The space occupied by the spermatic cord and its coverings as it passes obliquely through the anterolateral abdominal wall in the male is called the inguinal canal. A similar inguinal canal is present in the female; it transmits the round ligament of the uterus toward its termination in the labia majora. For the sake of convenience, the description given here will be based on the male. In general, it can be said that the canal and the structures described in relation to it are much the same in the female, although somewhat narrower.
The inguinal canal is an oblique tunnel, 3 to 5 cm long, through the muscular and deep fascial layers of the anterior abdominal wall that lie parallel to and just above the inguinal ligament. The canal extends between the deep inguinal ring, located in the transversalis fascia approximately halfway between the anterior superior spine of the ilium and the pubic symphysis, and the superficial inguinal ring, located in the aponeurosis of the external abdominal oblique muscle just superior and lateral to the pubic tubercle. The deep inguinal ring can be described as a funnel-shaped opening in the transversalis fascia, because it is the site at which this fascia is continued onto the spermatic cord to become the innermost covering of the cord, the internal spermatic fascia. The inferior epigastric vessels are just inferomedial to the deep inguinal ring, and the most lateral part of the inferior border of the transversus muscle is just superolateral to this ring. The superficial inguinal ring is formed by a splitting apart of the fibers of the external abdominal oblique aponeurosis, with those fibers that pass superomedial to the ring going to intermingle with similar ones of the opposite side and attach to the anteroinferior surface of the symphysis pubis. This portion of the external oblique aponeurosis is called the medial crus of the superficial ring. The fibers of the external oblique aponeurosis that pass inferolateral to the superficial inguinal ring are the lateral crus of the ring, which, in a sense, is the medial end of the inguinal ligament.
The lower border of the external abdominal oblique aponeurosis is folded under upon itself, with the edge of the fold (and variable added fibrous strands) forming the inguinal ligament. The fascia lata on the anterior aspect of the thigh is closely blended to the full length of the inguinal ligament. Its lateral half, folded deep to the aponeurosis, is firmly fused with the iliac fascia as the iliacus muscle passes into the thigh. As to the medial half of the inguinal ligament, the folded edge is actually formed by the fibers of the aponeurosis rolling under in such a way that the fibers forming the inferolateral margin of the superficial inguinal ring become the most inferior fibers at the attachment to the pubic bone and thus attach most interiorly on the pubic tubercle, whereas the fibers that were originally more inferior attach higher up on the tubercle and in sequence along the medial part of the pecten pubis for a variable distance, with the lowest fibers in the aponeurosis attaching farthest laterally on the pecten. The portion of the aponeurosis that runs posteriorly and superiorly from the folded edge to the pecten pubis can be called the pectineal part of the inguinal ligament, or the lacunar ligament. The fibers of the external oblique aponeurosis, described above, are attached to the pubic tubercle and the pecten pubis and continue, to a varying extent, beyond these points of attachment. Those which continue from the pecten pubis superiorly and medially superficial to the conjoined tendon reach the midline and blend somewhat with the external oblique aponeurosis of the opposite side. They are called the reflected inguinal ligament.
Lateral to the superficial inguinal ring, variable fibrous strands course roughly perpendicular to the fibers of the external oblique aponeurosis and are blended with the fibers of the superficial surface of this aponeurosis. These fibers, called the intercrural fibers, can be thought of as helping to prevent the split between the two crura of the external oblique aponeurosis (the superficial inguinal ring) from extending farther laterally.
Another structure that is frequently described as being formed by fibers from the external abdominal oblique aponeurosis, and which has considerable clinical significance as a firm structure to which sutures can be anchored in the surgical repair of hernia, is the pectineal ligament (Cooper ligament). This ligament runs along the sharp edge of the pecten pubis and has the effect of heightening this ridge. It is often described as being formed by fibers of the lateral part of the pectineal portion of the inguinal ligament (lacunar ligament) which, as they approach the pecten, turn sharply superolaterally to run along it. The pectineal ligament can also be interpreted as a building up of the periosteum along the pecten pubis, which is more in keeping with what appears to be the situation in many cadavers.
The origins and insertions of the internal abdominal oblique muscle and the transversus abdominis muscle have been described previously, but certain details in regard to the portions of these muscles related to the inguinal canal merit additional description. The exact amount of the turned-under edge of the external abdominal oblique aponeurosis (and the adjacent iliac fascia to which this edge of the aponeurosis is closely related) from which these two muscles take origin is quite variable, and it may be difficult to separate muscles in this area. The origin of the internal oblique muscle, more times than not, extends far enough medially so that some fasciculi of the muscle are anterior to the spermatic cord as its constituent structures come together at the deep inguinal ring, thus reinforcing this area to a certain extent. The origin of the transversus abdominis muscle (if it can be adequately separated) usually does not extend medially beyond the lateral border of the superficial inguinal ring, if it extends even that far. Because the conjoined tendon inserts on the pecten pubis and the crest of the pubis and thus along a line that angles from the pecten onto the crest, the part of this tendon inserting on the pecten is in one plane and that inserting on the crest is in a somewhat different plane. The part of the conjoined tendon inserting on the pecten pubis is partially fitted to the contour of the spermatic cord, and it approaches the pecten from posterior to the spermatic cord to meet the lacunar ligament (pectineal part of the inguinal ligament), which approaches the pecten from below the spermatic cord.
The inguinal canal and the structures within it can be further elucidated by thinking of this tubular tunnel as having a roof, a floor, and anterior and posterior walls, although, of course, because the tunnel is shaped to accommodate a cylindrical structure (the spermatic cord), no sharp boundary between any of the four walls can be established. It should be further remembered that the openings at the ends of the tunnel are not in planes perpendicular to the long axis of the tunnel but are in planes that form an acute angle with the long axis of the tunnel, so that the posterior wall of the canal extends farther medially than does the anterior wall and the anterior wall extends farther laterally than does the posterior wall. The two openings, of course, are the deep inguinal ring in the transversalis fascia at the internal end of the canal and the superficial inguinal ring in the aponeurosis of the external abdominal oblique muscle at the external end of the canal. The external abdominal oblique aponeurosis, strengthened by the intercrural fibers, is present in the entire length of the anterior wall of the canal. For approximately the lateral one quarter to one third of the canal, fibers of the internal oblique muscle, which arise from the inguinal ligament and related iliac fascia, form the anterior wall of the canal deep to the external oblique aponeurosis. Superficial to the external oblique aponeurosis lie the superficial fascia and the skin, which continue medially beyond the anterior wall of the canal above the superficial inguinal ring. The floor (inferior boundary) of the canal is formed in its medial two thirds to three quarters by the rolled-under portion of the external oblique aponeurosis together with the lacunar ligament (pectineal portion of the inguinal ligament), forming a shelf upon which the spermatic cord rests. The transversalis fascia is present for the entire length of the posterior wall of the canal. Toward the medial end of the canal, and thus reinforcing the part of this wall posterior to the superficial inguinal ring, is the reflected inguinal ligament to the extent present just anterior to the conjoined tendon of the transversus and internal oblique muscles. A quite variable expansion from the tendon of the rectus abdominis muscle (called by some authors the inguinal falx) fuses, to a variable extent, with the posterior aspect of the conjoined tendon. All of the reinforcing structures just described are, of course, anterior to the transversalis fascia. Posterior or deep to the transversalis fascia are the loose extraperitoneal fascia and peritoneum, which continue across posterior to the deep inguinal ring. At the lateral end of the canal, the inferior epigastric artery and vein are posterior to the canal in the extraperitoneal fascia as they are in relation to the medial (inferomedial) margin of the deep inguinal ring. Overlying these vessels, a thickening in the transversalis fascia is variably present. A slight depression in the parietal peritoneum, as seen from within, is apt to be present at the site of the deep inguinal ring. The roof of the inguinal canal can be said to be formed by the most inferior fasciculi of the internal oblique muscle as they gradually pass in a slightly arched fashion, from a position at their origin anterior to the canal to a position at their insertion (by way of the conjoined tendon) posterior to the canal. At the lateral end of the canal, the lower fasciculi of the transversus abdominis arch similarly over the canal. It should be pointed out that, although the description above of a roof and a floor of the canal can serve a useful purpose in talking about the canal, the anterior and posterior walls of the canal, in a sense, come together superior and inferior to the canal, and the roof and much of the floor are, perhaps, manufactured for descriptive purposes.
The weakest area in the anterolateral wall in relation to the inguinal canal is the superficial inguinal ring, which, to a varying extent, is reinforced by the reflected inguinal ligament, the conjoint tendon, and the expansion laterally and inferiorly from the tendon of the rectus abdominis muscle to the pecten pubis. This generally weakened area, the inguinal (Hesselbach) triangle, through which a direct inguinal hernia will pass, is a triangle bounded superolaterally by the inferior epigastric vessels, superomedially by the lateral margin of the rectus, and inferiorly by the inguinal ligament.
Developmentally, the inguinal canal is established as an outpouching in the inferior part of the anterior abdominal wall, the processus vaginalis, containing all of the layers from the parietal peritoneum outward, in preparation for the descent of the testes from their origin along the posterior abdominal wall through the inguinal canal and into the scrotum. Originally, the process was straight in an anterior-posterior direction, but further regional development causes it to become oblique. The processus vaginalis normally loses its connection with the parietal peritoneum of the abdominopelvic cavity, and all that remains of this is the double-walled serous sac, the tunica vaginalis, that partially surrounds the testis. The outpouchings of the other layers remain as coverings of the spermatic cord and testis which are picked up by the spermatic cord as it passes through the successive layers of the anterolateral abdominal wall. The covering acquired from the transversalis fascia is called the internal spermatic fascia. The spermatic cord is typically described as having passed inferior to the lower border of the transversus abdominis. The covering derived from the internal abdominal oblique muscle is the cremasteric muscle and fascia. The covering of the spermatic cord and testis procured from the external abdominal oblique muscle is the external spermatic and intercrural fasciae.