Inguinal Canal
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.
