main roles of the abdominal wall:

  • 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 andominal viscera upwards
  • Involved in any action (coughing, vomiting) that increases intra-abdominal pressure

Layers of the Abdominal Wall

The layers of the abdominal wall consist of (external to internal):

  1. Skin
  2. Superficial fascia (or subcutaneous tissue)
  3. Muscles and associated fascia
  4. Parietal peritoneum

We shall now look at these individual layers in more detail.

Fig 1.0 - The layers of the abdomen.

Fig 1.0 – The layers of the abdomen.


The Superficial Fascia

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. This continuous with the superficial fascia in other regions of the body.
  • Below the umbilicus: It is divided into two layers; the fatty superficial layer (Camper’s fascia) and the membranous deep layer (Scarpa’s fascia). Superficial vessels and nerves run between these two layers of fascia.

Muscles of the Abdominal Wall

There are five muscles in the abdominal wall. They can be divided into two groups:

  • Vertical muscles – There are two vertical muscles, situated near the mid-line of the body.
  • Flat muscles – There are three flat muscles, situated laterally.

The Flat Muscles

There are three flat muscles; the external oblique, internal oblique and transversus abdominis. They are located laterally in the abdominal wall, stacked upon one another.

These muscles act to flex, laterally flex and rotate the trunk. Their fibres run in differing directions and cross each other – strengthening the abdominal wall and decreasing the risk of herniation.

External Oblique

The largest and most superficial flat muscle in the abdominal wall. Its fibres run inferomedially. As the fibres approach the mid-line, they form an aponeurosis (a broad flat tendon).

In the mid-line, the aponeuroses of all the flat muscles become entwined, forming the linea alba. This is a fibrous structure that extends from the xiphoid process of the sternum to the pubic symphysis.

Internal Oblique

This muscle lies deep to the external oblique. It is smaller and thinner in structure, with its fibres running superiormedially (perpendicular to the fibres of the external oblique). Near the midline it forms aponeurotic fibres which contribute to the linea alba.

Transversus Abdominis

The deepest of the flat muscles, with transversely running fibres. Like the other flat muscles, it contributes aponeurotic fibres to the linea alba. Deep to this muscle is a well formed layer of fascia, called the transversalis fascia.

Fig 1.1 - Lateral view of the abdominal wall. The external oblique and its aponeurotic fibres are visible.

Fig 1.1 – Lateral view of the abdominal wall. The external oblique and its aponeurotic fibres are visible.

Fig 1.2 - Lateral view of the abdominal wall. The internal oblique is visible -  note that its fibres are perpendicular to those of the external oblique.

Fig 1.2 – Lateral view of the abdominal wall. The internal oblique is visible – note that its fibres are perpendicular to those of the external oblique.


The Vertical Muscles

Rectus Abdominis

This is a 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 called the linea sumilunaris.

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 low body fat.

As well as assisting the flat muscles in compressing the abdominal viscera, the rectus abdominus also stabilises the pelvis during walking, and depresses the ribs.

Pyramidalis

This is a small triangle shaped muscle, found superficially to the rectus abdominus. It is located inferiorly, with its base on the pubis bone, and the apex of the triangle attached to the linea alba. It acts to tense the linea alba.

Fig 1.3 - Anterior view of the abdominal cavity. The pyrimidalis muscle is not visible

Fig 1.3 – Anterior view of the abdominal cavity. The pyrimidalis muscle is not visible


The Rectus Sheath

The rectus sheath is formed by the aponeuroses of the three flat muscles, and encloses the rectus abdominus and pyramidalis muscles. It has an anterior and posterior wall for most of its length:

  • The anterior wall is formed by the aponeuroses of the external oblique, and of half of the internal oblique.
  • The posterior wall is formed by the aponeuroses of half the internal oblique and of the transversus abdominus.

Approximately midway between the umbilicus and the pubic symphysis, all of the aponeuroses move to the anterior wall of the rectus sheath. At this point, there is no posterior wall to the sheath; the rectus abdominus is in direct contact with the transversalis fascia.

The area of transition between having a posterior wall, and no posterior wall is known as the arcuate line.

Clinical Relevance: Surgical Incisions

There are various incisions used to gain access to abdominal cavity. In assessing which incision is best, the surgeon must consider:

  • Direction of muscle fibres (you want to split the muscle fibres rather than cut them)
  • Location of nerves
  • Ease of access to the desired viscera

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 abdominus 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 catherised), 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.

Fig 1.5 - Common surgical incisions into the abdominal wall.

Fig 1.5 – Common surgical incisions into the abdominal wall.

Surface Anatomy

The abdominal cavity contains many organs, many of which can be palpated directly, 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 midway between the xiphoid process and the pubis symphysis.

The rectus abdominus gives rise to some 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 abdominus 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 planes, and two vertical planes.

Fig 1.6 - The nine areas of the abdomen.

Fig 1.6 – The nine areas of the abdomen.

There are two 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.

The two 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 called mid-clavicular lines.

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