Welcome to this lecture on “The Abdominal
Wall”. This slide lists the objectives that
you should be able to answer at the conclusion
of this presentation.
First, list the muscles of the anterolateral
and posterior abdominal walls.
Describe the attachments, innervation and
action for each muscle.
Describe the rectus sheath and how the arcuate
line is formed.
Describe the arterial supply to the abdominal
Describe the anatomy of the inguinal canal.
Define the boundaries of the inguinal triangle.
And lastly, anatomically distinguish a direct
from an indirect inguinal hernia.
And then we will proceed to the summary slide
and identify the important take-home messages.
And then lastly, provide attribution for the
images that were used throughout this presentation.
We will stop momentarily here to point you
out on the body map our area of attention or
focus. Here is the anterior abdominal wall.
So, we will be looking at this region of the
body, outwards here laterally, and then we will
also want to take a look at the posterior
abdominal wall as well.
This slide depicts the muscles of the anterolateral
abdominal wall. These muscles consist of three
pair of flat muscles and then we have two
pairs of vertically oriented muscles. The
flat muscles are the external abdominal oblique,
the internal abdominal oblique, the transversus
abdominis, and then our two vertically oriented
muscles would be the rectus abdominis and
then, a very small pyramidalis. And again,
all these muscles listed here are paired,
so we have right and left muscles.
Let’s take a moment to look at each one
of these muscles individually and when we
do so, our interest here will be on describing
the attachments of the muscles, their innervation
and their actions.
So, our first stop will be the external abdominal
oblique and I want to highlight the fact that
we can see those fibers here. These fibers
are running in the same direction as the external
intercostal muscle fibers that are part of
the thoracic wall. That means they are running
down and in and they will attach to these
whitish sheath that we see here and this is
the aponeurosis of the external abdominal
oblique. And then that aponeurosis will course
toward the midline and help to form what is
known as the rectus sheath. And we see the
rectus sheath right in through this particular
The attachments of the external
abdominal oblique points of origin are going
to be here more superiorly. The external abdominal
oblique is going to arise from the outer surfaces
of ribs 5 through 12. And then the insertion
of the rectus will be to the aponeurosis,
ending in the midline here, the linea alba,
and also, to the iliac crest. The innervation
of the external abdominal oblique will be
from anterior rami of the inferior six thoracic
spinal nerve. So, we are looking at anterior
rami from T7 all the way down through T12.
The actions of the external abdominal oblique.
Let’s first think about both the right
and the left external abdominal obliques contracting
that will help flex the trunk of the body,
like so. We can also consider these working
unilaterally. So, if just the right external
abdominal oblique contracts, it can help the
body laterally flex to bend toward the same
side. It can also help to rotate the torso
and for the external abdominal oblique, if
it’s contracting, it will rotate the right
side. If it’s the right one contracting,
it will rotate your right anterior abdominal
wall toward the left side of your body like
In addition, the rectus abdominis as well
as the other two flat muscles of the anterolateral
abdominal wall will help to compress the abdominal
viscera, as well as protect the abdominal viscera.
The internal abdominal oblique lies immediately
deep to your external abdominal oblique and
if we take a look at our diagram here or illustration,
the internal abdominal oblique is shown right
along in through here. The cut edge of the
external abdominal oblique is right along
here. And now, we see the fibers of the internal
abdominal oblique, they are running down and
out. This is the same orientation that the
internal intercostal muscle fibers have within
the thoracic wall.
Here, more anteriorly, we can see that the
muscle fibers are ending in the aponeurosis
of the internal abdominal oblique and that
aponeurosis will extend toward the midline
of the anterior abdominal wall and within
the linea alba and will also contribute to
the formation of the rectus sheath.
The attachments of the internal abdominal
oblique more posteriorly, and out of view
here, would be to the thoracolumbar fascia.
It also will have a point of origin from the
iliac crest and it is going to attach for
its point of origin along the inguinal ligament
and the inguinal ligament is formed by the
aponeurosis of the external abdominal oblique.
Its insertion points will be to the inferior
ribs and we are looking at the inferior three
or four ribs. So, that would include 12, 11
and 10 and perhaps even rib 9. It would also
insert into the linea alba as well as to more
inferior portions associated with the pubic
The innervation of the internal abdominal
oblique will be by the anterior rami of the
inferior six thoracic spinal nerves T7 through
T12 and it also picks up additional innervation
from the anterior rami of L1, the first lumbar
spinal nerve. The actions of the internal
abdominal oblique, if both are contracting,
both will help to flex the trunk, just like
the external abdominal oblique. The internal
abdominal obliques help to protect and compress
the abdominal viscera, just like the external
abdominal oblique. If just one side is contracting,
say the right internal abdominal oblique,
it will help to bend toward the same side
or laterally flexed. And it will help rotate
the anterior abdominal wall in this
direction. So, say the right one is contracting,
the right side of the interior abdominal wall
rotates toward the same side thereby pulling
the left over toward the right.
The transverse abdominis muscle is the deepest
flat muscle of the anterolateral abdominal
wall. It is visualized over here on this side
of the image, this portion that we see here,
the muscle being reflected that belongs to
the internal abdominal oblique. We can see
the transverse or horizontal orientation of
the muscle fibers of the transversus. They
end within an aponeurosis as well. This aponeurosis
extends toward the midline and will also contribute
to the formation of the rectus sheath.
The attachments of the transversus abdominis
are going to be posteriorly, so out view here,
to the thoracolumbar fascia. In addition to
the ileac crest, points of attachment shared
with the other two flat muscles, will also
have an attachment to the inguinal ligament,
that inferior ending or termination of the external
abdominal oblique and will also have points
of origin from the costal cartilages of your
inferior ribs, usually the inferior most costal
cartilages of the six ribs here.
Its insertion will be into its aponeurosis
which extends into the linea alba, will also
attach to pubic bone structures such as the
Innervation of the transversus will be the
inferior six spinal nerves again, T7 through
T12, and their anterior rami and it also will
receive a contribution from L1. Actions of
the transversus abdominis because of the transverse
or horizontal orientation of these muscle
fibers, the primary function of this muscle
is to compress the abdominal viscera as well
as to protect the abdominal viscera.
Let´s talk about what happens if an oblique strain
gets injured. This happens typically when one of both
obliques are overstretched or partially torn.
This can be causes by flexion. lateral flexion,
or twisting. A typical symptom of this injury
is a sudden, sharp pain around the rib cage.
This pain is exacerbated when sneezing,
coughing and deep breathing.
This slide is introducing us to the concept
of the rectus sheath. The rectus sheath is
a sheath that will surround or invest the
rectus abdominis muscle. The rectus sheath
is going to be formed by the aponeurosis of
the three flat abdominal muscles that were
just described - the external abdominal oblique,
the internal abdominal oblique as well as
our transversus abdominis.
The orientation of these aponeurotic components
will vary depending on, if or above the umbilicus
or for about half way between the umbilicus
and the pubic bones. If we take a look at
some of the profiles that we have here. We
have two profiles that are at and above the
umbilicus and then this axial section that
we see in through here is at a point that’s
about half way between the umbilicus and the
If we look here at this upper portion of the
figure, we see our rectus abdominis right
in through here. We see that it is completely
invested anteriorly and posteriorly by aponeurotic
fibers. So, it’s completely surrounded by
a rectus sheath at this point. Same thing
is through of what this middle axial transverse
section. Contribution to the rectus sheath
is from here, the aponeurosis of the external
abdominal oblique. It completely will run
anterior to your rectus abdominis muscle,
meet in the mid line here, the area of the
linea alba, and then intermix with the aponeurotic
fibers from the opposite external abdominal oblique.
Your internal abdominal oblique is this middle
muscle layer. It has its aponeurosis beginning
right about here and the aponeurosis of the
internal abdominal oblique will split. Some
of the fibers will run anterior to the rectus
and then meet and intermix with the opposite
fibers of the internal abdominal oblique aponeurosis.
And then the other half of the aponeurosis
of the internal abdominal oblique will run
posterior to the rectus and then intermix
in the linea alba with those from the opposite
The transversus abdominis is your deepest
layer and we see it running right along here.
It too has an aponeurosis and at this level
of an axial section, all of its fibers are
running posterior to the rectus, will then
intermix with those on the opposite side in
the area of the linea alba.
Now, if we get inferior enough with our axial
section, we will see a difference in the orientation
of these aponeurotic fibers or sheaths and
if we take a look here, here is your external
abdominal oblique, internal abdominal oblique,
transverse abdominis and if we come more toward
the midline, we see all the aponeurotic fibers
converging here and they will all run anterior
to the rectus abdominis.
So, there is no longer a posterior component
or contribution or lamina to the rectus sheath;
it is devoid in this inferior location. When
you see a transition of the rectus sheath
lying anterior and posterior to where you
only have it lying anterior to the rectus
abdominis, you will see an arcuate line. That
arcuate line is depicted in this image and
if you remove your rectus from the view, the
right rectus has been cut here, cut here and
the two ends partially reflected.
Here is the rectus sheath that lies posterior
to the rectus abdominis. Here is the inferior
margin of where that posterior lamina ends
and then you see that is the arcuate line
that defines that termination. And then if
you go below that, this area here, is devoid
of the posterior portion of the sheath or
lamina and all those aponeurotic fibers at
this level are running anterior to your rectus
At this point below the arcuate line, the
membrane that you see here belongs to the
transversalis fascia. Now, the rectus abdominis
muscle specifically will have attachments,
will have its own innervations as well as
produce some various actions.
The attachments for origin are going to be
inferior and those points of origin will include
the pubic bone as well as the pubic symphysis.
The points of insertion will run more superiorly
and those will include the costal cartilages
of ribs 5 through 7 typically and the midline
structure being the xyphoid process. Innervation
to the rectus abdominis will be by anterior
rami from the inferior six spinal nerves,
so T7 through T12 and the actions of the rectus
are to help produce flexion of the trunk of
the body as well as to compress abdominal viscera.
This brings us to the last muscle or last
pair of muscles that belong to the anterolateral
abdominal wall. And this will be the pyramidalis
muscle. This is relatively unimportant functionally
and it may be absent in some individuals,
but it is this very small muscle mass that
we see here inferiorly located in the illustration.
It lies deep to the rectus sheath and would
occupy a region between the rectus abdominis
muscle itself and the anterior lamina of the
rectus sheath. You can see it is shaped like
a pyramid. This is just the left one. You
would have a right one on the opposite side
Attachments of this muscle are from the pubic
bone and pubic symphysis and it inserts into
the linea alba which is this region right
in through here. Functionally, it is said
to tense the linea alba, but again, this is
relatively unimportant. Its innervation is
from the anterior rami from your 12th thoracic