Relevant Anatomy

Anatomy of the Abdominal Wall

The abdominal wall is composed of 7 layers.  They are (from anterior to posterior) as follows:

Approximately midway between the umbilicus and the symphysis pubis is an anatomic landmark known as the Semicircular Line of Douglas.  Because the anatomy of the abdominal wall differs above and below this landmark, a description is appropriate and necessary when designing incisions or attempting repairs in this part of the belly. 

Above the Semicircular Line of Douglas, the anterior sheath is formed by the fusion of the external oblique aponeurosis and the anterior leaf of the internal oblique aponeurosis.  The posterior sheath is formed by the fusion of the posterior leaf of the internal oblique aponeurosis and the aponeurosis of the transversus. 

Below the Semicircular Line of Douglas, all three aponeuroses cross anterior to the rectus muscle, leaving only the peritoneum and the transversalis fascia between the rectus muscles and the contents of the abdomen.

Anatomy of the Inguinal Canal

The inguinal canal is a short passage that runs obliquely and inferomedially through the inferior part of the anterior abdominal wall. It runs parallel and slightly superior to the inguinal ligament. It is bounded by two walls (anterior and posterior), a roof, and a floor.

The anterior wall is formed by the anterior aponeurosis of the external oblique muscle with the addition of a small amount of the internal oblique laterally. 

The posterior wall is formed by transversalis fascia with medial reinforcement by the conjoint tendon, which is the tendon of both the internal oblique and transversus abdominis muscles.

Its floor is formed by the superior surfaces of both the inguinal and lacunar ligaments

Its roof is formed by the arching fibers of the internal oblique and transversus abdominis muscles.

The inguinal canal has two openings:  the deep and superficial inguinal rings. 

The superficial (external) inguinal ring is a slitlike opening in the aponeurosis of the external oblique muscle. The base of this triangle is formed by the pubic crest and its sides are formed laterally by a part of the external oblique aponeurosis, attached by the inguinal ligament to the pubic tubercle, and medially by the part of the external oblique aponeurosis attached to the pubic bone and crest. The sides are prevented from spreading apart by intercrural fibers, named by the medial and lateral crura which it holds together.

From the superficial rings emerge the spermatic cords in males or round ligaments in females, as well as the ilioinguinal nerve, which supplies the skin on the superomedial aspect of the thigh.

The deep (internal) inguinal ring consists of a simple slit in the transversalis fascia, located laterally to the inferior epigastric artery. Its base is the midpoint of the inguinal ligament and its lateral edge is the origin of the transversus abdominis muscle.

Blood Supply and Nerves

No discussion of the anatomy of the lower abdominal wall and inguinal region would not be complete without mention of the blood supply and nerves which traverse the area.

The Inferior Epigastric Artery and Vein originate from the external iliac artery and vein and course superiorly entering the rectus sheath.  There, they course deep to the rectus abdominis muscles supplying the rectus abdominus and medial portion of the anterolateral abdominal wall.

The Ilioinguinal Nerve originates from the ventral ramus of the 1st lumbar nerve.  It passes between the 2nd and 3rd layers of abdominal muscles, so continue on though the inguinal canal.  This nerve supplies innervation tot he skin of the scrotum (labia majorus in women), the mons pubis, and adjacent medial aspect of the thigh.  It also has distributions to the internal oblique and transversus muscles of the abdomen.

During an open hernia repair, careful dissection of the ilioinguinal nerve is important for two reasons.  (1)  Because of the nerve distributions cited above, injury during incision or closure can result in pain following the L1 dermatome (including the scrotum or labium majorum).  (2)  Because the ilioinguinal nerve has motor distributions to the internal oblique (which are inserted into the lateral border of the conjoint tendon), division of the nerve paralyzes these muscle fibers, weakening the conjoint tendon, which can precipitate a direct inguinal hernia.

Hesselbach's Triangle

Hesselbach's triangle is an anatomic area of the abdominal wall bounded by:

Laterally:  The inferior epigastric vessels

Medially:  The lateral border of the rectus muscle

Inferiorly:  The inguinal ligament

This space is important because it is through Hesselbach's Triangle that Direct Hernia's leave the abdomen.  Indirect Hernias protrude lateral to Hesselbach's Triangle.

Terms

The following terms are important to consider when talking about hernias:

Reducible:  Refers to a hernia that can be moved back into place easily (either manually or spontaneously)

Irreducible / Incarcerated:  Refers to a hernia that cannot be pushed back.  It does not necessarily mean that the hernia is strangulated or that obstruction is occurring (although both are possible).  If spontaneously reduced, and patient feels better, patient should be admitted for close observation.

Strangulated:  Refers to a hernia in which the blood supply to the incarcerated viscus is compromised.  Skin overlying the hernia is typically erythematous and/or pale, and nausea, vomiting, fever, and abdominal pain are usually present (or imminent.  A strangulated hernia should not be reduced, but if it is reduced (by patient or spontaneously), the patient should be opened and the dead bowel removed.