Anterior abdominal wall
The anterior abdominal wall forms the anterior limit of the abdominal viscera. It runs, superiorly from the xiphoid process and costal cartilages of the 7th, 8th, 9th and 10th ribs to the iliac crest, inguinal ligament, anterior superior iliac spine, pubic tubercle, pubic crest and pubic symphysis inferiorly. Generally, from superficial to deep, it is comprised of the skin, superficial fascia (adipose and membranous tissues), deep fascia, layers of muscles, extraperitoneal tissue and the peritoneum. Most of these structures, especially the muscles, span the anterolateral abdominal wall and there is no definite boundary between the anterior and lateral abdominal walls, thus the anterior abdominal wall is sometimes referred to as the anterolateral abdominal wall.
The anterior abdominal wall is highly distensible and is involved in various functions ranging from support of movements of the abdominal viscera to protection of the abdominal cavity. It is more flexible than the posterior abdominal wall, and supports lateral bending, flexion, extension or protrusion and twisting. It plays a role in the maintenance of posture, and increases in intra abdominal pressure to support defecation, parturition and micturition.
The anterior abdominal wall is also important because it is used in clinical surface anatomy for the localization of abdominal viscera. For these purposes, the abdominal wall is divisible into four quadrants by two imaginary lines; a midline (vertical) and a horizontal line which passes through the umbilicus. The quadrants are named as follows; the right upper quadrant, left upper quadrant, right lower quadrant and left lower quadrant. This form of dividing the abdomen into four quadrants is known as the four region scheme.
Similarly, the anterior abdominal wall can also be divided into nine regions by two imaginary vertical lines called mid-clavicular lines (two vertical lines passing through the midpoint of the clavicle on the left and right side), and two imaginary horizontal lines. These regions are of clinical importance. The mid-clavicular lines extend downwards to reach the mid inguinal point. The superior horizontal line is known as a transpyloric line (or transpyloric plane). This line is so named because of its relation to the pylorus of the stomach. It runs horizontally at the halfway point between the jugular notch and top of the pubic symphysis, passing through the pylorus of the stomach. The lower horizontal line can be drawn to join the tubercles of the right and left iliac crests of the hip bones. It is called the transtubercular plane or intertubercular line (intertubercular plane). Therefore, these four lines divide the anterior abdominal wall and the abdominopelvic cavity into a middle epigastric, umbilical, suprapubic or hypogastric, right hypochondrium, left hypochondrium, right flank (right lumbar), left flank (left lumbar), right groin (right iliac, inguinal) and left groin (left iliac, inguinal) regions. This division of the abdomen into nine regions is aptly known as the nine region scheme.
Structures of the Anterior Abdominal Wall
The layers of the anterior abdominal wall are mentioned below:
Skin and Fascia of the Anterior Abdominal Wall
The skin is the most superficial layer of the anterior abdominal wall. It shows creases which represent the lines of orientation of collagen fibers in the dermis of the skin. These lines are referred to as Langer's lines. The Langer’s lines are of clinical relevance and incisions along them heal faster and better leaving little or no scaring; while those across them leave large and more pronounced scars. In pregnant women, obese people and those with abdominal distention, there are dark elongate lines called stretch marks or striae distansae usually on the umbilical and hypogastric regions. Stretch marks during pregnancy is specifically known as striae gravidarum.
Lying just below the skin are two layers of superficial fascia called Camper’s fascia and Scarpa’s fascia. The Camper’s fascia is a subcutaneous tissue containing variable amounts of fatty tissue. This fatty tissue is generally more in females and also in the right and left lower quadrants. Next to the Camper’s fascia is the Scarpa’s fascia. The Scarpa’s fascia is a membranous fatty layer containing fibrous tissue and very little fat. It runs inferiorly into the lower limbs where it changes its name to the fascia lata of the thigh. It is also continuous with the superficial perineal fascia called Colles’ fascia and also with the fascia which invests the scrotum and penis in males.
Deep to the two superficial fascia are layers of deep fascia. The deep fascia of the anterior abdominal wall directly invests the muscular layers of the anterior abdominal wall. They are made up of elastin fibres, dense fibrous connective tissue and divide the groups of muscles of the anterior abdominal wall into fascial components. Because this layer of fascia contains elastin fibres, it helps the anterior abdominal wall in playing a role in increasing the intra-abdominal pressure to support defecation, parturition and micturition.
Muscular Layers of the Anterior Abdominal Wall
There are five (bilaterally paired) muscles in the anterior abdominal wall. These muscles are arranged into three muscular layers by the deep fascias. However, these muscles extend into the lateral aspects of the abdominal wall and are more precisely referred to as muscles of the anterolateral abdominal wall. Of these five muscles, three are flat. These are: the external oblique muscle, the internal oblique muscle and the transverse abdominal muscle. All three muscles attach anteriorly to a strong sheet of aponeurosis which interlaces at the linea alba (Latin word for: white line). Also, the posterior attachment of these flat muscles are onto another tough tendinous aponeurosis of the rectus muscle. This aponeurosis is called the rectus sheath. The rectus sheath houses the remaining two muscles of the anterior abdominal wall. The remaining two muscles are vertical muscles and they are referred to as the rectus abdominis muscle and the pyramidalis muscle. The following is a short description of these five muscles:
- The external oblique muscles are a pair of broad, thin, superficial muscles that lie on the lateral sides of the anterior abdominal wall. The location and structure of the external abdominal obliques gives them many different possible actions. The external obliques get their name from their position in the abdomen external to the internal abdominal obliques and from the direction of their fibers, which run diagonally across the sides of the abdomen. The external abdominal obliques have their origins along the lateral ribs 5 through 12 and insert into the xiphoid process, linea alba of the abdomen, the pubis, and the iliac crest of the pelvic bones. Their shape is roughly rectangular with the long axis running anterior to posterior along the linea alba. The posterior most fibres form the free posterior border. Above and medial to the pubic tubercle , a small triangular defect in its aponeurosis is termed as superficial inguinal ring. The inferior border of the aponeurosis between the anterior superior iliac spine and pubic tubercle, folds backward and forms the inguinal ligament. The lacunar ligament extends backward and upwards towards pectineal line from the medial end of the inguinal ligament. Muscle fibers in the external obliques run medially and inferiorly from the origins to the insertions across the lateral sides of the abdomen and end just lateral to the rectus abdominis muscles.
- The internal oblique muscles lie deep to the external oblique muscles, and they are of the anterior and lateral parts of the abdomen. They originate in the lumbar fascia, anterior two-thirds of the iliac crest, and the lateral two-thirds of the inguinal ligament and insert into the inferior borders of the lower three ribs and their costal cartilages, xiphoid process, and aponeurosis of the rectus sheath as well as the conjoined tendon to the pubic crest and pectineal line. Most of its fibres run at a right angle to those of external oblique.
- The transverse abdominal muscle lies deep to the internal oblique muscle. It is the deepest of the flat muscles, and originates from the lateral third of the inguinal ligament, the anterior three-fourths of the inner lip of the iliac crest of the pelvic bone, the inner surfaces of the cartilages of the 7th to 12th ribs, and interdigitates with the diaphragm and the lumbodorsal fascia. It attaches to the xiphoid process, linea alba and pubic symphysis. A conjoint tendon is formed by the joining of the lowest tendinous fibres of the internal oblique and transverse abdominal muscle.
- The rectus abdominis muscles are a pair of long straight muscles which run vertically on each side of the anterior abdominal wall. The term rectus abdominis is the Latin meaning “straight abdominal”, and indicates that the muscle fibers run in a straight vertical line through the abdominal region of the body. They are contained in the rectus sheath. The rectus abdominis muscles are separated at the midline by the linea alba. They extend along the entire length of the abdomen adjacent to the umbilicus. Each muscle consists of a string of four fleshy muscular bodies connected by narrow bands of tendon known as tendinous intersections. These intersections are present usually at three levels: at the level of xiphoid process, the level of umbilicus and midway between them. The muscle originates and extends from the xiphoid process of the sternum and costal cartilages of the 5th to 7th ribs to insert into the pubic bone, particularly from the pubic symphysis, pubic crest, and tubercle.
- The pyramidalis muscle is a small triangular muscle lying anterior to the rectus abdominis muscle. 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 at a point midway between umbilicus and pubis through the anterior pubic ligament. Superiorly, the fleshy portion of the muscle passes upward, diminishing in size as it ascends, and ends by a pointed extremity which is inserted into the linea alba, therefore when it contracts, it tenses the linea alba.
The rectus sheath is a strong fibrous sheath around the rectus abdominis and pyramidalis muscle. It is formed by the decussation of the aponeurotic fibres of the flat abdominal muscles. The linea alba is present between the sheaths of both sides. Along with the muscles, it contains superior and inferior epigastric vessels, anterior rami of the lower six thoracic nerves and lymph vessels.
Nerves and vessels of the anterior abdominal wall
Superior epigastric artery , a branch of internal thoracic artery , runs in the rectus sheath behind the rectus muscle and supplies it. It ends by anastomosing with the inferior epigastric artery. The Inferior epigastric artery is a branch of external iliac artery and enters the rectus sheath after piercing the fascia transversalis. It runs upwards, supplies the lower central part of anterior abdomen and anastomoses with superior epigastric artery. The lower two posterior intercostal arteries and the deep circumflex iliac arteries also contribute in the arterial supply. There is a network of superficial veins that radiate out from the umbilicus and a few small paraumbilical veins which connect the network. The deep veins follow the arteries of the same name. Muscles of the anterior abdominal wall are supplied by lower six thoracic nerves, the iliohypogastric nerve and the ilioinguinal nerve. The pyramidalis is supplied by 12th thoracic nerve.
There are superficial and deep lymph vessels of the anterior abdominal wall. The superficial lymph vessels above the umbilicus drain into anterior axillary group of lymph nodes and below the umbilicus they drain into superficial inguinal lymph nodes. The deep vessels follow the arteries and drain into internal thoracic, external iliac, posterior mediastinal, and para-aortic (lumbar) nodes.
Functions of Anterior Abdominal Wall Muscles
The external and internal oblique muscles flex the trunk laterally and play their part in rotation. The rectus abdominis muscles help stabilize the pelvis and also flex the trunk. All the muscles of the anterolateral abdominal wall relax the diaphragm as it descends during inspiration which helps the abdominal viscera to accommodate. The muscles pull down the ribs and sternum during coughing and sneezing, and assist in the act of forced expiration. The tone of these muscles supports and protects the abdominal viscera. During the process of micturition, defecation, vomiting and parturition these muscles increase the abdominal pressure (with the glottis of the larynx closed) by contracting together with the diaphragm.