Anterior abdominal wall

Question 1

What are the 3 muscle layers of the anterolateral abdominal wall?

Answer 1

• The 3 muscle layers of the body wall are separate in the flanks, where they are known as the external oblique, internal oblique and transversus abdominis muscles.
• The layers have fused ventrally to form the rectus abdominis muscle.

Question 2

Outline external oblique.

Answer 2

• The muscle arises by eight digitations, one from each of the lower eight ribs just lateral to their anterior extremities.
• The lower four slips interdigitate with the costal fibres of latissimus dorsi and the upper four with digitations of serratus anterior.
• From its fleshy origin the muscle fans out to a very wide insertion, much of which is aponeurotic.
• The muscle has a free posterior border which extends from the twelfth rib to its insertion by fleshy fibres into the anterior half of the outer lip of the iliac crest.
• Muscular fibres are replaced by an aponeurosis below a line joining the anterior superior iliac spine to the umbilicus, and medial to a vertical line drawn from the tip of the ninth costal cartilage.
• The aponeurotic fibres, directed obliquely downwards and forwards, interdigitate with each other across the front of the rectus abdominis along the whole length of the linea alba.
• The free horizontal upper border of this aponeurosis extends from the fifth rib to the xiphisternum. It is the only structure in the anterior sheath of the rectus muscle above the costal margin.
• The free posterior border of the muscle forms the anterior boundary of the lumbar triangle (of Petit) that is floored in by the internal oblique and bounded behind by the anterior border of latissimus dorsi and below by the iliac crest. The triangle may be the site of a rare lumbar hernia.
• The lower border, lying between the anterior superior iliac spine and the pubic tubercle, forms the inguinal ligament (of Poupart).
• Its edge is rolled inwards to form a gutter; the lateral part of this gutter gives origin to part of the internal oblique and transversus abdominis muscles.
• The fascia lata of the thigh is attached to the inguinal ligament and when the thigh is extended the fascia lata pulls the inguinal ligament downwards into a gentle convexity.
• Just above and lateral to the pubic tubercle is an oblique, triangular gap, the superficial inguinal ring, in the aponeurosis. The base of the gap is the pubic crest, and the margins are the crura of the ring.
• From the medial end of the inguinal ligament the triangular lacunar ligament (of Gimbernat) extends horizontally backwards to the pectineal line on the pubis.
• A fibrous band, the pectineal ligament (of Astley Cooper) extends laterally further along the pectineal line.
• The crescentic free lateral edge of the lacunar ligament is the medial margin of the femoral ring.
• From the pubic tubercle, fibres that may be traced upwards and medially, behind the spermatic cord, interdigitate in the linea alba with those of the opposite side. This is the reflected part of the ligament.
• Near the apex of the superficial inguinal ring are fibres running at right angles to those of the aponeurosis, the intercrural fibres, that prevent the crura from separating.

Question 3

Outline internal oblique.

Answer 3

• Fleshy fibres of the muscle arise from the whole length of the lumbar fascia, from the intermediate area of the anterior two-thirds of the iliac crest and from the lateral two-thirds of the inguinal ligament.
• From the lumbar fascia the muscle fibres run upwards along the costal margin, to which they are attached, becoming aponeurotic at the tip of the ninth costal cartilage.
• Below the costal margin, the aponeurosis splits around the rectus muscle, the two layers rejoining at the linea alba.
• Halfway between the umbilicus and the pubic symphysis the posterior layer ends in a curved free margin, the arcuate line.
• Below this point, the aponeurosis passes wholly in front of the rectus muscle, to the linea alba.
• The muscle fibres that arise from the inguinal ligament are continued into an aponeurosis that is attached to the crest of the pubic bone and, more laterally, to the pectineal line.
• This aponeurosis is fused with a similar arrangement of the transversus aponeurosis to form the conjoint tendon.
• The internal oblique therefore has a free lower border, which arches over the spermatic cord: laterally the margin consists of muscle fibres in front of the cord; medially the margin consists of tendinous fibres behind the cord.

Question 4

Outline transversus abdominis.

Answer 4

• The muscle arises in continuity from the lateral third of the inguinal ligament, the anterior two-thirds of the inner lip of the iliac crest, the lumbar fascia, the twelfth rib, and from the inner aspects of the lower six costal cartilages where it interdigitates with the diaphragm.
• The muscle fibres become aponeurotic and pass behind the rectus to fuse with the internal oblique aponeurosis in the linea alba.
• Below the arcuate line the aponeurosis passes wholly in front of the rectus muscle.
• In the upper part of the abdomen the outer margin of the aponeurosis is more medial, and muscular fibres lie behind the lateral part of rectus abdominis.
• The lower fibres of the aponeurosis curve downwards and medially with those of the internal oblique as the conjoint tendon, to insert on the pubic crest and the pectineal line.

Question 5

Outline rectus abdominis.

Answer 5

• Rectus abdominis arises by two heads: a medial from in front of the pubic symphysis and a lateral from the upper border of the pubic crest.
• The lower parts of the two muscles are narrower and lie edge to edge.
• The upper parts are broader and are separated from each other by the linea alba.
• They are inserted on to the front of the fifth to seventh costal cartilages.
• Typically three tendinous intersections are found in the muscle, one at the umbilicus, one at the xiphisternum, and one between these two; one or two incomplete intersections are some- times found below the umbilicus.
• The tendinous intersections blend inseparably with the anterior layer of the rectus sheath.
• They occupy only the superficial part of the rectus and do not penetrate to the posterior surface of the muscle, which is thus not connected to the posterior layer of the sheath.
• The contracting rectus abdominis can be seen as bulgings between the tendinous intersections in an individual who is not too fat.

Question 6

Outline pyramidalis.

Answer 6

• The small triangular pyramidalis muscle arises from the body of the pubis and the symphysis between rectus abdominis and its sheath.
• It converges with its fellow into the linea alba 4cm or so above its origin.
• Between the two recti all the aponeuroses that form the rectus sheath fuse to form the linea alba, a strong midline fibrous structure which is firmly attached to the xiphoid process above and the pubic symphysis below.
• Above the symphysis it is very narrow, for here the two recti are in contact with one another behind it. * From just below the umbilicus to the xiphisternum it broadens out between the recti. Here the fibres form a tough felted membrane.
• The umbilicus is a defect in the linea alba through which fetal umbilical vessels pass.

Question 7

Describe the rectus sheath.

Answer 7

• The aponeurosis of the internal oblique splits into anterior and posterior layers to enclose the rectus muscle.
• The external oblique aponeurosis fuses with the anterior layer to form the anterior layer of the sheath, and the transversus aponeurosis fuses with the posterior layer to form the posterior layer of the sheath.
• From halfway between the umbilicus and the pubic symphysis all three aponeuroses pass in front of the muscle.
• The aponeuroses of internal oblique and transversus fuse completely but that of the external oblique fuses only to the most medial part of the sheath.
• The posterior layer of the sheath has a free lower margin concave downwards, the arcuate line or semicircular line (of Douglas).
• Superiorly the posterior layer of the sheath is attached to the costal margin (seventh, eighth and ninth costal cartilages). * Above the costal margin the anterior layer of the sheath consists only of the external oblique aponeurosis.
• The splitting of the internal oblique aponeurosis along the lateral border of the rectus muscle forms a relatively shallow groove, the semilunar line. It curves up from the pubic tubercle to the costal margin at the tip of the ninth costal cartilage in the transpyloric plane.
• Detailed studies indicate that the aponeuroses of external oblique, internal oblique and transversus abdominis are each bilaminar, giving six layers in all; three form the anterior and three the posterior layers of the rectus sheath. These layers decussate across the midline.
• There may not be a well-defined arcuate line but a gradual diminution of aponeurotic fibres with increasing thickness of the transversalis fascia. The lower thickened part of the transversalis fascia, between the iliac crest and pubis just above the inguinal ligament, is called the iliopubic tract.

Question 8

What are the contents of the rectus sheath?

Answer 8

• Rectus abdominis
• Pyramidalis
• Lower 6 thoracic nerves (T7-11 intercostal nerves) and their accompanying posterior intercostal vessels
• Superior and inferior epigastric vessels

Question 9

What are the nerve supplies for the anterior abdominal wall muscles?

Answer 9

• The rectus muscle and external oblique are both supplied by the lower intercostal and subcostal nerves (T7–T12), and the internal oblique and transversus by those same nerves but with the addition of the iliohypogastric and ilioinguinal nerves (L1).
• The lowest fibres of the internal oblique and transversus that continue medially as the conjoint tendon receive the L1 innervation, which thus helps to maintain the integrity of the inguinal canal (see below).
• Pyramidalis is supplied by the subcostal nerve (T12).

Question 10

What are the movements of the anterior abdominal wall muscles?

Answer 10

• The muscles of the anterior abdominal wall have four main roles:
  (1) to move the trunk
  (2) to depress the ribs (expiration)
  (3) to compress the abdomen (evacuation, expiration, heavy lifting)
  (4) to support and protect the viscera (intestines only).
• The abdominal wall, moving to and fro with breathing, conforms to the volume of the abdominal contents.
• Its shape is determined by the tonus of its own muscles.
• The subumbilical pull of healthy flank muscles keeps its lower part flat by holding back the lower recti.

Question 11

Outline the borders and contents of the inguinal canal.

Answer 11

• The inguinal canal is an oblique intermuscular slit about 4 cm long lying above the medial half of the inguinal ligament.
• It commences at the deep inguinal ring and ends at the superficial inguinal ring.
• Contents - transmits the spermatic cord and ilioinguinal nerve in the male and the round ligament of the uterus and ilioinguinal nerve in the female.
• Anterior wall - external oblique aponeurosis assisted laterally by the internal oblique muscle.
• Floor - inrolled lower edge of the inguinal ligament, reinforced medially by the lacunar ligament.
• Roof - lower edges of the internal oblique and transversus muscles.
• Posterior wall - strong conjoint tendon medially and the weak transversalis fascia throughout.

Question 12

What are the contents of the spermatic cord?

Answer 12

• Vas deferens.
• Arteries - testicular, artery to vas, cremasteric.
• Veins - pampiniform plexus.
• Nerves - genital branch of the genitofemoral nerve, autonomic nerves (sympathetic twigs).
• Lymphatics - essentially those from the testis
  draining to para-aortic nodes, but including some from the coverings which drain to external iliac nodes.
• The obliterated remains of the processus vaginalis.
• In the female they are replaced by the obliterated processus vaginalis, the round ligament and lymphatics from the uterus.