Abdominal Wall, Peritoneum and Intestines - LO 2
2. Describe the layers of the abdominal wall.
To avoid injury to neurovasculature and close an incision with minimal disruption, abdominal wall anatomy should be thoroughly understood. There are distinct differences between the anterolateral & anterior abdominal walls.
Anterolateral abdominal wall
The more complex wall is the anterolateral. From superficial to deep, the layers are organized in the manner depicted in the illustration below.
The superficial fascia in the area inferior to umbilicus is unique, because there are two distinct layers instead of one fatty layer in many other areas of the body: superficial fatty (Camper's) fascia and deep membranous (Scarpa's) fascia. The deep membranous layer serves to better reinforce this portion of the wall which is lacking bony support.
There are three anterolateral abdominal muscles: external oblique m., internal oblique m., and transversus abdominis m. These muscles play a role in lateral flexion of the trunk and most importantly, maintenance of tone of the abdomen.
The transversalis fascia is deep fascia that covers the inner surface of the transversus abdominis m., and extends medially to the anterior abdominal wall.
There is a variable amount of extraperitoneal fat between the transversalis fascia and parietal peritoneum, typically small amounts anterolaterally and increased amounts posteriorly.
The peritoneum, a serous membrane lining the abdominopelvic cavity, consists of 2 layers: visceral & parietal peritoneum. Parietal peritoneum lines the walls of the cavity, whereas visceral peritoneum covers viscera suspended within the cavity.
A more realistic image of the layers is depicted below:
Anterior abdominal wall
The anterior abdominal wall differs in the muscles and tendons present. The two most prominent structures are the rectus sheath and rectus abdominis m.
The rectus sheath is the aponeuroses of the three anterolateral abdominal muscles, and encloses the rectus abdominis m. The rectus sheath is complete anteriorly, but is incomplete in posteroinferior portion, leaving only transversalis fascia. The linea alba (the 'white line' of the median rectus sheath) spans between the xiphoid process and the pubic symphysis, and is a medial-most attachment point of the three abdominal oblique mm.
The rectus abdominis mm. are straight/vertical muscles that spans from the inferior rib cage to the pubic bone. They are active in trunk flexion and compression of abdominal viscera. There are tendinous intersections dispersed throughout the muscle that attach to the anterior rectus sheath. When these muscles are tensed or hypertrophied, the muscle bulges around the intersections creating a ‘six pack’ appearance.
On the deep surface of the anterior abdominal wall, there are three types of peritoneal folds: median umbilical fold, medial umbilical fold, and lateral umbilical (epigastric) fold.
The median umbilical fold is located in the midline (extending from umbilicus to the apex of the urinary bladder) and is created by the median umbilical ligament (a remnant of urachus).
The medial umbilical folds are located lateral to the median umbilical fold. The fossae in between are the supravesicular fossae. The medial umbilical folds are created by the medial umbilical ligaments (obliterated umbilical aa.).
The lateral umbilical folds are located lateral to the medial umbilical folds, and the fossae in between are the medial inguinal fossae. Direct inguinal hernias are associated with the medial inguinal fossae. The fossae lateral to the lateral umbilical folds are the lateral inguinal fossae, and these are associated with indirect inguinal hernias. The lateral umbilical folds are created by the inferior epigastric vessels.
