Abdomen

What you will be able to do

Two ways to say where something is

The nine regions are made by two horizontal planes and two vertical lines. The upper plane crosses the lowest points of the costal margin, the lower one the tubercles of the iliac crests. The vertical lines drop from the middle of each clavicle.

The four quadrants are simpler: the midline, and a line through the umbilicus. Use the regions for precision, the quadrants when speed matters.

Figure 1 · The nine regions and the four quadrants

The wall, from the skin inwards

Work through the wall in order and every later problem becomes easier. Below the umbilicus the superficial fascia splits in two.

One word does a great deal of work from here on. An aponeurosis is a flat sheet of tendon: the same material as the cord-like tendon of a limb muscle, but spread out. That is how a muscle can grip along a whole line instead of at a single point. Each of the three flat muscles is fleshy at the side and turns into an aponeurosis as it reaches the front.

Figure 2 · The layers of the wall and the rectus sheath

Illustration to be added

Three cross-sections of the anterior abdominal wall seen from above, midline at the centre, the panels running left to right in the order given. Panel one, below the umbilicus: skin, fatty then membranous superficial fascia, external oblique, internal oblique, transversus abdominis, transversalis fascia, extraperitoneal fat and parietal peritoneum, each a separate tint named on a leader line, the flat muscles fleshy at the flank and aponeurotic in front. Take the section below the umbilicus because the membranous layer exists only there. Panel two, above the arcuate line: aponeuroses splitting to pass in front of and behind rectus abdominis, meeting the other side at the linea alba. Panel three, below the arcuate line: all three in front, only transversalis fascia and peritoneum behind, inferior epigastric vessels against the back of the muscle. Letter each panel with its own title, 'Below the umbilicus', 'Above the arcuate line' and 'Below the arcuate line', since nothing else distinguishes them. Muscle brick, fascia navy, fat ivory, peritoneum a thin teal line.

Three muscles, three directions

Plywood is made by gluing thin sheets together with the grain of each turned across the one below. A single sheet splits along its grain; a stack has none.

The wall is built the same way. It resists a pull from any direction, and each aponeurosis interweaves in the midline, tying the two halves together.

Figure 3 · Three muscles, three fibre directions

Rectus abdominis and its sheath

Rectus abdominis runs from the pubic crest up to the fifth, sixth and seventh costal cartilages and the xiphoid process, and is the strong flexor of the trunk. Three features matter.

The linea alba is the midline seam where the aponeuroses of both sides interweave. It is broad above the umbilicus, narrow below, and nearly bloodless, so a midline incision crosses no muscle belly.

The tendinous intersections are three or four fibrous bands crossing the muscle. They are fused to the front wall of the sheath but not to the back. That is what gives the six-pack its blocks. It is also why a bleed inside the sheath stays put as a firm, tender lump instead of spreading: the intersections wall it off.

The arcuate line lies about midway between the umbilicus and the pubic symphysis. Below it there is no aponeurotic back wall at all.

The nerves of the wall

The wall is segmental, like the chest wall it continues from. The anterior rami of T7 to T11 run on into it, followed by T12. Two more come from L1, and those reach the groin. All travel in one plane, between internal oblique and transversus.

That single plane has consequences. Because every one of these nerves lies in it, local anaesthetic placed there can numb a wide strip of wall from one injection. A transverse incision runs along the nerves and slips between them; a vertical one crosses them, and the muscle beyond loses its supply. A denervated strip thins and can bulge later, which is why the placement of an incision is not just a matter of the scar.

The inguinal canal

The testis descends into the scrotum during development and drags a passage through the wall with it. That passage is the inguinal canal, present in both sexes, and a permanent weak point.

It is about four centimetres long and runs downwards, forwards and medially, above the medial half of the inguinal ligament. The deep ring is a hole in the transversalis fascia above the midpoint of that ligament, lateral to the inferior epigastric vessels. The superficial ring is a gap in the external oblique aponeurosis, just above and medial to the pubic tubercle.

The conjoint tendon in that table needs naming properly. It is where the lower fibres of internal oblique and transversus fuse and turn down to the pubic crest. They are the same two muscles that arch over the canal as its roof, now doubling back behind it.

That gives each ring something solid on one side of it. The deep ring has internal oblique arching in front of it, laterally; the superficial ring has the conjoint tendon behind it, medially. The two rings are three or four centimetres apart, not opposite one another. So a rise in pressure inside the abdomen drives the front wall of the canal onto the back and shuts it, which is why the canal does not leak every time you cough.

It carries the spermatic cord in the male and the much smaller round ligament of the uterus in the female. In both, the ilioinguinal nerve passes through and leaves by the superficial ring.

Figure 4 · The inguinal canal and the two inguinal hernias

Illustration to be added

Three panels of the right groin. Panel one: external oblique intact, the inguinal ligament running from the anterior superior iliac spine to the pubic tubercle, and the triangular superficial ring above and medial to the tubercle. Panel two: external oblique cut and reflected, showing the arching lower fibres of internal oblique as the roof, the deep ring in the transversalis fascia, the conjoint tendon reinforcing the posterior wall medially, and the cord along the canal; draw the inferior epigastric vessels prominently, as the reference line for the figure. Panel three: the posterior wall from inside, an indirect sac entering the deep ring lateral to those vessels and a direct sac bulging medial to them. Muscle brick, fascia navy, artery red, vein blue, nerves gold, bone ivory, no text.

Hernia, and the vessel that defines it

The femoral hernia passes below the inguinal ligament rather than above it. The pubic tubercle separates them: an inguinal hernia appears above and medial to it, a femoral hernia below and lateral. Femoral hernias are commoner in women and more likely to become trapped.

The peritoneum

Picture pressing your fist into a partly inflated balloon. Your fist is the organ, the layer wrapped around it is the visceral peritoneum, the outer layer lining the wall is the parietal peritoneum. Between them is a potential space holding a film of fluid, not a container of organs.

The greater sac is the main cavity; the lesser sac is a flattened recess behind the stomach. They communicate through one opening, whose front edge carries the bile duct, hepatic artery and portal vein.

A mesentery is a double fold of peritoneum slinging an organ from the back wall and carrying its vessels to it. The greater omentum hangs from the stomach as a fatty apron and wraps itself around inflamed areas.

Figure 5 · The peritoneum in section

Illustration to be added

Two panels. Panel one: a sagittal section of the trunk with the peritoneum traced as one unbroken line, from the front wall up over the liver, down behind the stomach to enclose the lesser sac, forwards and down as the four layers of the greater omentum, back to the transverse colon and its mesocolon, around the small bowel on its mesentery, and into the pelvis. Keep that line one colour so a reader can follow it, and shade the two sacs in different tints. Panel two: a cross-section at the kidneys seen from below, the peritoneum passing in front of the kidneys, aorta, inferior vena cava, pancreas and the ascending and descending colon. Peritoneum teal, bowel warm tan, organs brick, artery red, vein blue, no text.

The organs, in outline

None of these positions is arbitrary. The gut began as a straight midline tube, then grew faster than the cavity holding it, herniated into the cord, rotated as it returned, and was pinned back against the wall where it came to rest. Everything below follows from that rotation and that pinning.

Read the table for the consequence, not the position. Where an organ sits tells you which rib to look behind, which incision reaches it, and where its pain will be felt.

Why the pain is not where the problem is

Sensation from the organs travels back to the cord alongside the sympathetic supply, and localises poorly. The brain reads it as coming from the wall of the same segments, so the pain appears in the midline, in a band, and vaguely. Which band depends on where the organ began in the embryo, not where it ended up.

Figure 6 · Referred pain from the three parts of the gut

One organ is regularly listed with the foregut and does not belong there. The spleen never formed part of the gut tube; it grew inside the fold of peritoneum behind the stomach. It keeps foregut company because it shares the same artery and the same sympathetic supply, which is why its pain is felt in the epigastrium. Blood collecting under the left diaphragm adds pain at the tip of the shoulder, and that is the phrenic nerve, not the gut.

This is why appendicitis pain moves. The appendix is a midgut structure, so inflammation first gives a dull central ache. When it reaches the parietal peritoneum against the wall, a somatic supply takes over: the pain shifts to the right iliac fossa, becomes sharp, and stays put.

What this means for a physiotherapist

Trunk control. The abdomen is a cylinder: diaphragm above, pelvic floor below, abdominal muscles in front and at the sides, thick fascia behind. When those walls tighten together the pressure inside rises and the lumbar spine is stiffened from the front, with almost no movement produced.

Breathing and coughing. Quiet breathing out is passive. Forced breathing out is not, and the abdominal muscles drive it, squeezing the contents up against the relaxed diaphragm. A cough is the same action performed fast against a closed throat, so a painful wall means a weak cough.

After surgery. The wound hurts, so the patient breathes shallowly and will not cough. Lung volumes fall and secretions collect where they are hardest to clear. That is the reasoning behind the treatment: sit them upright, get them moving early, teach breathing that reaches the lower chest, and show them how to hold a folded towel over the wound so they can build the pressure a cough needs.

A groin lump appearing on coughing is a hernia until proved otherwise, and a tender, tense, irreducible one needs urgent medical review rather than exercise. When wall pain and organ pain are hard to separate, ask the patient to tense the wall and palpate again: wall pain usually worsens.

What goes wrong here

Where students get this wrong

Drawing the three flat muscles running the same way

They run in three different directions, and that is the point.

Naming a hernia from where the lump ends up

They are defined by the inferior epigastric vessels. The scrotum is a clue, not a definition.

Putting the deep ring at the mid-inguinal point

The deep ring lies above the midpoint of the inguinal ligament, measured to the pubic tubercle. The mid-inguinal point, measured to the symphysis, marks the femoral artery.

Expecting a posterior rectus sheath all the way down

Below the arcuate line only transversalis fascia and peritoneum lie behind the muscle.

Picturing the organs floating in the peritoneal cavity

Organs are wrapped by peritoneum or lie behind it, never loose within it.

Treating the abdominal muscles as trunk flexors only

They also generate pressure, stiffen the spine, control rotation and drive forced expiration.

Check yourself

Ten questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Reviewed by the Physiotherapist India Team. · Human Anatomy contents