CLINICAL ANATOMY OF ANTERIOR ABDOMINAL WALL & RECTUS SHEATH

Abdomen

Structure of Abdominal Cavity

• Superiorly it is formed by diaphragm which separates the abdominal cavity from the thoracic cavity
• Inferiorly the abdominal cavity is continuous with the pelvic cavity through the pelvic inlet

Structure of Abdominal Wall
   Anteriorly:

• The abdominal wall is formed above by lower part of the thoracic cage
• Below by the rectus abdominis, external oblique, internal oblique, and transversus abdominis muscles and fasciae

Structure of Ant. Abdominal Wall

• It is made up of skin, superficial fascia, deep fascia, muscles, extraperitoneal fascia and parietal peritoneum
• The abdominal walls are lined by a fascial envelope and the parietal peritoneum

Skin

• Natural lines of cleavage in the skin are constant and run almost horizontally around the trunk
• An incision along a cleavage line will heal as a narrow scar, while one that crosses the lines will heal as a wide scar

Cutaneous Nerve Supply

• Is derived from the anterior rami of the lower six thoracic and first lumbar nerves
• Thoracic nerves are the lower five intercostal and the subcostal nerves
• First lumbar nerve is represented by the iliohypogastric and ilioinguinal nerves 

Blood Supply

• Skin near the midline is supplied by branches of the superior epigastric artery (br. of int. thoracic artery) and the inferior epigastric artery ( br. of external iliac artery)
• Skin of the flanks is supplied by branches from the intercostal, lumbar, and deep circumflex arteries

Superficial Fascia

• Fatty layer or fascia of camper is continuous with the superficial fat over the rest of the body and may be extremely thick in obese patients
• The membranous layer or scarpa’s fascia is thin and fades out laterally and above
• Becomes continuous with the superficial fascia of the back and the thorax

Superficial Fascia

• Inferiorly the membranous layer passes onto the front of the thigh, where it fuses with the deep fascia
• In the midline inferiorly forms a tubular sheath for the penis or clitoris
• Below in the perineum, enters the wall of the scrotum or labia majora
• From there it passes to be attached on each side to the margins of pubic arch, here it is called Colle’s fascia

Superficial Fascia

• Posteriorly it fuses with the perineal body and the margin of the perineal membrane
• The fatty layer is represented as a smooth muscle in the scrotum, the dartos muscle
• The membranous layer persists as a separate layer

Deep Fascia

• Deep fascia in the anterior abdominal wall is merely a thin layer of connective tissue covering the muscles
• It lies immediately deep to the membranous layer of the superficial fascia

Muscles

• Consists of Three broad thin sheets that are aponeurotic in front
• From exterior to interior they are:
• External oblique, internal oblique, and transverse
• A wide vertical muscle, the rectus abdominis
• They lie on either side of the midline anteriorly
• As the aponeurosis of three sheets pass forward, they enclose the rectus abdominis to form the rectus sheath
• The cremaster muscle which is derived from the lower fibers of internal oblique, passes inferiorly as a covering of the spermatic cord and enters scrotum

External Oblique Muscle

• Is a broad, thin, muscular sheet
• Origin: Lower 8 ribs
• Insertion: Xiphoid process, linea alba, pubic tubercle, iliac crest
• Nerve Supply: Lower 6 thoracic nerves, iliohypogastric & ilioinguinal nerves
• Action: Supports abdominal contents, assist in forced expiration, micturition, defecation, parturition, vomiting 
• A triangular shaped defect in the external oblique aponeurosis lies immediately above and medial to the pubic tubercle, known as superficial inguinal ring
• Between the anterosuperior iliac spine and the pubic tubercle, the lower border of the aponeurosis is folded backward on itself, forming the inguinal ligament

Internal Oblique Muscle

• Origin: Lumbar fascia, iliac crest, lateral two-thirds of inguinal ligament
• Insertion: Lower three ribs and costal cartilages, xiphoid process, linea alba, symphysis pubis
• Nerve Supply: Lower six thoracic nerves, iliohypogastric & ilioinguinal nerves
• Action: Supports abdominal contents, assist in forced expiration, micturition, defecation, parturition, vomiting 

Transversus Abdominis

• Origin: Lower six costal cartilages, lumbar fascia, iliac crest, lateral third of inguinal ligament
• Insertion: Xiphoid process, linea alba, symphysis pubis
• Nerve Supply: Lower six thoracic nerves, iliohypogastric & ilioinguinal nerves
• Action: Compresses abdominal contents

Rectus Abdominis

• Origin: Symphysis pubis and pubic crest
• Insertion: 5th, 6th and 7th costal cartilages and xiphoid process
• Nerve Supply: Lower six thoracic nerves
• Action: Compresses abdominal contents, flexes vertebral column, accessory muscle of expiration

Lymph Drainage

• Lymph drainage of the skin of the anterior abdominal wall above the umbilicus is upward to the anterior axillary (pectoral group of nodes)
• Below the level of umbilicus drains downward and laterally to the superficial inguinal nodes
• Swelling in the groin is may be due to enlarged superficial inguinal node

Venous Drainage

• Venous blood is collected into a network of veins that radiate from the umbilicus
• The network is drained above into the axillary vein via the lateral thoracic vein
• Below into the femoral vein via the superficial epigastric and the great saphenous veins
• Few small veins, the paraumbilical veins form a clinically important portal-system venous anastomosis

Caput Medusae

• The superficial veins around the umbilicus and the paraumbilical veins connecting them to the portal vein may become grossly distended in case of portal vein obstruction
• The distended subcutaneous veins radiate out from the umbilicus, producing in severe cases the clinical picture called Caput Medusae 

Nerves

• Nerves of the anterior abdominal wall supply the skin, muscles and the parietal peritoneum
• They are derived from the anterior rami of lower six thoracic and the first lumbar nerves
• Inflammation of parietal peritoneum causes pain in the overlying skin and also a reflex increase in tone of the abdominal musculature in the same area 

Rectus Sheath

• Is a long fibrous sheath
• Encloses the rectus abdominis and pyramidalis muscle (if present)
• Contains the anterior rami of lower six thoracic nerves and the superior and inferior epigastric vessels and lymph vessels
• Formed mainly by aponeurosis of three lateral abdominal muscles
•    For description it is considered at three levels:
• Above the costal margin the anterior wall is formed by the aponeurosis of the external oblique and posterior wall is formed by the thoracic wall
• That is the 5th , 6th and 7th costal cartilages and the intercostal spaces
• Between the costal margin and the level of the anterosuperior iliac spine, the aponeurosis of the internal oblique splits to enclose the rectus muscle
• The external oblique aponeurosis is directed in front of the muscle
• Transversus aponeurosis is directed behind the muscle

• Between the level of the anterosuperior iliac spine and the pubis, the aponeurosis of all three muscles form the anterior wall
• The posterior wall is absent
• The rectus muscle lies in contact with the fascia transversalis
• The posterior wall of the rectus sheath is not attached to the rectus abdominis muscle
• The anterior wall is firmly attached to it by the muscle’s tendinous intersections 

Linea Alba

• The rectus sheath is separated from its fellow on the opposite side by a fibrous band called the linea alba
• Extends from the xiphoid process to the symphysis pubis.