Figure 1: The rough position of the inguinal canal and its surrounding features.
The superficial inguinal ring is a triangular aperture 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 fibre, named by the medial and lateral crura which it holds together. The superficial ring contains the spermatic cord in the male and the uteral ligament in the female, as well as the ilioinguinal nerve, which supplies the skin on the superomedial aspect of the thigh.
The deep 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. The deep inguinal ring is less easy to located than the superficial ring and often only exists as a dimple in the peritoneum.
Formation and Function of the Inguinal Canal
Inguinal canals develop in both male and female embryos, however they are only functional in the male. Their function is to allow the testes, which develop in the lumbar region of the abdomen between the transversalis fascia and the peritoneum, to migrate to the scrotum. This starts to occur at around the twenty-eighth week and it takes about four weeks for the testes to descend to the scrotum. The testes are guided by the gubernaculum, a ligament linked from the testes through the anterior abdominal wall into the scrotum.
The processus vaginalis, a pouch of peritoneum, follows the gubernaculum pushing out the anterior abdominal before it, and eventually forms the covering for the spermatic cord. This descent through the three different layers of the anterior abdominal wall, the external and internal oblique and transversus abdominis muscles, occurs at an oblique angle and not directly.
This results in a reduced weakness in the abdominal wall which would not have existed if a direct hole had been formed. Instead the apertures in the different layers of the abdomen are not in a direct line and therefore the different muscle layers act as a shutter mechanism to restrain any herniation of the abdomen contents.
In the female, while the ovaries have to descend from a position similar to the testes, on the posterior abdominal wall, to a point just below the top of the pelvis, they do not normally enter the inguinal canals. However the processus vaginalis, which usually disappears completely, can remain in the female and therefore cause an indirect inguinal hernia. The gubernaculum attaches to the uterus and splits to become the ovary and uteral ligaments.
Inguinal Hernias
A hernia is a perfusion of a viscus through its coverings. Most hernias, including inguinal hernias, occur through parts of the abdominal wall e.g. umbilical hernias in embryos.
Figure 2: Schematic for an Abdominal Hernia.
Due to the gap created in the abdominal wall by the inguinal canal, both inguinal and scrotal hernias are very common in males. While females can have inguinal hernias the weakness in the abdominal wall is much smaller due to the presence of fat in the canal.
There are two types of inguinal hernias, direct and indirect, with indirect hernias being the commoner form. Indirect inguinal hernias occur when the contents of the hernia is surrounded by all three layers of the spermatic cord, which form the hernial sac and are in fact the remains of the processus vaginalis. They can either be congenital or acquired, the congenital form occurring if the processus vaginalis has not been complete removed, and therefore remains to act as a weak spot in the abdominal wall. Indirect congenital herniation is usually brought about by straining of the abdominal muscles in activities such as coughing, lifting or pushing.
Direct inguinal hernias protrude through the front of the posterior wall of the inguinal canal. They do not pass through the deep inguinal ring but instead pass through any gap remaining between the inguinal ligament and the conjoint tendon before going through the superficial inguinal ring. The sac of the hernia consists of peritoneum unless it passes through the very fibres of the conjoint tendon in which case it is additionally covered by transversalis fascia and said fibres.