Other Structrues
Inguinal Canal
The inguinal canal is an oblique passage through the lower part of the anterior abdominal wall. In males, it allows structures of the spermatic cord to pass to and from the testis and abdomen. (Normal spermatogenesis takes place only if the testis leaves the abdominal cavity to enter a cooler environment in the scrotum.) In females, the smaller canal allows the round ligament of the uterus to pass from the uterus to the labium majus. The canal is about 1.5 in. (4 cm) long in the adult and extends from the deep inguinal ring, a hole in the transversalis fascia, downward and medially to the superficial inguinal ring, a hole in the aponeurosis of the external oblique muscle (see Figs. 6.11 and 6.15). It lies parallel to and immediately above the inguinal ligament. In the newborn, the deep ring lies almost directly posterior to the superficial ring so that the canal is considerably shorter at this age. Later, as the result of growth, the deep ring moves laterally.
The deep inguinal ring, an oval opening in the transversalis fascia, lies about 0.5 in. (1.3 cm) above the inguinal ligament midway between the anterior superior iliac spine and the symphysis pubis (see Figs. 6.11 and 6.15). The inferior epigastric vessels, which pass upward from the external iliac vessels, lie medial to the deep ring. The margins of the ring give attachment to the internal spermatic fascia (or the internal covering of the round ligament of the uterus).
The superficial inguinal ring is a triangular-shaped defect in the aponeurosis of the external oblique muscle and lies immediately above and medial to the pubic tubercle (see Figs. 6.10, 6.12, and 6.15). The margins of the ring, sometimes called the crura, give attachment to the external spermatic fascia.
A common frustration for students is the inability to see these rings as actual openings. Remember that the internal spermatic fascia is attached to the margins of the deep inguinal ring and the external spermatic fascia is attached to the margins of the superficial inguinal ring. Thus, the edges of the rings cannot be observed externally. Compare this arrangement with seeing the openings for the fingers from inside a glove with not seeing the openings when looking at a glove from the outside.
Walls
The inguinal canal has four walls: anterior, posterior, superior (roof), and inferior (floor).
Anterior wall. Formed by the external oblique aponeurosis and reinforced laterally by the origin of the internal oblique from the inguinal ligament (see Figs. 6.10 and 6.15). This wall is strongest where it lies opposite the weakest part of the posterior wall, namely, the deep inguinal ring.
Posterior wall. Formed by the conjoint tendon medially and the transversalis fascia laterally (see Figs. 6.11 and 6.15). This wall is strongest where it lies opposite the weakest part of the anterior wall, namely, the superficial inguinal ring.
Superior wall (roof). Formed by the arching lowest fibers of the internal oblique and transversus abdominis muscles (see Fig. 6.14).
Inferior wall (floor). Formed by the upturned lower edge of the inguinal ligament and, at its medial end, the lacunar ligament (see Fig. 6.14).
Mechanics
The inguinal canal is a site of potential weakness in both sexes. It is interesting to consider how the design of this canal attempts to lessen this weakness.
- Except in the newborn infant, the canal is an oblique passage with the weakest areas (the superficial and deep rings) lying some distance apart.
- The fibers of the internal oblique muscle immediately in front of the deep ring reinforce the anterior wall of the canal.
- The strong conjoint tendon immediately behind the superficial ring reinforces the posterior wall of the canal.
- On coughing and straining, as in micturition, defecation, and parturition, the arching lowest fibers of the internal oblique and transversus abdominis muscles contract, flattening out the arched roof so that it is lowered toward the floor. The roof may actually compress the contents of the canal against the floor so that the canal is virtually closed (Fig. 6.24).
- When great straining efforts may be necessary, as in defecation and parturition, the person naturally tends to assume the squatting position; the hip joints are flexed, and the anterior surfaces of the thighs are brought up against the anterior abdominal wall. By this means, the lower part of the anterior abdominal wall is protected by the thighs (see Fig. 6.24).
Inguinal Canal Development
A peritoneal diverticulum called the processus vaginalis forms before the descent of the testis and the ovary from their site of origin high on the posterior abdominal wall (L1) (Fig. 6.25). The processus vaginalis passes through the layers of the lower part of the anterior abdominal wall and, as it does so, acquires a tubular covering from each layer. It traverses the transversalis fascia at the deep inguinal ring and acquires a tubular covering, the internal spermatic fascia (see Fig. 6.11). As it passes through the lower part of the internal oblique muscle, it takes with it some of its lowest fibers, which form the cremaster muscle. The muscle fibers are embedded in fascia, and thus the second tubular sheath is known as the cremasteric fascia (see Fig. 6.11). The processus vaginalis passes under the arching fibers of the transversus abdominis muscle and therefore does not acquire a covering from this abdominal layer. On reaching the aponeurosis of the external oblique, it evaginates this to form the superficial inguinal ring and acquires a third tubular fascial coat, the external spermatic fascia (see Figs. 6.11 and 6.12). The inguinal canal is formed in this manner in both sexes. (In the female, the term spermatic fascia should be replaced by the covering of the round ligament of the uterus.)
Meanwhile, a band of mesenchyme, extending from the lower pole of the developing gonad through the inguinal canal to the labioscrotal swelling, condenses to form the gubernaculum
In males, the testis descends through the pelvis and inguinal canal during the 7th and 8th months of fetal life. The normal stimulus for the descent of the testis is testosterone, which is secreted by the fetal testes. The testis follows the gubernaculum and descends behind the peritoneum on the posterior abdominal wall. The testis then passes behind the processus vaginalis and pulls down its duct, blood vessels, nerves, and lymph vessels. The testis takes up its final position in the developing scrotum by the end of the 8th month.
Because the testis and its accompanying vessels, ducts, and so on, follow the course previously taken by the processus vaginalis, they acquire the same three coverings as they pass down the inguinal canal. Thus, three concentric layers of fascia cover the spermatic cord: the external spermatic fascia, the cremasteric fascia, and the internal spermatic fascia.
In females, the ovary descends into the pelvis following the gubernaculum (see Fig. 6.25). The gubernaculum attaches to the side of the developing uterus, and the gonad descends no farther. That part of the gubernaculum extending from the uterus into the developing labium majus persists as the round ligament of the uterus. Thus, in females, the only structures that pass through the inguinal canal from the abdominal cavity are the round ligament of the uterus and a few lymph vessels. The lymph vessels convey a small amount of lymph from the body of the uterus to the superficial inguinal nodes
Spermatic Cord
The spermatic cord is a bundle of structures that pass through the inguinal canal to and from the testis (Fig. 6.26; also see Fig. 6.15). It begins at the deep inguinal ring lateral to the inferior epigastric artery and ends at the testis.
Structures
The structures of the spermatic cord are as follows: Vas deferens Testicular artery Testicular veins (pampiniform plexus) Testicular lymph vessels Autonomic nerves Remains of the processus vaginalis Genital branch of the genitofemoral nerve
Vas Deferens (Ductus Deferens)
The vas deferens is a cordlike structure located in the posterior aspect of the spermatic cord (see Figs. 6.12 and 6.26). It can be palpated between finger and thumb in the upper part of the scrotum. It is a thick-walled muscular duct that transports spermatozoa from the epididymis to the urethra.
Testicular Artery
A branch of the abdominal aorta (at the level of the second lumbar vertebra), the testicular artery is long and slender and descends on the posterior abdominal wall. It traverses the inguinal canal and supplies the testis and the epididymis (see Fig. 6.26).
Testicular Veins
An extensive venous plexus, the pampiniform plexus, leaves the posterior border of the testis (see Fig. 6.26). As the plexus ascends, it becomes reduced in size so that at about the level of the deep inguinal ring, a single testicular vein is formed. This runs up on the posterior abdominal wall and drains into the left renal vein on the left side and into the inferior vena cava on the right side.
Lymph Vessels
The testicular lymph vessels ascend through the inguinal canal and pass up over the posterior abdominal wall to reach the lumbar (para-aortic) lymph nodes on the side of the aorta at the level of the first lumbar vertebra (Fig. 6.27).
Autonomic Nerves
Sympathetic fibers run with the testicular artery from the renal or aortic sympathetic plexuses. Afferent sensory nerves accompany the efferent sympathetic fibers.
Processus Vaginalis
The remains of the processus vaginalis are present within the cord (see below).
Genital Branch of the Genitofemoral Nerve
This nerve supplies the cremaster muscle (see Fig. 6.26; also see Chapter 7, Nerves of the Posterior Abdominal Wall).
Spermatic Cord Coverings (Spermatic Fasciae)
The coverings of the spermatic cord are three concentric layers of fascia derived from the layers of the anterior abdominal wall. Each covering is acquired as the processus vaginalis descends into the scrotum through the layers of the abdominal wall (see Fig. 6.25). See the Embryology Notes above (Development of the Inguinal Canal) for an explanation of the formation of the spermatic fasciae. External spermatic fascia derived from the external oblique aponeurosis and attached to the margins of the superficial inguinal ring Cremasteric fascia derived from the internal oblique muscle Internal spermatic fascia derived from the transversalis fascia and attached to the margins of the deep inguinal ring