In this video I want to talk about the muscles of the torso.
Some of the muscles, superficial chest are actually for movement of the upper extremity.
Then after we remove the superficial chest we will look at the intercostal muscles, muscles
for breathing.
And then we will move into muscles of the abdomen.
So let's start with superficial chest.
Superficial chest really has three muscles.
We have pectoralis major muscle which is a large convergent muscle.
Has origin off of the anterior clavicle and the sternum, and the first six ribs.
And it will insert out into the humerus on the lateral lip of the intertubercular groove.
Pectoralis major functions as an adductor and a flexor of the arm.
If I remove the pectoralis major, we can see underneath pectoralis major, the pectoralis
minor muscle.
Pectoralis minor muscle is also for movement of the upper extremity.
It has origin from ribs two, three, four, and on this model five.
But usually we see a three slip pectoralis minor, but there is mild variation, so this
is acceptable.
But notice, pectoralis minor origin from the ribs but insertion not on the arm or the humerus,
but on the coracoid process of the scapula where it functions to pull the scapula forward.
So pectoralis major, pectoralis minor.
Muscles for movement of the scapula.
Now if I rotate this model somewhat laterally I can see coming around from the anterior
medial border of the scapula and inserting into the ribs in sort of a tooth like fashion,
the muscle fibers of the serratus anterior muscle.
Serrated means tooth like.
And you can see the tooth like edge of the serratus anterior muscle which comes from
the scapula.
Again that muscle is a muscle for movement of the scapula.
It is not actually a primary muscle for breathing, however it can be used for movement of the
ribs during respiration if the scapula is fixed.
If we look deep to those muscles I can see that there are muscles between the ribs known
as intercostal muscles.
Now this torso model doesn't do the best depictation of the intercostal muscles.
To learn your intercostal muscles you have to follow the fiber directions.
The fiber directions of the external intercostal muscles are down and in.
Whereas the fiber muscles or fiber directions of the internal intercostal muscles are up
and in.
We say inferomedial versus superomedial.
Now the external intercostal muscles do not make it all the way to the sternum.
There is a membranous connective tissue between them and the sternum.
So they did that a little bit right on his torso model.
Here we can see some fiber direction down and in.
This is an external intercostal muscle.
We should see some here, here, here.
And we do see some here but they have really worked them quite a bit laterally.
Here is an external intercostal muscle, here's some more, here’s some more.
They should make it more medially than what is shown on this model.
The internal intercostal muscles then are deep to those and their fiber direction is
described as superomedial, or going up into the sternum and the next set of ribs.
External intercostal muscles are muscles of inspiration, muscles which allow you to breathe
in.
The internal intercostal muscles are muscles of expiration, for breathing out.
Moving down into the abdominal cavity.
We can see on the right side of this torso there's a muscle that is covered in the abdominal
cavity by what is known as the rectus sheath.
A sheet of connective tissue overlying the rectus abdominis muscle.
On the left side of the torso they have removed the rectus sheath so you can see the muscle
portions of the rectus abdominis muscle.
Rectus abdominis muscle has origin down at the body of the pubic bone and the pubic crest.
It actually inserts into the sternum at the xiphoid process, the sternum and in the lower
ribs 5, 6, 7, and maybe even the coastal cartilages of rib eight.
Rectus abdominis does something that most of our other muscles do not do.
It has what are called tendinous intersections along the way.
This is not actually an insertion point, we have a muscle belly then a tendinous intersection.
Another muscle belly and a tendinous intersection, and again, and again.
This is what, if you are in shape, if you haven't spackled it over with subcutaneous
fat, gives your abdomen that six pack abs that they talk about in the gym.
However if you count, there certainly would not be six pack but the rippling.
So I would prefer to say the washboard abs.
But notice if I were to skin a cadaver I would not see the rectus abdominis initially because
it is underneath this connective tissue sheath called the rectus sheath.
Now lateral to the edge of the rectus sheath, we will see where the rectus sheath ends is
an area known as the linea semilunaris.
Now we can see the abdominal musculature.
We have three layers of abdominal musculature laterally.
We have the external abdominal oblique.
Like the external intercostal muscles its fiber direction is described as running down
and in, or inferomedially.
So we can see the fiber direction here of the external abdominal obliques.
The internal abdominal oblique, like the internal intercostals, is described as running superomedial,
or up and in.
So if I rotate to this left side of this torso model we can see here the fiber direction
is up and in.
So that tells us that on this model not only did they remove the rectus sheath on this
side, they have also removed the external abdominal oblique muscle.
So we can tell because of the fiber direction is up and in, this must be the internal abdominal
oblique muscle.
There is one more muscle of the abdominal wall that is called the transversus abdominis.
It's fiber direction runs directly across or transversely across the abdomen.
So if I remove the breast plate of this torso model we should be able to see the fiber direction
of the transversus abdominis.
And here we can see, posteriorly, the rectus sheath, and we can see a fiber direction going
crosswise.
That is transverses abdominis.
Now if we look into the posterior portion of this abdominal cavity, and if you could
see carefully on this side we have three layers.
We have the external abdominal oblique, the internal abdominal oblique, and the transversus
abdominis.
On this side they only show us two layers.
Remember we've removed the external abdominal oblique, so here we see the internal abdominal
oblique, I know because following the fiber direction superomedially, and the transversus
abdominis.
Now the abdominal wall also has muscles posteriorly.
Called the posterior abdominal wall.
And in this model we can see, on the posterior abdominal wall the quadratus lumborum, a four
sided shaped muscle in the posterior abdominal wall.
We can see right next to the quadratus lumborum the psoas major muscle.
The psoas major muscle runs from vertebrae's T 12 all the way down to L5 and their spinous
processes and then crosses anteriorly to the pelvis to actually be a muscle used for moving
the lower extremity.
So this muscle inside our posterior abdominal wall actually flexes at the hip.
There is a psoas minor muscle.
Psoas minor muscle is absent in roughly 40% of people.
It is a small muscle, it only insert into the pelvis, so if you don't have a psoas minor
muscle it is not a great consequence.
And then the last muscle which is posterior abdominal wall is the iliacus muscle.
The iliacus muscle comes from the iliac fossa and joins the psoas major to become what is
known as the iliopsoas muscle.
So iliacus also functions for flexion at the hip.