Author: Dr Peter de Souza
Last modified: 17 December 2020


Now we're going to take a look at some of the muscles of the thoracic wall. First of all, we'll take a look at some of the muscles which attach to the thoracic cage. And then we'll look at some of the smaller muscles which lie between the ribs and the intercostal spaces.


Outside the ribcage, we've got three muscles really. We've got this serratus anterior, which you can see laterally and anteriorly here. And then at the back, we've got this serratus posterior and we've got superior and inferior muscle.

We’ll start with the superior muscle.   As you can see here, the serratus posterior superior originates on the spinous processes of the vertebra. It originates from C7 to T3 on these spinous processes and it inserts on the upper borders of the 2nd to 5th ribs.


If I just remove the scapula there and just temporarily remove the serratus anterior, you can see the insertion point of this muscle on the ribs.   This muscle is innervated by the corresponding intercostal nerves. It attaches to the 2nd to 5th ribs and it's innervated by the 2nd to 5th intercostal nerves. Its function is to elevate the ribs.   You can imagine if it contracts, the angle of the muscles indicates that it's going to pull the ribcage upwards and elevate the ribs.   This assists in inspiration.


Just taking a look a bit lower down at the serratus posterior inferior, this muscle originates from the spinous processes of T11 to L2. It inserts on the lower ribs, so ribs 9-12. And again, it's innervated by corresponding intercostal nerves – so intercostal nerves which comes from T9 to T12. And its function is opposite to the superior serratus posterior.   It depresses the lower ribs. This helps enforce expiration.


Coming back round to the front, you can see the serratus anterior muscle here on either side. This muscle originates on the upper ribs. It originates from the first 8-9 ribs. It inserts on the medial border of the scapula, on the costal surface medially on the scapula.


I've just isolated this muscle on the skeleton here and you can see the medial border of the scapula here. The costal surface of the scapula is the anterior surface of the scapula which lies close to the rib cage.   The serratus anterior originates on the medial side of the scapula on its costal surface.


This muscle is innervated by the long thoracic nerve. Its function is to protract and to stabilize the scapula. It works to pull the scapula forward around the thorax and it also keeps the scapula pressed against the thorax.


Now we're going to take a look at some of the muscles that lie in the intercostal spaces.   You’ve got three layers of muscles that lie in this intercostal space.


The intercostal spaces are the spaces between the ribs. The three muscles that you have is the external intercostal, the internal intercostal and then you've got the innermost intercostal muscle.   That’s from superficial to deep.


It's useful to know the orientation of the muscle fibers of these three layers. The external intercostal muscle has fibers which are oriented inferiorly and medially.   The external oblique fibers run like this – towards the midline and inferiorly.


And what's particular interesting about this muscle is that anteriorly where it's about to meet the sternum, it doesn't continue as muscles, but it forms this aponeurosis called the external intercostal membrane. This aponeurosis connects it to the sternum.   It’s got a little seat of aponeurosis called the external intercostal membrane.


Underneath the external intercostal muscle, you've got the internal intercostal muscle. These fibers are oriented in the opposite direction. They're oriented in superior direction and medial direction.   They’re like this. This doesn't have the aponeurosis which connects it to the sternum.


And deep to the internal intercostal muscle, when you remove the internal intercostal muscle layer, you see the innermost intercostal muscle layer. These fibers are oriented in the same direction as the internal intercostal muscles – so superiorly and medially.


The way to remember the orientation of these fibers is with a simple mnemonic, which is the same as the mnemonic I used in the anterior abdominal wall muscles tutorial. 'Hands in pockets' and 'hands on tits'. If I just draw some hands which are in pockets – it's a pretty crappy drawing, but it's the best I can do unfortunately – this is the forearm and this is the hand. This is the angle your forearms would be if they were in your pockets.   Pockets has the little 'e' in it, so this is the direction the 'external' intercostal muscles would be oriented.


And if your hands are on your tits – so another pair of badly-drawn hands – your forearms would be angled upwards and inwards in the same direction as the internal intercostals.   Tits has the letter 'I' in it, so the 'internal' intercostal muscles are oriented in the superior medial direction the same as the angle of your forearms.


Those are the three layers of intercostal muscles. Underneath the innermost intercostal muscles, you've got the endothoracic fascia, which separates the intercostal muscles from the underlying pleural cavity containing the lungs.   Underneath the endothoracic fascia, you've got the parietal pleura, then the visceral pleura and then finally, you've got the lung.


In terms of the actions of these muscles, the external intercostal muscle is important in inspiration whereas the internal intercostal muscle is important in expiration. And the innermost intercostal acts with the internal intercostal muscle.   Remember they have the fibers oriented in the same direction so the muscle actions are similar.   They’re active during expiration.


And these three muscles are innervated by the associated intercostal nerves, so the intercostal nerves from T1 to T11.


I've removed from the muscles from this model. I want to point out something that's quite important. Between the internal intercostal muscle and the innermost intercostal muscles, you've got a neurovascular bundle.   You’ve got the intercostal vein, artery and nerve which lie in the subcostal groove underneath the rib.


If we just show you this skeleton model again, we're just going to rotate round to the back and if I just zoom in a little bit on this ribcage, I can show you the subcostal groove.


At the inferior margin of the rib, you've got these little grooves at this level and you've got these neurovascular bundles which run in this the costal groove.   In this groove, you've got nerves, arteries and veins.   From superior to inferior, you've got the vein at the top, the intercostal vein, then you've got the intercostal artery and the intercostal nerve. The easy way to remember that is that it spells 'van', V-A-N from superior to inferior – vein, artery, nerve.


Just returning to this model, you can see those structures here.   You’ve got the vein, artery and nerve, but these are shown a little bit out-of-place. They are mostly covered by the rib and lie in the costal groove. But sometimes, the lowest structure, the nerve can lie out of the costal groove and be unprotected.   These neurovascular structures lie between the innermost and the internal intercostal muscle.


Just to give you a better idea of how the structures are organized from superficial to deep, I'm just going to draw a little diagram on the side.   The first muscle you've got is the external intercostal. Then underneath that, you've got the internal intercostal. And then the next muscle down is the innermost intercostal. And between these two muscles, you've got the neurovascular structures.


At the top, you've got the vein. And then you've got the artery. And then you've got the nerve.


And then underneath the innermost intercostal, you've got the endothoracic fascia. And then just underneath this, you've got the pleura.   You’ve got the parietal pleura. And then you've got the visceral pleura. And then you've got the lung tissue under that.


Just coming back to this model again, we've got three more muscles that we need to talk about. These are quite minor muscles, but I'll just talk about them very quickly.


Rotating down to the back, I've dissected away the serratus posterior and the intercostal muscles from the right-hand side. You can see these 12 small muscles here. These muscles are called the levatores costarum muscles and they attach from the transverse processes onto the rib below.


For instance, the 4th thoracic vertebra attaches to the 5th rib.   The transverse process of T4 attaches to the rib 5. You've got this on other side.


I've just isolated the levatores costarum muscles on this skeleton. You can see the 12 small muscles. These 12 muscles function to assist in elevation of the thoracic ribcage.


The next muscle is this muscle called the subcostales muscle. As you can see, it attaches from one rib to the next below or the one two below.   This muscle lies on the internal surface of the ribs.


And the final muscle which I'm going to talk about is this one which also lies internally to the rib cage. This is called the transversus thoracis muscle. This muscle originates inferiorly on the body of the sternum and also on the xiphoid process and it originates on some parts of these costal cartilages. It inserts internally on ribs 2-6, on the costal cartilages of ribs 2-6. Its function is to pull the ribs down.


Those are the muscles which make up the thoracic wall.