Regions and Planes of the Abdomen

The anterior abdominal wall can be subdivided into various regions and planes, and the wall is composed of many layers with varying blood supply and innervation. The abdomen can be divided into quadrants or into a series of 9 sections, by two different methods. The layers of the abdominal wall consist of the skin, superficial fascia, and muscles.

Anatomically, the abdomen is divided to facilitate clinical assessment and localization: ^[1]

• Four Quadrants - Created by the median sagittal plane and the transumbilical plane, resulting in the right upper, left upper, right lower, and left lower quadrants.
• Nine Regions - Established by two horizontal planes (subcostal and intertubercular) and two vertical midclavicular planes, yielding the right and left hypochondriac, epigastric, right and left lumbar (flank), umbilical, right and left iliac (inguinal), and hypogastric (pubic) regions (see below).

Three most often used regions when dividing the anterior abdomen into nine areas.

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Abdominal planes

The anatomic planes of the abdominal wall are made up of multiple muscular and fascial layers that interdigitate and unite to form a sturdy, protective musculofascial layer that protects the visceral organs and provides strength and stability to the body's trunk. This anatomy varies with respect to the different topographic regions of the abdomen; thus, a firm understanding of these layers, their blood supply, and their innervation is essential for surgical management of the abdomen.

The abdominal cavity is the largest hollow space in the body. It is bound cranially by the diaphragm and xiphoid process of the sternum and the costal cartilages of ribs 7-10; caudally, by the anterior ilium and the pubic bone of the pelvis; anteriorly, by the abdominal wall musculature; and posteriorly, by the L1-L5 vertebrae.

Imaging studies have refined our understanding of key abdominal planes: ^[2]

• Transpyloric Plane - Located between the lower border of L1 and upper L2 vertebrae in most individuals, this plane intersects critical structures such as the superior mesenteric artery, hepatic portal vein formation, left renal hilum, and tip of the ninth rib.
• Subcostal Plane - Typically aligned with the lowermost bony point of the rib cage, it contains the origin of the inferior mesenteric artery.
• Supracristal Plane - Found at the L4 vertebral level, often used in clinical practice to locate lumbar puncture sites

Abdominal regions

The abdominal wall has few anatomic landmarks. The flat abdominal plane is broken up only by the costal margins, anterior superior iliac spines, and the umbilicus. Thus, many attempts have been made over the years to describe what surface anatomy cannot.

The most common and widely accepted system for identification of the various regions of the abdomen is the simple division of the abdomen into four quadrants by a vertical and horizontal line bisecting the umbilicus and forming the right and left upper and lower quadrants (see the image below). ^[3]

Division of the abdomen into four quadrants.

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Advances in abdominal anatomy have introduced the concept of a unified mesenteric model, challenging traditional peritoneal-based descriptions. According to this model: ^[4]

• All digestive organs are connected to a single continuous mesentery, which organizes them into two domains: a mesenteric domain (includes the mesentery and all abdominal digestive organs) and a nonmesenteric domain (consists of the urogenital system, musculoskeletal frame, and great vessels).
• This simplified framework aligns with clinical observations and enhances our understanding of abdominal organ positioning and surgical implications. 

Langer's lines

As elsewhere on the human body, the abdominal skin is transgressed by Langer's lines, also called cleavage lines. This is a term used to define the direction within the skin along which the skin has the least flexibility and corresponds to the alignment of the collagen fibers within the dermis. Across the superior half of the anterior abdominal skin, these lines are oriented in a transverse direction. Toward the inferior half of the abdominal skin, these lines begin to assume a slightly more oblique course in an inferior medial direction toward the groin, paralleling the inguinal crease.

While Langer's lines indicate areas of least skin flexibility, they are not always synonymous with relaxed skin tension lines (RSTLs) or other anatomical guides such as Kraissl's lines. RSTLs, which result from dynamic forces acting on living skin, are often more clinically relevant for surgical planning as they minimize tension and scarring more effectively than Langer's lines. ^{[5, 6]}

Studies have shown that biodynamic excisional skin tension (BEST) lines, which are determined based on real-time intraoperative measurements, can differ from traditional Langer's lines and may be more effective in certain surgical contexts. ^[6]

Innervation

The abdominal skin is innervated in a segmental pattern by the anterior rami of the T7-L1 thoracoabdominal nerves.

T7-T9 thoracoabdominal nerves innervate the skin above the umbilicus; T10 thoracoabdominal nerves innervate the skin around the umbilicus; and T11 thoracoabdominal nerves plus cutaneous branches of the subcostal (T12), iliohypogastric, and ilioinguinal (L1) nerves supply the skin inferior to the umbilicus, including areas around the pubis and parts of the inguinal region. ^[3]

These nerves traverse between the internal oblique and transversus abdominis muscles before piercing the rectus sheath to reach the overlying skin. This anatomical arrangement makes them susceptible to entrapment, particularly at points where they pass through fibrous or muscular structures such as in cases of abdominal cutaneous nerve entrapment syndrome. ^[7]

The superficial fascia of the abdominal wall is the next layer encountered just deep to the skin. It consists of connective tissue that contains a variable amount of fat. This layer can vary in thickness from less than 1 cm to greater than 15 cm, depending on a person's body habitus. Studies show that in males, the mean thickness of the subcutaneous fatty layer is approximately 17.5 mm, while it is about 24.8 mm in females. Additionally, the membranous layer of the superficial fascia has a mean thickness of around 847.4 ± 295 μm. These measurements can vary significantly among individuals depending on factors such as body fat distribution and overall body habitus. ^{[8, 9]}

The superficial fascia consists of loosely packed collagen fibers, abundant elastic fibers, and adipose tissue interspersed with connective tissue. Elastic fibers provide stretchability and recoil, while collagen fibers contribute to tensile strength. Hydroxyproline content, indicative of collagen levels, is higher in the superficial fascia of the upper abdomen than that of the lower abdomen. ^[10]

Camper and Scarpa fasciae

Superior to the umbilicus, the superficial fascia consists of a single layer. Inferior to the umbilicus, it splits into two layers. The more superficial and fattier layer is Camper's fascia. The deeper and more fibrous layer is Scarpa's fascia. Scarpa's fascia contains very little fat and is continuous with both the superficial fascia of the thigh known as the fascia lata and the superficial fascia of the perineum known as the Colles' fascia.

The fasciae are displayed in the image below.

Muscular layers of the abdomen. Ext. = exterior; fasc. = fascia; Inf. = inferior; Int. = interior; m. = muscle.

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Studies have suggested a more complex three-layer organization of the anterior abdominal wall as follows: ^[11]

• A superficial fatty zone
• A membranous layer (Scarpa's fascia)
• A deep fatty zone 

The abdominal wall is composed of five paired muscles: two vertical muscles (the rectus abdominis and the pyramidalis) and three layered, flat muscles (the external abdominal oblique, the internal abdominal oblique, and the transversus abdominis muscles). These muscles and their fascial attachments interdigitate and unite to form a sturdy, protective musculofascial layer that gives strength and support to the anterolateral abdominal wall (see the images below).

Layers of the abdominal wall.

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Muscular layers of the abdomen. Ext. = exterior; fasc. = fascia; Inf. = inferior; Int. = interior; m. = muscle.

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The abdominal wall muscles function synergistically to protect visceral organs, increase intra-abdominal pressure during activities such as coughing, defecation, or childbirth and facilitate movements such as trunk rotation, flexion, lateral bending, and stabilization during dynamic activities. ^[12]

External abdominal oblique muscle

The external abdominal oblique muscle is the largest and most superficial of the three paired, flat abdominal muscles. It arises from the lower eight ribs and interdigitations of the serratus anterior muscle. As the external abdominal oblique courses in an inferior medial direction, its muscle fibers change from thick muscle to a fibrous aponeurosis that inserts medially in the linea alba. Inferiorly, the external abdominal oblique aponeurosis folds back on itself to form the inguinal ligament between the anterior superior iliac spine and the pubic tubercle before inserting onto the pubic tubercle and the anterior half of the iliac crest. Just medial to its insertion on the pubic tubercle, the aponeurosis divides and forms the superficial (or external) inguinal ring. ^[13]

The external abdominal oblique is innervated in a segmental pattern by the anterior rami of the inferior six thoracoabdominal nerves (T7-T12). ^[3]

Internal abdominal oblique muscle

The internal abdominal oblique muscle is the intermediate layer of the three paired, flat abdominal muscles. It originates broadly from the anterior portion of the iliac crest, lateral half of the inguinal ligament, and thoracolumbar fascia. The internal abdominal oblique inserts on the inferior border of the 10th-12th ribs, the linea alba, and the pubic crest via the conjoint tendon. The muscle fibers of the internal abdominal oblique course upward in a superomedial orientation, perpendicular to the muscle fibers of the external abdominal oblique.

Like the external abdominal oblique, the internal abdominal oblique forms a broad aponeurosis that fuses into the midline and contributes to the rectus sheath. Superior to the arcuate line (see the image below), the internal abdominal oblique aponeurosis splits anteriorly and posteriorly to enclose the rectus muscle and helps form the rectus sheath. However, inferior to the arcuate line, the internal abdominal oblique aponeurosis does not split and only passes anterior to the rectus muscle as part of the anterior rectus sheath.

The arcuate line. a. = artery; Ant. = anterior; Ext. = exterior; Inf. = inferior; Int. = interior; m. = muscle; Post. = posterior; v = vein.

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The inferior aponeurotic fibers of the internal abdominal oblique muscle course over the spermatic cord, through the inguinal canal, and the medial fibers fuse with the aponeurosis of the transversus abdominis muscle to form the conjoint tendon. ^[14]

The internal oblique is innervated in a segmental pattern by the anterior rami of the inferior six thoracoabdominal nerves (T7-T12) and first lumbar nerves (iliohypogastric and ilioinguinal nerves).

Of note, all the neurovascular structures supplying the abdominal muscles run in the plane between the internal abdominal oblique muscle and the transversus abdominis muscle, except for the iliohypogastric and ilioinguinal nerves. Initially, they lie on the anterior surface of the quadratus lumborum, then pass laterally into the plane between the transversus abdominis and the internal abdominal oblique. Above the anterior superior iliac spine, they penetrate the internal abdominal oblique to run between this muscle and the aponeurosis of the external abdominal oblique muscle.

Transversus abdominis muscle

The transversus abdominis muscle is the deepest of the three paired, flat abdominal muscles. It originates on the internal surfaces of the 7th-12th costal cartilages, thoracolumbar fascia, anterior three fourths of the iliac crest, and lateral third of the inguinal ligament.

As with the other flat muscles, the transversus abdominis forms a broad aponeurosis that helps make up the rectus sheath before it fuses in the midline to the linea alba. Above the arcuate line, the transversus abdominis aponeurosis contributes to the posterior rectus sheath. Below the arcuate line, it is fused with other flat muscles as the anterior rectus sheath (see the image above).

As its name implies, the muscle and aponeurotic fibers run in a transverse direction, except for the inferior most aponeurotic fibers. These fibers curve in an inferomedial direction to unite with the aponeurosis of the internal abdominal oblique to form the conjoint tendon, which attaches onto the pubic crest and the pectineal (Cooper) ligament. The inferior aponeurotic fibers are fused to the underlying transversalis fascia, thus forming the posterior wall of the inguinal canal. A small triangular opening in this posterior wall is known as the deep or internal inguinal ring. It is at this opening that the spermatic cord is formed (by the ductus deferens, testicular vessels, and genital branch of genitofemoral nerve) and through which all indirect inguinal hernias develop. ^[15]

The transversus abdominis is innervated in a segmental pattern by the anterior rami of the inferior six thoracoabdominal nerves (T7-T12) and first lumbar nerves (iliohypogastric and ilioinguinal nerves).

Rectus abdominis muscles

The rectus abdominis muscles are paired, long muscles that run just lateral to the linea alba in a vertical direction from the xiphoid process of the sternum and costal cartilage of the 5th-7th ribs to the pubic symphysis. These muscles function to tense the abdominal wall, flex the trunk, stabilize the pelvis, and aid in childbirth, defecation, micturition, and forced expiration.

Each muscle is divided along its course by three or four transverse fibrous bands known as tendinous intersections, which essentially divide the muscle into a series of interconnected muscles. This results in one's "abs" or "six-pack." The rectus muscles are contained within the rectus sheath, which is formed by the aponeuroses of the external abdominal oblique, internal abdominal oblique, and transversus abdominis muscles.

The rectus muscles have a dual blood supply. The superior epigastric artery and vein, which are direct continuations of the internal thoracic vessels, supply the superior half of the rectus muscles. The inferior epigastric artery and vein, which arise from the external iliac vessels just proximal to their passage under the inguinal ligament, supply the inferior portion of the rectus muscles and run superiorly until they anastomose with the superior epigastric vessels. In addition, there are numerous small, segmental contributions from the lower six intercostal vessels (see the image below). ^[14]

Abdominal wall vasculature.

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The rectus muscle is innervated in a segmental pattern by the anterior rami of the T7-T12 thoracoabdominal nerves. ^[3] The anterior cutaneous branches of the intercostal nerves can get entrapped as they pass through the fascia of rectus abdominis. This may cause numbness on the anterior abdominal wall that may be associated with painful paresthesia. This is called the rectus abdominis syndrome. ^[1]

Pyramidalis muscle

The pyramidalis muscle is a small, triangular muscle that lies anterior to the inferior aspect of the rectus abdominis muscles. It originates at the pubic symphysis and attaches superiorly at the linea alba. This muscle functions to tense the linea alba and aid in midline stabilization. The pyramidalis muscle is generally considered insignificant in humans and is in fact absent in about 20% of the population. ^[14]

The pyramidalis muscle is innervated by the subcostal nerve (T12). Its blood supply is derived from branches of the inferior epigastric artery. ^[1]

Despite its limited mechanical force (less than 1% of that generated by the rectus abdominis), it has been suggested to play a minor role in stabilizing the rectus sheath. The pyramidalis muscle serves as a surgical landmark for identifying the linea alba during midline incisions such as those performed in classical cesarean sections. It has been explored as a source of skeletal muscle stem cells for regenerative therapies, including post-prostatectomy stress urinary incontinence and microsurgical transfer for treating small wounds. ^{[16, 17]}

Arcuate line

Approximately midway between the umbilicus and the pubic symphysis is an arching, transverse anatomic line known as the arcuate line or the semicircular line of Douglas. Superior to this line, the anterior rectus sheath is composed of the fusion of the aponeuroses of the external abdominal oblique and the anterior leaf of the internal abdominal oblique aponeuroses. (The internal abdominal oblique aponeurosis splits to envelop the rectus muscle at this level.)

The posterior rectus sheath above the arcuate line is composed of the fusion of the posterior leaf of the internal abdominal oblique aponeuroses and the transversus abdominis aponeuroses. Inferior to the arcuate line, the anterior rectus sheath is composed of the fusion of all three muscle aponeuroses, and little or no posterior sheath exists, because only the thin transversalis fascia runs posterior to the rectus muscle (see the image below). ^{[3, 18]}

The arcuate line. a. = artery; Ant. = anterior; Ext. = exterior; Inf. = inferior; Int. = interior; m. = muscle; Post. = posterior; v = vein.

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Linea alba

The linea alba is a dense, tendinous line created by the decussating aponeuroses of the external abdominal oblique, internal abdominal oblique, and transversus abdominis muscles at the abdominal midline.

The linea alba is a three-dimensional meshwork of collagen fibers organized into distinct zones: oblique fibers (lamina fibrae obliquae), transverse fibers (lamina fibrae transversae), and occasionally irregular fibers (lamina fibrae irregularium) in its deeper layers. It lacks significant blood supply or innervation, making it an ideal site for surgical incisions such as midline laparotomies. ^{[19, 20]}

Between the rectus muscles, it extends superiorly from the xiphoid, continuing inferiorly, where it passes superficially in front of the rectus muscles to attach to the symphysis pubis. Deeper fibers pass behind the rectus abdominis, attaching to the posterior pubic crest to create a triangular lamella known as the "adminiculum lineae albae." ^[13]

Above the umbilicus, the well-formed linea alba is wider, progressively narrowing and becoming vaguer below the umbilicus to its inferior attachments. The midline laparotomy incision, dividing the linea alba, is one of the most common surgical approaches for abdominal exposure and the most common site of incisional hernia formation. ^[15]

Widening of the superior linea alba can cause a noticeable midline bulge known as diastasis recti or separation of the rectus muscles. Diastasis recti is a common and normal condition in newborns and is quite common in women who have had multiple pregnancies.

Studies have highlighted that the linea alba has a rhomboid shape with a maximum width at the umbilicus. Its width is greater above the umbilicus than that below. Its length averages 37.5 ± 3.6 cm and increases with BMI. It is shorter in women than in men. The inter-rectus distance varies along its length: it is widest at the umbilicus and narrows inferiorly toward the pubis due to structural changes in the rectus sheath below this transition zone. ^{[21, 22]}

The transversalis fascia is a thin layer of connective tissue lining most of the abdominal cavity between the posterior surface of the transversus abdominis and superficial to the extraperitoneal fat and peritoneum. Superiorly, this fascia continues with the inferior diaphragmatic fascia, inferiorly with the iliac and pelvic fascia, posteriorly with the thoracolumbar fascia, and laterally to the iliac crest.

Above the arcuate line, the transversalis fascia contributes to the posterior sheath along with the posterior leaf of the internal abdominal oblique and the transversus abdominis muscles (see the image below). Below the arcuate line, the transversalis fascia forms the posterior sheath alone. With attachments to the posterior margin of the inguinal ligament, it contributes to the internal spermatic fascia and to the formation of the femoral sheath and becomes the only layer contributing to the inguinal floor. At the deep inguinal ring, the structures of the spermatic cord in males and the structures of the round ligament of the uterus in females pass through the transversalis fascia.

The arcuate line. a. = artery; Ant. = anterior; Ext. = exterior; Inf. = inferior; Int. = interior; m. = muscle; Post. = posterior; v = vein.

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The transversalis fascia and transversus abdominis form a little fascial triangle (2-3 mm) on the lateral border of quadratus lomborum. This triangle contains T12 and L1 nerves. This triangular space is particularly relevant in procedures such as nerve blocks (e.g., transversalis fascia plane blocks) and hernia repairs, where its role in guiding anesthetic spread or reinforcing surgical repairs is increasingly recognized. ^[23]

In hernia repairs, particularly for inguinal hernias, modifications involving suturing or reinforcing transversalis fascia have shown promise in reducing recurrence rates by strengthening weak areas of the inguinal floor. Additionally, its involvement in defining spaces such as Retzius' space during laparoscopic procedures highlights its importance in surgical anatomy. ^{[23, 24]}

The inguinal ligament, or the Poupart ligament, is the thickened, inferior border of the external abdominal oblique aponeurosis that extends from the anterior superior iliac spine to the pubic tubercle and forms the floor of the inguinal canal (see the image below). At the medial aspect, some fibers divide posteriorly, attaching to the pectineal line to form the lacunar ligament or the ligament of Gimbernat, whereas other fibers extend superomedially to join the rectus sheath and linea alba, forming the reflected inguinal ligament. Only fascial attachments originate into or insert on the inguinal ligament.

Inguinal anatomy.

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The inguinal ligament also contributes to the inguinal (Hesselbach) triangle, forming the inferolateral border, whereas the epigastric vessels and the lateral edge of the rectus abdominis muscle form the superolateral and medial borders. Direct inguinal hernias are seen within the triangle, whereas indirect inguinal hernias form lateral to the epigastric vessels. ^[18]

The inguinal canal is a natural space created within the layers of the lower anterior abdominal wall just above the inguinal ligament (see the image below).

Inguinal anatomy.

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In the newborn period, the inguinal canal is short and lengthens over time to its final oblique position. In adults, the canal measures approximately 3-5 cm and extends from the deep or internal inguinal ring to the superficial or external inguinal ring. The deep ring, formed by an opening in the transversalis fascia, corresponds to the middle of the inguinal ligament midway between the anterior superior iliac spine and the pubic tubercle. The superficial inguinal ring is formed by an opening in the aponeurosis of the external abdominal oblique muscle, with the base at the pubic crest and a lateral and medial crus dividing and fusing by intercrural fibers, creating a triangular opening and lateral margin to the superficial ring.

The inguinal canal is formed superiorly by the lower edge of the internal abdominal oblique and transversus abdominis muscles, inferiorly by the inguinal and lacunar ligaments, and anteriorly by the external abdominal oblique aponeurosis. The lateral third of the anterior wall is further reinforced by a portion of the internal abdominal oblique. The posterior wall is formed by the conjoint tendon (the union of internal abdominal oblique aponeurosis and transversus abdominis aponeurosis) and the transversalis fascia.

The canal is reinforced medially by the internal abdominal oblique aponeurosis, whereas laterally, the transversus abdominis aponeurosis and the transversalis fascia form the wall in approximately 75% of individuals, with the remainder being formed only by transversalis fascia. This remains the most important boundary of the inguinal canal. Weakness or defects in the posterior wall may lead to formation of inguinal hernias. In males, the inguinal canal tends to be more developed, containing the spermatic cord and ilioinguinal nerve, whereas in females, it remains less developed and contains the round ligament of the uterus, in addition to the ilioinguinal nerve. ^[25]