Abdominal wall

Surface anatomy

Let’s first take a look at the surface anatomy of the anterolateral abdominal wall, before we dive into its layer description. The anterolateral abdominal wall spans the anterior and lateral sides of the abdomen. It can be divided into several topographical areas, which are used to describe the location of abdominal organs and the pain associated with them:

• Four quadrants, which are divided by the horizontal transumbilical and vertical median planes. The four resulting areas are called right upper, left upper, right lower and left lower quadrants.
• Nine abdominopelvic regions, which are divided horizontally by the superior subcostal plane, which passes right under the costal margins of the 10th ribs, and the inferior intertubercular plane, which connects the tubercules of the iliac crest. Vertically they are divided by the two midclavicular planes which pass through the midpoint of each clavicle and halfway between the pubic symphysis and the anterior superior iliac spine. The four planes create nine abdominal regions as you see in the picture: hypochondriac (right, left) and epigastric regions superiorly, flanks (right, left) and umbilical region in the middle, groin (right, left) and hypogastric region inferiorly.  

Check out the following resources which explain the abdominal topography and region division in detail using graphic diagrams:

Fascia 

The skin is the most superficial layer of the anterior abdominal wall. In pregnant women, obese people and those with abdominal distention, it can contain elongated lines called stretch marks or striae distensae, usually situated in the umbilical and hypogastric regions. The superficial fascia is located just below the skin and consists of connective tissue. In the anterior abdominal wall, superior to the umbilicus, it is similar and continuous to the superficial fascia of the body and is made up mostly of one layer. However, inferior to the umbilicus, it is divided into two layers:

• Superficial Camper’s fascia, which is a thicker fatty layer that can have a variable degree of thickness. For example, it is greatly increased in obese individuals and very thin in people with low body fat.
• Deep Scarpa's fascia, which is a thinner and denser membranous layer overlying the muscle layer of the abdominal wall. It is firmly attached to the linea alba and pubic symphysis and fuses with the fascia lata (deep fascia of the thigh) right below the inguinal ligament. 

In men, the Camper’s fascia continues over the penis and blends with the Scarpa’s fascia to form the superficial fascia of the penis. The latter extends further on into the scrotum, where it contains smooth muscle fibers and becomes the dartos fascia. Scarpa’s fascia continues into the perineum to form the superficial fascia of the perineum, called Colles’ fascia. In women it continues into the labia majora and anterior perineum.

Read about necrotizing fasciitis here. for more information about the fascial layers of the anterolateral abdominal wall.

Muscles 

If we go deeper into the anterolateral abdominal wall, beneath the superficial fascia we will find the muscular layer. It consists of five paired muscles and their respective aponeuroses. The muscles of the anterolateral abdominal wall are divided into two main groups:

• Lateral flat muscle group situated on either side of the abdomen, which includes three muscles: external oblique, internal oblique and transversus abdominis. 
• Anterior vertical muscles situated bilaterally to the median fibrous structure called linea alba. They are called rectus abdominis and pyramidalis muscles.

Lateral abdominal muscles

Let’s take a look at the lateral abdominal muscles first. These flat muscles are part of the tension system of the abdominal wall musculature. Through abdominal press they increase the intra-abdominal pressure and thereby support emptying processes (e.g. defecation, micturition) and exhalation (expiratory breathing muscles). Their unilateral contraction results in torso rotation.

The external oblique muscle is the outermost muscle, whose fibers run inferomedially. Right beneath it sits the internal oblique muscle whose fibers run superomedially. The most profound lateral muscle is the transversus abdominis which consists of horizontal fibers. The transverse fascia is located below the transversus abdominis. 

The external oblique muscle is a lateral flat muscle that courses from the 5th to the 12th rib ventromedially until the anterior layer of the rectus sheath. At its origin, it is tightly connected with the serratus anterior and latissimus dorsi muscles. Ventrally the external oblique muscle builds a large aponeurosis which extends medially to the linea alba and caudally to the iliac crest and the pubic bone. Its inferior margin forms the inguinal ligament.

Originating from the thoracolumbar fascia, iliac crest and iliopectineal arch, the internal oblique muscle inserts cranially at the lower costal cartilages and ventrally at the linea alba. In men, caudal fibers extend to the spermatic cord and merge to form the cremaster muscle. The semilunar lines (linea semilunaris) are formed by the divisions of the internal oblique aponeurosis and correspond with the lateral margins of the rectus abdominis muscle. They extend from the tip of the 9th costal cartilage to the pubic tubercle. 

The transversus abdominis muscle is the deepest of the three lateral abdominal muscles. It runs from the inner surface of the lower costal cartilages, thoracolumbar fascia, iliopectineal arch and iliac crest horizontally to the linea alba. Caudal fibers are also involved in the formation of the cremaster muscle. The transversalis fascia separates the anterior abdominal wall from the extraperitoneal fat. Posteriorly, the transversalis fascia is continuous with the thoracolumbar fascia.

The aponeuroses of these muscles form the rectus sheath, which is divided into anterior and posterior layers. The latter is only present in the superior three quarters of the rectus sheath and its inferior limit is demarcated by the horizontal arcuate line. This is where the inferior epigastric artery and vein perforate the rectus abdominis. Above the arcuate line the anterior layer consists of the aponeuroses of the internal and external oblique , while the posterior layer is made up the aponeuroses of the internal oblique and transversus abdominis muscles. Below the arcuate line the anterior layer of the rectus sheath is built by the aponeuroses of all three lateral abdominal muscles whereas the posterior layer is only covered by the transverse fascia and the peritoneum.

Is it a daunting experience trying to memorize all the muscles of the abdominal wall? Use our muscle anatomy reference charts to quickly memorize all the attachments, innervation and functions of the abdominal wall muscles.

Anterior abdominal wall muscles

The anterior muscle group includes the rectus abdominis and pyramidalis muscles. These are almost completely enveloped by the thick rectus sheath formed by the aponeuroses of the lateral abdominal muscles. The only exception is the posterior side of the lowest fourth of the rectus abdominis muscle, below the arcuate line, which is covered only by the transversalis fascia and parietal peritoneum. Immediately deep to the rectus sheath is the transversalis fascia, below which lie the two deepest layers of the abdominal wall: extraperitoneal fat and peritoneum.

The rectus abdominis muscles are a pair of long, straight muscles which run vertically on either side of the anterior abdominal wall. They are separated by the linea alba. The term rectus abdominis means “straight abdominal” in Latin, indicating that the muscle fibers run in a straight vertical line through the abdominal region of the body. Each muscle consists of a string of four fleshy muscular bodies connected by three narrow bands of tendon known as tendinous intersections. The shape of these segments is often visible through the superficial fascia and skin in those with low body fat, resulting in a ‘six-pack’ shape.

The pyramidalis muscle is a small triangular muscle lying anterior to the rectus abdominis muscle that can be absent in approximately 20% of the population. It is contained in the rectus sheath and originates from the bony pelvis, where it is attached to the pubic symphysis and pubic crest through tendinous fibers. The fibres run superiorly and medially to insert into the linea alba, tensing it during muscular contractions.

Deep to the transversalis fascia and superficial to the parietal peritoneum lie structures which create several peritoneal folds divided by fossae. The median umbilical ligament, a remnant of the urachus, lies in the median line and forms the median umbilical fold of the parietal peritoneum. Lateral to it on both sides lie the supravesical fossae, laterally bounded by paired medial umbilical ligaments which are remnants of the umbilical arteries. Each median umbilical ligament is covered by a medial umbilical fold. Medial inguinal fossae are located lateral to the medial umbilical folds. The lateral umbilical folds are formed by the inferior epigastric vessels. Lateral to them lay the lateral inguinal fossae.

Check out the following study unit for more information about the anatomy of the anterolateral abdominal muscles:

Mnemonic

If you like using memory devices for your learning, we have one for the anterolateral abdominal wall muscles. Just remember TIRE Pump, and you're all set!

Transversus abdominis
Internal oblique
Rectus abdominis
External oblique
Pyramidalis

Feel like you have mastered the muscles of the anterior abdominal wall? Why not test yourself with our quiz!

Neurovasculature

As we’ve seen, the anterolateral abdominal wall is a large structure made up of multiple layers of skin, connective tissue and muscles. These require abundant blood supply, which is provided by numerous blood vessels. The arteries of the anterolateral abdominal wall can be divided into superficial and deep layers. The superficial branches include:

• Musculophrenic artery, which is a branch from the internal thoracic artery. It supplies the superior part of the superficial anterolateral abdominal wall.
• Superficial epigastric artery and lateral to it the superficial circumflex iliac artery. They are branches of the femoral artery and supply the inferior part of the wall.

The deep layers of the anterolateral abdominal wall are supplied by the following:

• Superior epigastric artery, a terminal branch of the internal thoracic artery. It runs in the rectus sheath behind the rectus muscle and supplies the superior part of the wall. 
• Inferior epigastric artery and deep circumflex iliac artery, both branches from the external iliac artery, supply the inferior part of the wall. The inferior epigastric artery enters the rectus sheath after piercing the fascia transversalis and ends by anastomosing with the superior epigastric artery.
• The tenth and eleventh intercostal arteries and subcostal artery supply the lateral part of the abdominal wall.

There is a network of superficial veins that radiate out from the umbilicus and a few small paraumbilical veins which interconnect the network. The deep veins follow the arteries of the same name. The skin and peritoneum of the anterolateral abdominal wall are innervated by the T7 to L1 spinal nerves, which run in an inferomedial direction. They give off lateral and anterior cutaneous branches along their course. Muscles of the anterior abdominal wall are supplied by lower six thoracic nerves, the iliohypogastric nerve and the ilioinguinal nerve.

Test yourself on the neurovasculature of the anterior abdominal wall with our quiz.

Inguinal region 

The anterior abdominal wall has naturally occurring paired canals in the lateral lower regions known as inguinal canals. These oblique intramuscular tunnels may range from 3 to 5 cm long in an adult. They serve as a conduit that allows the passage of the male gonads from their intra-abdominal point of origin to their final destination in the scrotal sac. Each inguinal canal originates superolaterally at the deep inguinal ring located at the medial half of the inguinal ligament of Poupart. The canal then terminates at the superficial inguinal ring, which is found about 1 cm superolateral to the pubic tubercle. 

In females, the round ligament of the uterus passes through each canal. Male inguinal canals convey the spermatic cord, which contains the ductus deferens, its related neurovasculature, lymphatics and connective tissue. Superficial and deep inguinal rings impose weak points in the abdominal wall, creating a predisposition to inguinal hernias.

Check out this resource which explains the complex and sometimes confusing anatomy of the inguinal canal:

Fascia

Analogously to the anterolateral abdominal wall, the fascia of the posterior abdominal wall lies immediately below the skin and subcutaneous tissue. The thoracolumbar fascia is a large, roughly diamond-shaped area of connective tissue formed by the thoracic and lumbar parts of the deep fascia. The thoracolumbar fascia is continuous with the transversalis fascia of the anterolateral abdominal wall and it is divided into three layers: anterior, middle and posterior. The intrinsic deep back muscles are enclosed between the posterior and middle layers, while the quadratus lumborum muscles lay between the middle and anterior layers. More profound to the anterior layer runs the psoas major muscle with its psoas fascia.

Muscles 

The posterior abdominal wall is supported by 12th thoracic and all five lumbar vertebrae, along with their corresponding intervertebral discs. Three or four muscles are present in the posterior abdominal wall, depending on the individual: psoas major, iliacus, quadratus lumborum and psoas minor muscles. The latter is variable, being present in about 40% of the population. Note that the quadratus lumborum is the only 'true' posterior abdominal muscle, while the others extend into the lower limb.

Psoas major is a triangular, bilaterally paired muscle that forms part of the floor of the paravertebral gutter. It joins the iliacus muscle to form the iliopsoas muscle, the strongest hip flexor of the human body. Iliopsoas is important for standing, walking and running. Quadratus lumborum muscle has medial attachments to the transverse processes of L1-L4 and superior attachments to the inferior border of the 12th rib. The paired muscle acts by stabilizing the caudal connections of the diaphragm, while its unilateral contraction leads to lateral flexion of the trunk. Bilateral contraction depresses the thoracic rib cage during inspiration and extends the trunk. Psoas minor is a variable muscle present in only about 40% of the population. Its contraction leads to weak trunk lateral flexion of the trunk.

Nerves and vessels 

Branches derived from the descending aorta supply the posterior abdominal wall. These include paired subcostal arteries, which run right below the 12th ribs and four pairs of lumbar arteries arising from the back of the aorta. A variable fifth pair of lumbar arteries arising from the median sacral artery can be present. Subcostal and lumbar arteries anastomose with one another and with the superior epigastric, lower intercostal and iliolumbar arteries, supplying the posterior abdominal wall and related structures. Venous drainage of the posterior abdominal wall is carried out mainly by the lumbar veins which empty into the inferior vena cava. 

There is a plethora of nerves and vessels coursing through the abdominal cavity which are closely related to the posterior abdominal wall. These include: abdominal aorta and its major branches, inferior vena cava with its tributaries, lumbar plexus, sympathetic ganglion chain and sympathetic plexus. The lumbar plexus is formed by the divisions from L1 – L4 spinal nerves with contribution of T12, which merge on the anterior surface of psoas major.

Learn more about the neurovasculature of the posterior abdominal wall using the following study units or take our quiz to test yourself on the structures of the posterior abdominal wall: