The abdominal wall is subdivided into the anterior wall, the right and left lateral walls, and the posterior wall. These walls are musculoaponeurotic, meaning they are composed of muscles and fascial layers, except for the posterior wall which is also made up by the lumbar vertebral column. This musculoaponeurotic wall functions to enclose and protect the abdominal viscera, stabilize and contribute to movements of the trunk, and also increase the intra-abdominal pressure which is needed during urination, defecation, vomiting, and assisting in childbirth.
Now, the anterior and lateral abdominal walls are collectively known as the anterolateral abdominal wall, mainly because the boundary between the two is not distinct. So the anterolateral abdominal wall extends from the thoracic cage down to the pelvis. More specifically, it’s bounded superiorly by the cartilages of the seventh through tenth ribs as well as the xiphoid process, and inferiorly by the inguinal ligament and superior margins of the anterolateral aspects of the pelvic girdle
The anterolateral wall is composed of many different layers. There’s 12 of them in total. The most superficial layer is the skin, which covers a superficial fatty layer of subcutaneous tissue, or fat, known as Camper fascia, which is a major site of fat storage. Deep to the Camper fascia, there is a membranous layer of subcutaneous tissue known as Scarpa fascia, which is continuous inferiorly with the superficial perineal fascia, or Colles fascia. And deep to the superficial fascial layers, there are 3 muscle layers, each covered in a layer of a deep fascia - so 6 layers in total. So right after Scarpa fascia, there’s the superficial investing fascia, followed by the most superficial muscular layer: the external oblique muscle. Then comes the intermediate investing fascia and the internal oblique muscle. And finally, there are the deep investing fascia and the transversus abdominis muscle.
Deep to the transversus abdominis is the transversalis fascia. And finally, for our two deepest layers, there is a thin layer of extraperitoneal fat which is just above the parietal peritoneum, which is the deepest layer of the abdominal wall and lines the abdominal cavity.
So just to recap, let’s take a quick break and see if you can recall the layers of the antero-lateral abdominal wall!
Now let’s talk muscles! The external oblique muscle, the internal oblique muscle, and the transversus abdominis muscle are considered the flat abdominal muscles, and the fibers of each have varying orientations. All three of these abdominal muscles continue anteriorly and medially as aponeuroses.
Aponeuroses are simply flat sheets of fibrous tissue that anchor muscles to bone, deep fascia, or other muscles. The aponeuroses from the left and right flat abdominal muscles fuse in the midline to form the linea alba, which is latin for ‘white line’ and runs from the xiphoid process to the pubic symphysis.
The external oblique is the largest and most superficial of the anterolateral abdominal muscles. It originates at the external surfaces of the fifth through twelfth ribs. and inserts on the linea alba, pubic tubercle, and the anterior half of the iliac crest. The posterior fibers of the external oblique are nearly vertical as they travel distally toward the iliac crest. The more anterior fibers, however, fan out medially, making most of the muscular fibers run inferomedially. To remember this, the orientation of the external oblique muscles fibers are in the same direction as your fingers are when your hands are in your pocket. These muscle fibers eventually become aponeurotic around the mid clavicular line, forming a sheet of tendinous fibers that cross at the linea alba. The inferior margin of the aponeurotic portion also forms the inguinal ligament connecting the anterior superior iliac spine to the pubic tubercle. The muscular portion of the external oblique contributes primarily to the lateral abdominal wall, and the anterior aponeurotic portion contributes to the anterior abdominal wall. The external oblique is innervated by the thoracoabdominal nerves, which are derived from the anterior rami of the T7 to T11 spinal nerves, as well as by the subcostal nerve, which is the anterior ramus of T12. The external oblique flexes and rotates the trunk, like when you’re doing those Russian twists at the gym. It also compresses and supports the organs within the abdominal cavity, particularly during expiration.
Deep to the external oblique muscles are the internal oblique muscles. Most of their fibers run perpendicularly to the external oblique muscle fibers, so they head inferolaterally. Try giving yourself a hug and placing your hands on your hips; the internal oblique fibers would run in the same direction as your fingers! The posterior portion of the internal oblique muscles originate from the broad connective tissue attached to the spine known as the thoracolumbar fascia, and also originates from the anterior two thirds of the iliac crest and tissue deep to the lateral third of the inguinal ligament. The internal oblique then inserts at the inferior border of the tenth through twelfth ribs posteriorly and the linea alba anteriorly. It is innervated by the thoracoabdominal nerves, which, again, are derived from the anterior rami of the T7 to T11, and also by the subcostal nerve,and branches of L1 anterior ramus. Similar to the external oblique, the internal oblique muscle compresses and supports abdominal viscera, and also helps to flex and rotate the trunk. More specifically, since many of their fibers are actually continuous at the linea alba, the right external oblique and the contralateral left internal oblique would work together to bring the right shoulder towards the left hip.
The deepest and final layer of the flat abdominal muscles is the transversus abdominis. Living up to its name, its fibers run transversely, except for the inferior fibers which run parallel to the internal oblique. This muscle originates from a number of structures including the internal surface of the seventh to twelfth costal cartilages, the thoracolumbar fascia, iliac crest, and connective tissue deep to the lateral third of the inguinal ligament. Along with the external and internal obliques, it inserts at the linea alba, as well as the pubic crest. It also has the same innervation as the internal oblique and the thoracoabdominal nerves, subcostal nerve and the nerves from the L1 anterior ramus. And similar to both oblique muscles, the transversus abdominis helps to compress the abdominal contents in order to increase intra-abdominal pressure , which is helpful during forced expiration, defecation and labour. Unlike the obliques, though, it doesn’t play a role in trunk movement.
Besides the flat abdominal muscles, there are also vertical abdominal muscles - namely the rectus abdominis and pyramidalis muscle. The rectus abdominis is a set of vertically oriented paired muscles that lies right at the midline of the anterior abdominal wall and originates at the pubic symphysis and pubic crests and inserts at the xiphoid process and fifth through seventh costal cartilages. It is innervated by the anterior rami of T7 to T12 via the thoracoabdominal and subcostal nerves. The rectus abdominis is a powerful flexor of the trunk, so you can thank this muscle when you do your crunches! It also helps stabilize the tilt of the pelvis, and just like the other abdominal muscles it compresses the abdominal viscera. The pair of rectus abdominis muscles is separated in the midline by the linea alba, which is a fibrous band composed of interweaving aponeuroses from the flat abdominal muscles. The rectus abdominis is mostly enclosed by the rectus sheath, where the anterior layer of the rectus sheath anchors the rectus muscle transversely by tendinous intersections, which create the bulges seen in people with well defined abs, or “a 6 pack”.
The second vertical abdominal muscle is the pyramidalis muscle which is a smaller, triangular or pyramidal shaped muscle. It lies anterior to the lower portion of the rectus abdominis and originates on the anterior surface of the pubis and inserts at the linea alba. Interestingly, up to 20% of people don’t have a pyramidalis muscle, but luckily it is not a critical organ considering its only function is to tense the linea alba.
Now, the fibrous rectus sheath is a strong, incomplete aponeurotic covering of the pyramidalis muscle and the rectus abdominis muscle. It also contains the superior epigastric and inferior epigastric arteries, which are an important blood supply for the abdominal wall, as well as other veins, lymphatic vessels, and nerves. The overall function of the rectus sheath is to protect the structures contained within it. The rectus sheath itself is formed from interweaving of the flat abdominal muscles’ aponeuroses with one another.
Now, the rectus sheath is divided into an anterior and posterior layer. However, the rectus sheath is not uniform throughout; so its composition is different in three main areas: above the costal margin, below the costal margin to the arcuate line, and then below the arcuate line to the pubic crest.
So, the anterior layer of the rectus sheath above the costal margin consists only of the external oblique aponeurosis. It doesn’t contain a posterior layer and therefore the rectus abdominis lies directly on the thoracic wall.
Between the costal margin to just below the umbilicus, the rectus sheath contains an anterior and posterior layer. The internal oblique aponeurosis splits into two layers - or laminae - at the lateral border of the rectus abdominis; the anterior lamina of the internal oblique aponeurosis passes anterior to the muscle and merges with the aponeurosis of the external oblique to form the anterior layer of the rectus sheath. The posterior lamina of the internal oblique aponeurosis passes posterior to the rectus abdominis and merges with the aponeurosis of the transversus abdominis to form the posterior layer of the rectus sheath.
Finally, the lower portion of the rectus sheath begins approximately one-third of the distance from the umbilicus, or belly button, to the pubic crest. Here, the anterior layer of the rectus sheath is composed of the aponeuroses from all three flat muscles as the aponeuroses making up the posterior rectus sheath pass anterior to the rectus abdominis, leaving only the transversalis fascia posteriorly.
The transition between the thin transversalis fascia covering the inferior quarter of the rectus abdominis and the posterior layer of the rectus sheath covering the superior three quarters is demarcated by an anatomical landmark known as the arcuate line.