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The Anterolateral Abdominal Wall 

  Most of the abdominal wall is muscular and extends between the thoraciccage and the bony pelvis.

  There are four important paired muscles in the anterior abdominal wall:three flat muscles (external oblique, internal oblique, and transversusabdominis) and one strap-like muscle (rectus abdominis).

  The combination of muscles and aponeuroses in the anterior abdominalwall affords considerable protection to the abdominal viscera, especiallywhen the muscles are in good physical condition.

  The flat muscles cross each other in such a way (similar to a three-plycorset) that strengthens the abdominal wall and diminishes the risk of protrusion of viscera (herniation) between the muscle bundles.

The Extern al Obliqu e Muscle (pp. 132-3)  

  This is the largest and most superficial of the three flat abdominalmuscles. It is located in the anterolateral aspect of the abdominal wall.

  Its fleshy part forms the anterolateral portion and its aponeurosis formsthe anterior part.

  Its fibres run inferoanteriorly and medially in the same direction as do theextended digits when they are in one's side pockets.

  Origin: external surfaces of 5th to 12th ribs.

  Insertion: linea alba, pubic tubercle and anterior half of the iliac crest.  Innervation: inferior six thoracic nerves and subcostal nerves.

  As the fibres pass medially, they become aponeurotic. This aponeurosisends medially in the linea alba.

  Inferiorly, it folds back on itself to form the inguinal ligament between theanterior superior iliac spine and the pubic tubercle.

  Medial to the pubic tubercle, the external oblique aponeurosis is attachedto the pubic crest. Some fibres of the inguinal ligament cross the linea albaand attach to the opposite pubic crest. These fibres form the reflex inguinal

ligament.

  Just superior to the medial part of the inguinal ligament, there is anopening in the aponeurosis called the superficial inguinal ring.

The Internal Obliq ue Mus cle (pp. 133, 135)  

  This is the intermediate layer of the three flat abdominal muscles.

  Origin: thoracolumbar fascia, anterior two-thirds of iliac crest, and lateralhalf of inguinal ligament.

  Insertion: inferior borders of 10th

to 12th

ribs, linea alba, and the pubic viathe conjoint tendon.

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  Innervation: ventral rami of inferior six thoracic and first lumbar nerves.

  Its fibres also become aponeurotic and the aponeurosis splits to form asheath for the rectus abdominis muscle.

  The inferior fibres of the aponeurosis arch over the spermatic cord as it lies

in the inguinal ring to attach to the pubic crest and pecten pubis.

  The most inferior tendinous fibres of the internal oblique muscle join theaponeurotic fibres of the transversus abdominis muscle to form theconjoint tendon, which turns inferiorly to insert into the pubic crest andpecten pubis.

The Transversu s Ab dom inis Musc le (p. 135)  

  This is the innermost of the three flat abdominal muscles.

  Origin: internal surfaces of 7th to 12th costal cartilages, thoracolumbarfascia, iliac crest, and lateral third of inguinal ligament.

  Insertion: linea alba with aponeurosis of internal oblique, pubic crest, andpecten pubis via conjoint tendon.

  Innervation: ventral rami of inferior six thoracic and first lumbar nerves.

  Its fibres run more or less horizontally, except to those of the internaloblique muscle. Muscle fibres of the transversus abdominis end in anaponeurosis which contributes to the formation of the rectus sheath.

Actio ns o f the Three Flat Abd omin al Muscles (pp. 135-6)  

  The anterolateral abdominal wall is unsupported and unprotected by bone.However the three-ply structure of its flat muscles and the extensiveaponeuroses form a strong expandable support, which providesconsiderable protection for the abdominal viscera.

  Normally, quite rhythmic movements of the anterolateral abdominal wall

accompany respiration. The anterolateral abdominal wall expands as itsmuscles relax. During expiration, the anterolateral abdominal wallpassively sinks in.

  However, in the forced expiration that occurs during coughing, sneezing,vomiting, and straining, all the anterior abdominal muscles act strongly incompressing the abdominal contents.

  Acting together , the flat abdominal muscles increase the intraabdominalpressure. This action produces the force required for defecation,micturition (urination), and parturition (childbirth).

  Acting separately, the flat abdominal muscles move the trunk. If the pelvis

is fixed, both external oblique muscles can flex the trunk. Acting

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separately, one external oblique muscle can laterally flex the trunk androtate it to the opposite side.

  If the thorax if fixed, both external oblique muscles tilt the anterior part of the pelvis superiorly and flex the trunk.

  Similarly, when the pelvis is fixed, one internal oblique muscle can flex thetrunk and rotate it to the same side.

  If the thorax is flexed, one internal oblique muscle can laterally flex thetrunk and rotate the pelvis to the opposite side.

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The Rectus Ab dom inis Mus cle (p. 136)  

  This is a long, broad, strap-like muscle and is the principle vertical muscle of the anterior abdominal wall.

  Origin: pubic symphysis and pubic crest.  Insertion: xiphoid process and 5th to 7th costal cartilages.  innervation: ventral rami of inferior six thoracic nerves.

  The two muscles are separated by the linea alba and lie close together

inferiorly.  The rectus abdominis is three times as wide superiorly as it is inferiorly.  The lateral border of the rectus muscles and its sheath are convex and

form a clinical important surface marking known as the linea semilunaris.

  Most of the rectus abdominis muscle is enclosed in the rectus sheath 

formed by the aponeuroses of the three flat abdominal muscles.  The anterior layer of the rectus sheath is firmly attached to the rectus

muscle at three or more tendinous intersections. When this muscle istensed in muscular persons, each stretch of muscle between the tendinousintersections is indicated by grooves in the skin between the muscle

bulges.  They are usually located at the level of the xiphoid process, umbilicus, and

halfway between this structures.

 Actions of the Rectus Abdominis Muscles (p.136) 

  In addition to helping the other abdominal muscles to compress theabdominal viscera, these muscles depress the ribs and stabilise the pelvis

during walking.

  The fixation of the pelvis enables the thigh muscles to act effectively.

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  Similarly, during lower limb lifts from the supine position, the rectusabdominis muscles contract to prevent tilting of the pelvis by the weight of the limbs.

The Lin ea Alba and Rectu s Sheath (pp . 136-7)  

  The rectus sheath is the strong, incomplete fibrous compartment for the

rectus abdominis muscle.  It forms by the fusion and separation of the aponeurosis of the flat

abdominal muscles.

  At its lateral margin, the internal oblique aponeurosis splits into twolayers, one passing anterior to the rectus muscle and the other passingposterior to it.

  The anteriorly layer joins with the aponeurosis of the transverse abdominismuscle to form the posterior wall of the rectus sheath.

  The fibres of the anterior and posterior wall of the rectus sheath interlacein the medial line to form a complex tendinous raphe, called the linea alba,which is an intermixture of the aponeurotic fibres of the oblique andtransverse abdominal muscles.

  It is narrow inferior to the umbilicus, but is wide superior to it. The grooveis visible in the skin superficial to it in thin muscular persons.

  The linea alba lies between the two parts of the rectus abdominis muscle;the umbilicus is located just inferior to its midpoint.

  Superior to the costal margin, the posterior wall of the rectus sheath isdeficient because the transversus abdominis muscle passes internal to thecostal cartilages and the internal oblique muscle is attached to the costalmargin. Hence, superior to the costal margin, the rectus muscle liesdirectly on the thoracic wall.

  The inferior one-fourth of the rectus sheath is also deficient because theinternal oblique aponeurosis does not split to enclose the rectus muscle.

  A crescentic border called the arcuate line marks the inferior limit of theposterior wall of the rectus sheath.

  The position of this line is usually midway between the umbilicus and the

pubic crest.  Inferior to the arcuate line, the aponeuroses of the three flat muscles pass

anterior to the rectus muscle to form the anterior layer of the rectussheath.