Main muscles of the trunk

Hey everyone! It's Nicole from Kenhub, and in this tutorial, we'll be discussing the main muscles of the trunk. Since this is a basics tutorial, let's just take some time to define a couple of terms, and the first term that we're interested in defining is the question, what is a muscle? A muscle is a bundle of fibrous tissue that contracts to produce movement, and you can feel your muscle when you tense your biceps, for example, which is highlighted in green in this image, and you can see the muscle moving up and down.

The second question we want to ask ourselves for this tutorial is what is the trunk? And the trunk is the part of the body that is not the head or the appendages. So if you look at this image of our anatomical male in the center just here, you can see that the rectangle we've highlighted is the part of the body that doesn’t include the arms and the legs or the head and that's, of course, the region that we're going to be looking at today.

So, in today's tutorial, we're going to be talking through four sets of major groups of muscles found within the trunk, firstly, the pectoralis muscles followed by the intercostal muscles followed by the anterior abdominal wall muscles and finally, by the posterior trunk muscles. And, of course, we'll summarize them all at the end. Then let's, of course, begin with the pectoralis muscles which are made up of the pectoralis major and the pectoralis minor muscle.

Let's, of course, begin with the pectoralis major muscle, and as you can see, the pectoralis major muscle is a large fan-shaped muscle of the shoulder joint. And the pectoralis major muscle has three points of origin. It has a clavicular part which has its origin on the medial half of the clavicle, it has a sternocostal part which has its origin on the sternum and on the second to seventh costal cartilages, and the abdominal part of the pectoralis major has its origin on the anterior layer of the rectus sheath.

So although this muscle has three points of origin, its fibers come together onto one insertion and that point is at the crest of the greater tuberosity of the humerus. Because of the different courses of its muscle fibers, we can also see that the pectoralis major has a recess at its point of insertion that is open at the top in order to prevent the muscle from overstretching – and you can see this little recess outlined in dark gray over here. So, the muscle is innervated by the medial and lateral pectoral nerves, C5 to T1, which are direct branches of the brachial plexus – and you can see these nerves in yellow on our right – the superior one being the medial pectoral nerve and the lower one being the lateral pectoral nerve.

In terms of its function, contraction of the entire muscle results in the adduction and internal rotation of the humerus whereas contraction of the clavicular and sternal parts of the muscle results in anteversion which is the tilting forward of a structure or organ without bending – and you can see that action demonstrated by my arrow just here. Something else to note is that when the shoulder girdle is fixed, the pectoralis major also assists in respiration.

The pectoralis minor muscle is also a fan-shaped muscle of the shoulder girdle but as you can see, it's a lot smaller than the pectoralis major that we just looked at and it lies posterior to the pectoralis major and has its origin on the third to fifth ribs. Its fibers insert on the coracoid process of the scapula and it's innervated by the medial and lateral pectoral nerves, C6 to T1, which we saw earlier and these, just to remind you, are direct branches of the brachial plexus.

Talking about its function, the contraction of the pectoralis minor muscle pulls the scapula anteriorly and inferiorly effectively resulting in the adduction and the depression of the scapula and this in turn results in a dorsomedial movement of the inferior angle of the scapula which you can see down here and, in addition, this muscle also assists respiration by elevating the third to fifth ribs when the scapula is fixed.

Alright now that we're finished talking about the pectoralis muscles, let's move on to talk about the intercostal muscles. As you can see, the intercostal muscles are muscles that occupy the intercostal spaces between the ribs and there are three of these – the external intercostal muscles, the internal intercostal muscles and the innermost intercostal muscles. Let's, of course, begin with the external intercostal muscles.

As you can see, the external intercostal muscles are comprised of eleven pairs of muscles which course obliquely from the inferior border of a rib forward and downward to the superior border of the rib below. In other words, each muscle has its origin from the inferior surface of the rib above and inserts on the superior surface of the neighboring rib below. The external intercostal muscles are innervated by the intercostal nerves which are branches of the thoracic nerves T1 to T11 and contraction of these muscles raises the ribs during inspiration. In addition, they also support the intercostal spaces and stabilize and the chest wall.

The second layer of intercostal muscles that we're going to be looking at are the internal intercostal muscles, and these muscles have their origin on the superior border of a rib and course obliquely forward and upward to insert on the inferior margin of the neighboring rib above. They are innervated by the intercostal nerves and their main function is to lower the ribs during expiration. And like the external intercostal muscles, the internal intercostal muscles also support the intercostal spaces and stabilize the chest wall.

Lastly, let's have a look at the innermost intercostal muscles, and these muscles are situated deep to the two intercostal muscles that we've already seen so that's why we're having a look at these muscles from a sagittal view. And the innermost intercostal muscles arise from the rib above from the inner margin of the costal groove and their fibers course downwards perpendicular to the fibers of the external intercostals and go on to insert on the superior border of the neighboring rib below. Like the other two intercostals, they're also innervated by the intercostal nerves and the function of the innermost intercostal muscles is to lower the ribs during expiration.

So one thing that's important to note about this region is that the intercostal nerves and the blood vessels pass through a fibrous space between the innermost and the internal intercostal muscles at the lower border of the rib above and this space is known as the intercostal tunnel. We're just going to pull this out for you to explore it a little bit more clearly. And in this zoom, you can see the innermost intercostal muscles highlighted in green. They're the innermost muscles, and in blue, you can see the internal intercostal muscles highlighted posterior to them. And in between that, you can see the intercostal tunnel with the intercostal nerves and the blood vessels running through them and on the anterior surface of the intercostal wall, you can see the tunnel exposed with what would be the innermost intercostal muscles running on top of that.

Alright, now let's move on to talk about the anterior abdominal wall muscles which are made up of the rectus abdominis, the external abdominal oblique muscle, the internal abdominal oblique, and the transversus abdominis. Beginning with the rectus abdominis, you can see this very straight muscle and the way to remember this is that the term rectus is a Latin word meaning "straight" and this, of course, means that the rectus abdominis is the straight muscle of the abdomen. And this muscle has its origin from the fifth to seventh intercostal cartilages as well as on the xiphoid process of the sternum and its fibers run vertically inserting onto the pubis between the pubic tubercle and the pubic symphysis.

Innervation to the rectus abdominis is supplied by the lower intercostal nerves, T5 to T12. With regards to its function, contraction of this muscle flexes the lumbar spine and straightens the pelvis. It's also active during expiration and helps in the maintenance of abdominal tone which is also known as abdominal press. And abdominal press increases the intra-abdominal pressure, for example, during defecation or vomiting.

So, because this muscle passes through the rectus sheath, it has about three to four tendinous intersections which adhere to the anterior layer of the rectus sheath and this gives the muscle a multi-bellied appearance that can be seen in individuals whose rectus abdominis muscles are well-defined as a result of training or exercise. And this washboard appearance of this muscle is what is commonly referred to as "abs" or as a "six-pack".

So, one small structure I want to mention that's related to the rectus abdominis muscle is a connective tissue structure that runs along the midline of the anterior abdomen, and this is known as the linea alba. The linea alba is formed from the aponeurosis of the muscles of the anterior abdominal wall which you can see outlined in blue and it's this vertically running connective tissue band that separates the left and right sides of the rectus abdominis muscle.

Let's move on now to the external abdominal oblique muscle. And the external oblique muscle is one of three muscles that belong to the anterolateral muscles of the anterior abdominal wall and it's the most superficial of the three muscles – the other two being the internal abdominal oblique muscle and the transversus oblique muscle which we'll look at after this. And the external oblique muscle has its origins on the outer surface of the fifth to twelfth ribs and its fibers course ventromedially and form a large aponeurosis which I've highlighted for you in blue just here which inserts onto the linea alba and the anterior layer of the rectus sheath medially and on the outer lip of the iliac crest, the pubic bone and the inguinal ligament caudally. And as the name suggests, the fibers of the external abdominis muscle run obliquely.

It's innervated by the lower intercostal nerves, T7 to T12, and carries out several actions. For example, the unilateral contraction of this muscle results in the bending of the trunk to the same side while at the same time rotating the trunk to the opposite side. The bilateral contraction of this muscle flexes the trunk and straightens the pelvis and the external oblique muscle also helps to maintain abdominal press and is active during expiration.

So the second of the three anterolateral muscles of the anterior abdominal wall is the internal oblique muscle and it's a flat muscle which lies deep to the external oblique muscle. It has its origins on the deep layer of the thoracolumbar fascia, the intermediate line of the iliac crest and on the inguinal ligament, and its fibers run obliquely inserting cranially on the lower border of the costal cartilages of the tenth to twelfth ribs on the anterior and the posterior rectus sheath and on the linea alba ventrally. And the internal abdominal oblique is innervated by the lower intercostal nerves, T7 to T12 and L1 as well as by the iliohypogastric and ilioinguinal nerves which are branches of the lumbar plexus.

Regarding its function, the unilateral contraction of this muscle, bends the trunk to the same side of the muscle and rotates the trunk to the same side as well. The bilateral contraction of this muscle, however, flexes the trunk and straightens the pelvis. It's also active during expiration and it helps maintain abdominal press. Something that's important to note is that the fibers of the internal abdominal oblique muscle run perpendicular to the fibers to the external abdominal oblique – and you can see that a bit more clearly in this image just here.

The third of the anterolateral abdominal muscles is the transversus abdominis muscle, and this muscle lies deep to the internal abdominal oblique muscle and its fibers run transversely. It has its origins on the inner surfaces of the seventh to twelfth costal cartilages, the thoracolumbar fascia, the inner lip of the iliac crest, the anterior superior iliac spine and on the lateral part of the inguinal ligament. It inserts into the posterior layer of the rectus sheath.

The caudal fibers of the transversus abdominis are involved in the formation of the cremaster muscle, which you can see just here, and its aponeurotic fibers also contribute to the linea alba. The transversus abdominis muscle is innervated by the lower intercostal nerves, T5 to T12, the iliohypogastric nerve, the ilioinguinal nerve, and the genitofemoral nerve. And in terms of its function, the unilateral contraction of the transversus abdominis rotates the trunk to the same side while the bilateral contraction of the muscle aids in expiration and the maintenance of abdominal press.

Let's move on now to talk about the posterior trunk muscles and there is some conceptual structural anatomy ideas I want you to understand first. So, the posterior trunk muscles are divided into extrinsic back muscles – that is muscles that help with respiration and movement and they also lie superficially – and they're also divided into an intrinsic layer which lie close to the vertebral column and this acts to control posture and move the vertebral column itself.

So the extrinsic and intrinsic back muscles can themselves be divided into smaller parts – the extrinsic back muscles being divided into superficial and intermediate layers of muscle while the intrinsic back muscles can be divided into a superficial, intermediate, and deep layer. And though we're going to go through them all now, really, the thing if you remember is that the posterior trunk muscles have an extrinsic layer that help with respiration and movement and an intrinsic layer that help to control posture and the vertebral column, then you should be pretty happy.

Let's, of course, begin with the superficial layer of the extrinsic back muscles and this layer contains the trapezius, the latissimus dorsi, the levator scapulae, and the rhomboid muscles which consist of the rhomboid major and the rhomboid minor. Let's, of course, begin with the trapezius muscle.

So, the trapezius muscle, as you can see in this image, is a large triangular flat muscle of the shoulder joint and this muscle is the most superficial of all the back muscles discussed in this tutorial, and because it's so broad, it can be divided into three parts. The first part is the descending part of the muscle, and the descending part of the muscle is the most superior part of the muscle and has its origins on the superior nuchal line and the external occipital protuberance of the occipital bone as well as the spinous processes of all the cervical vertebrae via the nuchal ligament and it has its insertion on the lateral third of the clavicle.

The second part of this muscle is the transverse part of this muscle which is also the middle part and it has its origins on the spinous processes of the first to fourth thoracic vertebrae, T1 to T4, and inserts on the acromion of the scapula. The third part of this muscle which is known as the ascending part is the most inferior part of the trapezius muscle and this part of the muscle has its origins in the spinous processes of the fifth to twelfth thoracic vertebrae and inserts onto the spine of the scapula. The trapezius muscle is innervated by the eleventh cranial nerve – the accessory cranial nerve – and the cervical plexus.

With regards to its function, the trapezius muscle's role is to stabilize and secure the scapula to the thorax and in addition, the three parts of this muscle facilitate various actions. So we're going to go through them now. So, first off, talking about the descending part of the muscle, the contraction of this part draws the scapula obliquely upwards and rotates the glenoid cavity inferiorly. When the shoulder girdle is in a fixed position, contraction of this part of the muscle also tilts the head to the same side and rotates it to the opposite side. Contraction of the transverse part of the muscle draws the scapula medially while contraction of the ascending part of the muscle depresses and draws the scapula medially.

Let's move on now to the latissimus dorsi muscle, and as you can see, it's a fairly wide muscle. In fact, it's the widest muscle in the human body and it covers most of the back muscles with the exception of the trapezius muscle. The latissimus dorsi, together with the teres major, forms the posterior axillary fold in the armpit and it's also here that this muscle is thickest and therefore easy to palpate.

The latissimus dorsi muscle can be divided into four parts depending on the points of origin starting with the scapular part. The scapular part of the muscle originates from the inferior angle of the scapula as you can see in the blue highlight. The second part of the latissimus dorsi is the costal part. Now, we kind of have to turn the body a little bit to this side in order for you to conceptualize this. So, we're looking at the skeleton from right posterolateral view where I'm just drawing a trapezoidal structure that represents the scapula and over here is the structure that represents the humerus. So, therefore, you can see the part of the latissimus dorsi on the right side of the body in outline and down the bottom in our dark gray oval, you can see where the costal part of the latissimus dorsi attaches to the rib cage.

The third part of the latissimus dorsi muscle that I want to look at is the vertebral part, and this part of the muscle has its origins on the spinous processes of the seventh to twelfth thoracic vertebrae, on the thoracolumbar fascia of the spinous processes of the lumbar vertebrae as well as on the sacrum. And the last part I want to talk about is the iliac part, and the iliac part of the muscle originates from the posterior third of the iliac crest. The fibers of this muscle come together to insert on the crest of the lesser tuberosity of the humerus and it's innervated by the thoracodorsal nerve which is a branch of the brachial plexus.

The latissimus dorsi muscle is a muscle of the shoulder girdle and therefore supports various movements of the shoulder and the contraction of this muscle results in internal rotation, adduction and retroversion of the humerus. The latissimus dorsi muscle is also an accessory muscle of expiration and when activated on both sides, it compresses the rib cage and this is part of its support of expiration. It's also sometimes referred to as the coughing muscle as it's particularly strained during coughing attacks.

Moving on now, let's talk about the levator scapulae muscle. And as you can see, this is a long muscle of the shoulder girdle and it has its origins on the transverse processes of the first and second cervical vertebrae and on the posterior tubercles of the third and fourth cervical vertebrae. Its fibers insert on the superior border and the medial border of the scapula. Nerve supply to this muscle comes from the dorsal scapular nerve which is a branch of the brachial plexus and it also receives direct branches from the spinal nerves C3 and C4. The main function of this muscle as the names suggests is to elevate or raise the scapula and contraction of the levator scapulae draws the scapula medially upward and moves the inferior angle of the scapula away from the trunk and when the scapula is in a fixed position, contraction of the levator scapulae results in lateral flexion of the neck.

Let's move on now to the rhomboids starting with the rhomboid major muscle. And the rhomboid major muscle is a diamond-shaped muscle as you can see and it has its origins just below that of the rhomboid minor on the spinous processes of the first to fourth thoracic vertebrae. In terms of its insertion, its fibers insert on the medial border of the scapula below the spine of the scapula and it's also innervated by the dorsal scapular nerve. The rhomboid major muscle contracts the scapula medially upward pulling the inferior angle of the scapula towards the vertebral column and returning the elevated arm to the neutral position. It also stabilizes the scapula during arm movement and at rest.

Moving on to the rhomboid minor muscle, the rhomboid minor muscle as you can see is also a diamond-shaped muscle and it has its origins on the spinous processes of the sixth and seventh cervical vertebrae and inserts on the medial border of the scapula above the spine of the scapula just like the rhomboid major. And it's innervated by the dorsal scapular nerve which is a branch of the brachial plexus. Contraction of the rhomboid minor draws the scapula medially upwards and draws the inferior angle of the scapula towards the vertebral column. In other words, it results in adduction and elevation of the scapula and this, for example, returns the raised arm to the neutral position or lowers an elevated arm. It also stabilizes the scapula during both arm movement and rest.

Let's now talk about the intermediate layer of the extrinsic back muscles. And this layer is pretty simple. It consists of the serratus posterior muscle which can be divided into a superior part and an inferior part, and we're going to, of course, begin with the superior part.

So, the serratus posterior superior muscle is one of the two serratus posterior muscles of the superficial back muscles as we mentioned before. It has its origins on the spinous processes of the C7 to T3 vertebrae and it inserts on the second to fifth ribs. The serratus posterior superior lies deep to the rhomboid muscles and it is innervated by the intercostal nerves. Contraction of this muscle supports inspiration as it elevates the second to fifth ribs and the bilateral contraction of this muscle also extends the thoracic vertebral column while unilateral contraction results in the rotation of the thoracic vertebral column to the opposite side.

Now, let's have a look at the serratus posterior inferior muscle which is, of course, the more inferiorly located of the two serratus posterior muscles. And the serratus posterior inferior has its origins on the spinous processes of the lower thoracic vertebrae, the upper lumbar vertebrae and on the thoracolumbar fascia. Its fibers ascend superolaterally inserting on the ninth to twelfth ribs and it's innervated by the intercostal nerves that arise from the anterior branches of the spinal nerves. Contraction of the serratus posterior inferior muscle depresses the ribs and helps during expiration and unilateral contraction rotates the spine to the opposite side while bilateral contraction of this muscle also extends the spine.

Alright, and that finishes our little section on the extrinsic muscles. Now let's turn to the intrinsic back muscles and if you remember, this also has a superficial layer. And this superficial layer is pretty simple. It is made up of the splenius muscle – just the splenius. And the splenius can be divided into two parts – the splenius capitis muscle and splenius cervicis muscle – and we're going to talk through both of these parts starting with the splenius capitis.

So, the splenius capitis muscle as you can see has its origin on the spinous processes of the C3 to T3 vertebrae and this intrinsic back muscle of the lateral tract inserts on the superior nuchal line and the mastoid process and it receives innervation from the lateral branches of the dorsal rami of the spinal nerve C1 to C6. The unilateral contraction flexes and rotates the head to the same side while bilateral contraction of the entire muscle extends the cervical spine and the head.

Let's move on now to the splenius cervicis muscle, and as you can see, it's quite long and looks a little bit similar to the splenius capitis. And the muscle has its origins on the spinous processes of the third to sixth thoracic vertebrae and inserts on the transverse processes of the atlas and axis. It's also innervated by the lateral branches of the dorsal rami of spinal nerves C1 to C6 and in the same way as the splenius capitis, unilateral contraction of this muscle flexes and rotates the head to the same side while the bilateral contraction results in the extension of the cervical spine and the head.

Okay, let's move on to the intermediate layer of the intrinsic back muscles, and this is quite an important layer. And this is layer is made up of one very major muscle called the erector spinae muscle which you may have heard of before as it's a pretty large muscle that lies on either side of the vertebral column.

The erector spinae muscle is mostly involved in the extension of the vertebral column and it can be divided into three columns. So, the first column is the iliocostalis muscle and that makes up the lateral column of the erector spinae muscle, the longissimus muscle which makes up the intermediate column, and the spinalis muscle which makes up the medial column. To make things more complicated, each of these three muscles can be divided into three parts – for example, a capitis cervicis and a thoracis part – but we'll be going through each of these as we go through each of this muscle and don't forget if you are struggling with anything, you can just go back and listen to that section again.

So let's begin with the iliocostalis muscle. And this muscle as you can see can be divided into a cervicis, thoracis and a lumborum part and we're, of course, going to begin with the cervical part. So, the iliocostalis cervicis has its origins on the third to seventh ribs and inserts on the transverse processes of the fourth to sixth cervical vertebrae. The thoracic part of the muscle is known as the iliocostalis thoracis and it has its origins on the seventh to twelfth ribs and inserts on the first to sixth ribs. And the lumbar part of this muscle is known as the iliocostalis lumborum and it has its origins on the sacrum, the iliac crest and the thoracolumbar fascia and it inserts onto the sixth to twelfth ribs, the deep layer of the thoracolumbar fascia and on the transverse processes of the upper lumbar vertebrae.

So even though this muscle can be broken into three parts based on the origins and their insertions, the entire muscle is innervated by the lateral branches of the dorsal rami of the spinal nerve C8 to L1 while unilateral contraction results in the bending of the spine laterally to the same side. The bilateral contraction of the entire iliocostalis muscle extends the spine.

The next muscle on our list is the longissimus muscle. And the longissimus muscle is situated medial to the iliocostalis muscle and as we mentioned before, it's considered the intermediate column. The longissimus muscle, of course, has three parts – the longissimus capitis, the longissimus cervicis and the longissimus thoracis. Let's begin with our longissimus capitis.

So the most superior part of this muscle is known as the longissimus capitis and it has its origins on the transverse processes of the first to third thoracic vertebrae as well as on the transverse and articular processes of the fourth to seventh cervical vertebrae and it inserts superiorly on the mastoid process of the occipital bone. The middle part of this muscle – the longissimus cervicis – has its origins on the transverse processes of the first to fifth thoracic vertebrae and inserts superiorly on the transverse processes of the second to fifth cervical vertebrae. The third part of this muscle which is the thoracic part is known as longissimus thoracis and it has its origins on the sacrum, the spinous processes of the lumbar vertebrae, the transverse processes of the lower thoracic vertebrae and it even shares a common tendon of origin with the iliocostalis on the iliac crest and inserts on the second to twelfth ribs, the costal processes of the lumbar vertebrae which is small protuberances for the attachment of muscles on the transverse processes and the transverse processes of the thoracic vertebrae which are around about here.

Putting all the muscle back together, the longissimus muscle receives innervation from the lateral branches of the dorsal rami of spinal nerves C1 to L5 and in terms of its function, the unilateral contraction of the muscle bends the spine laterally to the same side while the bilateral contraction of the entire longissimus muscle extends the spine. And a couple more words on some of the longissimus muscle's parts. So the most superior part of our muscle – the longissimus capitis – flexes and rotates the head to the same side upon unilateral contraction and extends the head when it contracts bilaterally.

Okay, the last of the three columns of the erector spinae muscle is, of course, the spinalis muscle and this muscle can be divided into three parts as well – the spinalis capitis which actually we can't see on this image and we're not going to talk about in this part but I just want to mention it because it is a part of the spinalis so the spinalis capitis followed by the spinalis cervicis which you can see just over here, and, of course, the spinalis thoracis.

So this time as we mentioned before, we're not going to talk about the spinalis capitis. We're going to just talk about the spinalis cervicis. So let's begin with that.

So, as you can see, the spinalis cervicis is the middle part of the spinalis muscle and it has its origin on the spinous processes of C6 to C7 while it inserts on the spinous processes of C3 to C4. In terms of its innervation, the spinalis cervicis gets its innervation from the cervical nerve C1 to C8 and in terms of its function which is only bilateral, it extends the cervical spine.

Moving onto the spinalis thoracis, the spinalis thoracis originates on the spinous processes of T10 to L3 while it inserts on the spinous processes of T1 to T8. In terms of its innervation, it's innervated by the dorsal rami of the thoracic and lumbar spinal nerves and again in terms of its bilateral function, it extends the cervical spine.

Let's move on now to our last layer of the intrinsic back muscles. And the deep layer of the intrinsic back muscles are divided into two sections. The first section being the transversospinalis muscles – again, don't get too caught up in this terminology but just note that there's certain divisions within intrinsic back muscles – and the first one of these is the semispinalis followed by the multifidus which is followed by the rotatores and that is followed by a minor deep layer which contains the interspinales and the levatores costarum.

Let's, of course, begin with the semispinalis which as you might guess can be divided into three parts – the semispinalis capitis, the semispinalis cervicis and the semispinalis thoracis. So beginning with the semispinalis capitis, you can see that it originates in the transverse processes of C3 to T6 and then it inserts into the occipital bone between the superior and inferior nuchal lines. In terms of its innervation, it is innervated by the dorsal rami of the cervical spinal nerves and in terms of its unilateral function, it flexes the cervical neck laterally and rotates the cervical vertebrae to the opposite side. In terms of its bilateral function, it extends the head.

Moving on down to the semispinalis cervicis, the semispinalis cervicis originates on the spinous processes of C2 to C7 and inserts on the transverse processes of T1 to T6. Similarly, the innervation arises from the dorsal rami of the cervical and thoracic spinal nerves while its unilateral function flexes the head laterally and extends the vertebral column bilaterally.

And moving on down one more time down to the semispinalis thoracis, we can see that the semispinalis thoracis originates on the transverse processes of T6 to T12 and then inserts into the spines of C6 to T4. Its innervation arises from the dorsal rami of the cervical and thoracic spinal nerve while its unilateral function flexes cervical and thoracic vertebrae laterally and rotates the cervical and thoracic vertebrae contralaterally. In terms of its bilateral function, it extends the vertebral column.

Let's move on now to the multifidus muscle, and as you can see with the multifidus muscle, there isn't really an origin and insertion point. Instead, we're just going to talk about the course as they really travel between the transverse and spinous processes of the cervical vertebrae except for the transverse processes of the C2 to C4 vertebrae which you can see just here. They can be found all the way from the axis all the way down to the sacrum. In terms of its innervation, it's innervated by the dorsal rami of the spinal nerves and in terms of its unilateral function, it flexes the spine to the same side while it rotates the spine to the opposite side. And in terms of its bilateral function, it extends the spine.

Let's move on now to the rotatores thoracis brevis and longi muscles. And, basically, the rotatores can be divided into two parts – a thoracis brevis and a longi section. And in terms of the rotatores brevis' origin, it originates on the transverse processes of the thoracic vertebrae and the spinous processes of the next higher vertebrae while the origin of the longi muscles originate on the transverse processes of the thoracic vertebrae and the spinous processes of the vertebrae two levels higher. In terms of its innervation, it receives innervation from dorsal rami of the spinal nerves and its unilateral function rotates the thoracic spine to the opposite side while the bilateral contraction results in the extension of the thoracic spine.

Let's move on now to the interspinales muscles and again like the multifidus muscles, we don't really have an origin or an insertion point as they course between the spinous processes of the cervical vertebrae and these are called the interspinales cervicis and the spinous processes of the thoracic vertebrae which are known as the interspinales thoracis. We also have the section between the spinous processes of the lumbar vertebrae known as the interspinales lumborum. And all of these muscles are innervated by the dorsal rami of the spinal nerves. In terms of its function, it extends the cervical, thoracic and lumbar spine. And don't forget that the interspinales is part of the minor deep layer of the deep layer of the intrinsic back muscles.

The other muscle that's part of the minor deep layer of the deep layer of the intrinsic back muscles is the levatores costarum and this has two parts – a levatores costarum brevis and a levatores costarum longi. So in terms of its origin, both of these parts originate in the transverse processes of the C7 to T11 vertebrae and insert in different places. So the brevis inserts into the costal angle of the next rib below while the longi inserts into the costal angle of the next rib above. In terms of its innervation, it's supplied by the dorsal and ventral rami of the spinal nerves C8 to T11 and the unilateral function bends the thoracic spine to the same side while rotating it to the other side while the bilateral function extends the spine.

So now we finished that, let's come to our summary and talk about what we've discussed today

So, we started off talking about the pectoralis muscles of which there were two – the major muscle which is a large fan-shaped muscle and the pectoralis minor which is also fan-shaped but considerably smaller. Then we talked about the intercostal muscles which help with respiration and the first one we talked about was the external intercostal muscle which is made up of eleven pairs of muscles, the internal intercostal muscle which lies below the external intercostal muscle, and the innermost intercostal muscle which itself lies deep to the internal intercostal muscle and has the intercostal tunnel.

We then moved on to talk about the anterior abdominal wall muscles starting with the rectus abdominis which is the straight muscle of the abdomen followed by the linea alba which is a dense line of connective tissue that runs down the middle of the rectus abdominis. We talked about the external oblique which is the most superficial of the anterolateral muscles found on the anterior abdominal wall followed by the internal oblique muscle whose fibers run perpendicular to the external oblique muscle and then we talked about the transversus abdominis whose fibers run as you can see horizontally across the abdomen.

We then moved on to talk about the posterior trunk muscles starting with the extrinsic back muscles and its superficial layer which consists of the trapezius which is the most superficial muscle of the posterior trunk muscles, the latissimus dorsi muscle which is the widest muscle in the body, the levator scapulae muscle which is a long muscle of the shoulder girdle and the rhomboids of which there are two – the rhomboid major which is diamond-shaped and the rhomboid minor which is also diamond-shaped.

We then moved on to talk about the intermediate layer which is made up of the serratus posterior and there are two of these – the serratus posterior superior and the serratus posterior inferior – and then we went on to talk about the intrinsic back muscles of which there was three layers – the superficial layer being the first one and that contained the splenius muscle which has two parts, the splenius capitis and the splenius cervicis and they are involved with extending the cervical spine and the head.

We then moved on to talk about the intermediate layer which is made up of the erector spinae muscle which is a major muscle that is involved with posture and the movement of the vertebral column and the erector spinae is made up of the iliocostalis which was the lateral column of the erector spinae and that's made up of the iliocostalis cervicis, the iliocostalis thoracis and the iliocostalis lumborum muscle. This was followed by the longissimus muscle which is part of the intermediate column of the erector spinae muscle and that's made up of the longissimus capitis, the longissimus cervicis and the longissimus thoracis. And, finally, we talked about the spinalis muscle which is made up also of three parts – the spinalis capitis which we can't see on this image, the spinalis cervicis and the spinalis thoracis.

We then moved on to talk about the deep layer of the intrinsic back muscles and the deep layer is itself made up of two further layers, one of which is the transversospinalis muscle group and that contains the semispinalis which again can be divided into three parts – the semispinalis capitis, the semispinalis cervicis and the semispinalis thoracis. The multifidus is also a member of the transversospinalis muscle as well as the rotatores which has two parts – the brevis and the longi. And then we talked about the minor deep layer which consists of the interspinales and the levatores costarum, again, which can be divided into a brevis and a longi.

And that's all we have time for today. Thanks for watching!