The Inguinal Canal
The inguinal canal is a short passage that extends inferiorly and medially, through the inferior part of the abdominal wall. It is superior and parallel to the inguinal ligament.
It acts as a pathway by which structures can pass from the abdominal wall to the external genitalia.
The inguinal canal also has clinical importance. It is a potential weakness in the abdominal wall, and therefore a common site of herniation.
Development of the Inguinal Canal
To fully comprehend the anatomy of the inguinal canal, we must first look at its development, and the role the inguinal canal plays in the development of the genitalia. We shall explore the inguinal canal in the context of male development.
During development, the testes establish in the posterior abdominal wall, and descend into the scrotum. A fibrous cord of tissue called the gubernaculum attaches the inferior portion of the gonad to the future scrotum, and guides them during their descent.
The inguinal canal is the pathway by which the testes are able to leave the abdominal cavity and enter the scrotum. In the embryological stage, the canal is flanked by an out-pocketing of the peritoneum, and the abdominal musculature. This out-pocketing (processus vaginalis) normally degenerates, but a failure to do so can result in an indirect inguinal hernia.
In women, there is also a gubernaculum, this attaches the ovaries to the uterus and future labia majora. Because the ovaries are attached to the uterus by the gubernaculum, they are prevented from descending as far as the testes, instead moving into the pelvic cavity. The gubernaculum then becomes the ovarian ligament, and round ligament of uterus.
‘Mid-Inguinal Point’ and ‘Midpoint of the Inguinal Ligament’
These two terms are mentioned frequently in this article, and are often (mistakenly) used interchangeably.
The mid-inguinal point is halfway between the pubic symphysis and the anterior superior iliac spine. The femoral artery crosses into the lower limb at this anatomical landmark.
The midpoint of the inguinal ligament is exactly as the name suggests. The inguinal ligament runs from the pubic tubercle to the anterior superior iliac spine, so the midpoint is halfway between these structures. The opening to the inguinal canal is located just above this point.
The inguinal canal is made up of:
- Anterior and posterior walls
- Superficial and deep rings (openings)
- Roof and floor (or superior and inferior walls)
We shall go through each component in turn.
- The anterior wall is formed by the aponeurosis of the external oblique, and reinforced by the internal oblique muscle laterally.
- The posterior wall is formed by the transversalis fascia.
- The roof is formed by the transversalis fascia, internal oblique and transversus abdominis.
- The floor is formed by the inguinal ligament (a ‘rolled up’ portion of the external oblique aponeurosis) and thickened medially by the lacunar ligament.
During periods of increased intra-abdominal pressure, the abdominal viscera are pushed into the inguinal canal. To prevent herniation, the muscles of the anterior and posterior wall contract, and ‘clamp down’ on the canal.
The two openings to the inguinal canal are known as rings. The deep (internal) ring is found above the midpoint of the inguinal ligament. which is lateral to the epigastric vessels. The ring is created by the transversalis fascia, which invaginates to form a covering of the contents of the inguinal canal.
The superficial (external) ring marks the end of the inguinal canal, and lies just superior to the pubic tubercle. It is a triangle shaped opening, formed by the evagination of the external oblique, which forms another covering of the inguinal canal contents. This opening contains intercrural fibres, which run perpendicular to the aponeurosis of the external oblique and prevent the ring from widening.
In men, the spermatic cord passes through the inguinal canal, to supply and drain the testes. In women, the round ligament of uterus traverses through the canal.
The walls of the inguinal canal are usually collapsed around their contents, preventing other structures from potentially entering the canal and becoming stuck.
Clinical Relevance: Direct and Indirect Inguinal Hernias
A hernia is defined as the protrusion of an organ or fascia through the wall of a cavity that normally contains it. Hernias involving the inguinal canal can be divided into two main categories:
- Indirect – where the peritoneal sac enters the inguinal canal through the deep inguinal ring.
- Direct – where the peritoneal sac enters the inguinal canal though the posterior wall of the inguinal canal.
Both types of inguinal hernia can present as lumps in the scrotum or labia majora.
Indirect Inguinal Hernias
This classification of hernia is the more common. It has a congenital origin – due to the failure of the processus vaginalis to regress.
The peritoneal sac enters the inguinal canal via the deep inguinal ring. The degree to which the sac herniates depends on the amount of processus vaginalis still present.
As the sac moves through the inguinal canal, it acquires the same three coverings as the contents of the canal.
Direct Inguinal Hernias
In contrast to the indirect hernia, this is acquired in origin, due to weakening in the abdominal musculature.
The peritoneal sac originates from an area medial to the epigastric vessels and bulges into the inguinal canal via the posterior wall.