Inguinal Canal Anatomy
The inguinal canal is about 4 cm long and passes obliquely through the flat muscles of the abdominal wall just above the medial half of the inguinal ligament (Fig. 4.19). In the male, the canal conveys the spermatic cord (comprising the ductus [vas] deferens and the vessels and nerves of the testis). In the female, the canal is narrower and contains the round ligament of the uterus.
The lateral end of the canal opens into the abdominal cavity at the midinguinal point, defined as midway between the pubic symphysis and the anterior superior iliac spine. In clinical practice, the midinguinal point serves as a guide to the deep inguinal ring and the femoral artery (Fig. 4.2). There may be individual variation in the relative positions of the deep inguinal ring, the femoral artery, and the bony landmarks, and some authors refer to the midinguinal point or the midpoint of the inguinal ligament as appropriate surface markings. The medial end of the canal opens into the subcutaneous tissues at the superficial inguinal ring, an aperture in the external oblique aponeurosis immediately superior to the pubic tubercle (Fig. 4.20). Continuous with the margins of the superficial ring is a thin sleeve surrounding the spermatic cord, the external spermatic fascia (Fig. 4.21).
The canal comprises a floor, a roof, and an anterior and a posterior wall. The gutter-shaped floor is formed by the inguinal ligament (Fig. 4.22), the in-turned lower edge of the external oblique aponeurosis. The ligament attaches laterally to the anterior superior iliac spine and medially to the pubic tubercle and the pectineal line of the pubis. The expanded medial end of the inguinal ligament, the lacunar ligament, lies in the floor of the medial end of the canal, and its concave lateral edge forms the medial boundary of the femoral ring (Fig. 4.23 and p. 262).
The roof is formed by the lowest fibres of internal oblique and transversus abdominis (Fig. 4.22). These fibres arch over the canal and pass medially and downwards to form the inguinal falx (conjoint tendon), which attaches to the crest and pectineal line of the pubis. The anterior wall of the canal is formed by the external oblique aponeurosis, supplemented laterally by fibres of internal oblique. These fibres arise from the lateral part of the inguinal ligament and cover the anterior aspect of the deep ring (Fig. 4.21). The posterior wall is formed by the transversalis fascia, reinforced medially by the conjoint tendon. Deep to the transversalis fascia are the inferior epigastric vessels, which lie just medial to the deep ring (Fig. 4.23). The inferior epigastric artery may be at risk during operations to repair inguinal hernias.
The inguinal canal is a site of potential weakness in the abdominal wall through which intra-abdominal structures may pass, producing an inguinal hernia (see below). However, several features of the canal’s anatomy minimize this weakness. The obliquity of the canal ensures that the superficial and deep inguinal rings do not overlie one another (Fig. 4.19). Furthermore, the strongest part of the anterior wall lies in front of the deep ring and the strongest part of the posterior wall lies behind the superficial ring. Hence, when pressure within the abdomen rises, the anterior and posterior walls of the canal are firmly opposed. In addition, when the abdominal muscles contract, the canal is compressed by the descent of fibres of internal oblique and transversus abdominis in its roof.
In the male, the canal contains the spermatic cord (Fig. 4.21). In the female, it transmits the round ligament of the uterus (Fig. 4.24), a fibromuscular cord running from the body of the uterus to the subcutaneous tissues of the labium majus. Lymphatics from part of the body of the uterus accompany the round ligament and terminate in the superficial inguinal nodes (Fig. 6.11).
In both sexes, the ilioinguinal nerve (Fig. 4.14) lies deep to the external oblique aponeurosis close to the inguinal ligament. The nerve runs medially in the anterior wall of the canal and emerges through the superficial ring (Figs 4.20 & 4.24).
The inguinal canal is the most common site for an abdominal hernia. Two types of inguinal hernia are recognized. The direct type pushes through the inguinal falx into the medial part of the canal. By contrast, the indirect (oblique) type traverses the deep ring and turns medially along the canal. Hernias of both types may emerge through the superficial ring and descend into the scrotum or labium majus. Direct and indirect hernias are distinguished by their relationships to the inferior epigastric vessels. A direct hernia lies on the medial side of these vessels, while the indirect type enters the inguinal canal lateral to them. The processus vaginalis normally closes but may remain patent in infancy, leaving a tubular channel connecting with the peritoneal cavity. Herniation along the patent processus, called an infantile inguinal hernia, is more common in the male child and may extend into the tunica vaginalis around the testis (p. 151).