okay let's move inferiorly in terms of our discussion of the skeletal muscle from the
thorax into the abdomen and skeletal muscle plays a huge role in terms of the abdomen but before we
really get into the specific muscles here let's talk about the muscle that's in between the thorax
and the abdomen and that is your diaphragm and we've all probably heard of our diaphragm and we
know very important in terms of breathing and I have here it's its main action this is the chief
muscle of inspiration or think of breathing in but really when you're talking about breathing this is
the main muscle in terms of breathing because the the main force in terms of expiration is
really gravity so the only muscle that's playing a role our active role most of the time is the
diaphragm this is one of those skeletal muscles which obviously has a voluntary component but
also an involuntary component so you're not always thinking about I need to contract my
diaphragm in order to breathe you do have a level of involuntary control here as well so let's look
at this actual dissection here you can see that the anterior portion of your thorax so the ribs
or the majority of your ribs have been removed and you're inside your thorax your lungs would
be right here your heart would be right here all right and then you see this double domed muscle
and this is your diaphragm so thorax is up here and then your abdomen is right here so if I were
to remove this region of peritoneum you'd be looking at your intestines alright and the
liver is hiding out right here right underneath the diaphragm so really serves to separate the
thorax as well as the abdomen now this is going to be innervated by your phrenic nerve if you've
ever heard C 3 4 & 5 keep the diaphragm alive so you have spinal nerve C 3 4 & 5 portions of
that coming together to innervate the diaphragm they're traveling down you can see it a little
bit right here to the diaphragm so as as we talked about very important terms of breathing when you
breathe or when you inspirate it's going to push the center portion of the diaphragm down okay so
it's moving not only the diaphragm down but it's kind of it's compressing on those at the
abdominal viscera as well all right so that's the main muscle that's really dividing the thorax and
the abdomen now let's talk about the muscle specific to the abdomen and the key here and
as you can see I have it in all caps we have no bones anteriorly so you have to have a very strong
complex of muscles as well as tendons in this area to protect the anterior portion of your abdomen
and the posterior portion of course you have your spinal column that will play a role in terms of
protection so let's talk about the muscles on this side we refer to these as the muscles of
the anterolateral abdominal wall a lot of times we think of them being more lateral but they do
extend anteriorly as well so the two main muscles here are your external and internal obliques okay
and they actually their muscle fibers are going to go in the same direction as the
external and internal intercostal muscles so external obliques think hands in the pocket in
terms of which way the muscle fibers work and the internal obliques are going to be perpendicular to
that and that crisscrossing is really important in terms of protection now deep to the internal
obliques you'll have your transversus abdominus and as its name would indicate the fibers are
going to be transverse and the main action of these muscles is to actually just support and
in times of contraction to compress the deep abdominal viscera or the viscera associated
with the gastrointestinal system additionally if you think about doing an oblique crunch in terms
of lateral flexion of the abdomen these muscles will play a role so let's look at this actual
image here you can see the superficial external oblique internal oblique transversus abdominus
and then you're actually getting into your parietal peritoneum which play a big role in turn
of allowing for the movement of your GI system so now all those muscles of course are going to have
tendons associated with them like all muscles do and they are going to coalesce anteriorly to
this very thick very important rectus sheath and that's what all this white portion right here is
a component of your rectus sheath so covering the superficial portion of your anterior abdomen so
all these tendons are just interlacing in this region and they're all going to meet up in the
middle and what we refer to as the linea alba and that's this particularly white portion right down
the middle you have your umbilicus right there in the middle so you're going to extend from the
xiphoid process remember that's the most inferior portion of your sternum all the way to the pubic
symphysis which is the anterior portion of your pelvic girdle so quite an expanse in terms of
that now if you were to reflect your rectus sheath which is what's happening here you're looking at
your straight muscle of the abdomen which is your rectus abdominus and it has a similar
length as what you had with the rectus sheath so here the rectus sheath is intact and we've
reflected it here now one thing I want you to note in terms of the rectus abdominis as you
have what's referred to as tendinous intersections and these are just portions of tendon that really
help to anchor the rectus abdominus to the rectus sheath and this is what will give that six-pack
or eight-pack appearance if those muscles have enlarged in those regions it's because these can
enlarge but the tendinous intersections cannot so it'll give that appearance of the six-pack
this is the main muscle in terms of flexion of the trunk so if you think of a typical crunch
or sit-up and you're flexing that way that is the rectus abdominis really playing a role but similar
to the anterolateral muscles it will play a role in terms of compression of the abdominal
viscera as well so that is the discussion in terms of the abdomen we are going to continue
we're going to move inferiorly towards the pelvic girdle and muscles associated with that