The thoracic wall surrounds the thoracic cavity, which is the anatomical region where viscera like the heart and lungs can be found.
The thoracic wall contains a variety of muscles, where many of the muscles that cover and attach to the thoracic wall have a primary action elsewhere, such as the arms, neck, and abdomen, and it is their secondary actions that affect the movements of the thoracic wall.
For example, the pectoralis major and pectoralis minor muscles overlay the thoracic wall but they are primarily involved with movements of the upper limb and their secondary action is to act on the thoracic wall as accessory muscles of respiration.
However, the thoracic wall also does have muscles that have their primary action in the thoracic wall, and these are called the true muscles. They are the serratus posterior, levatores costarum, subcostals, transversus thoracis, and the innermost, internal, and external intercostals.
Let’s start with the two serratus posterior muscles. There’s a serratus posterior superior muscle and serratus posterior inferior muscle on each side. The Serratus posterior superior muscles have their proximal attachment at the nuchal ligament and the spinous processes of C7 through T3 vertebrae and their distal attachment at the superior borders of the 2nd through 5th ribs. Its main action is the elevation of the superior 4 ribs during inspiration.
The serratus posterior inferior muscles have their proximal attachment at the spinous processes of T11 through L2 vertebrae and their distal attachment at the inferior borders of 9th through 12th ribs near their angles. Its main action is depressing the inferior ribs in order to prevent them from being pulled upwards by the diaphragm.
Both of these muscles have also been shown to have a role in proprioception, meaning that they might make you aware of the position of the body. For example, when you’re lying in bed with your eyes closed, these muscles, among others, could send signals to the brain regarding where you are and what’s your position in relation to the bed.
Next, the levatores costarum muscles are 12 triangle shaped muscles with a proximal attachment to the transverse processes of C7 through T11 vertebrae and distal attachments to the subjacent ribs between the tubercle and the angle of that rib. Their main action is to elevate the ribs.
Then there are the intercostal muscles, which are a group of muscles that occupy the space between the ribs. The external intercostal muscles form the superficial layer, while the internal intercostal muscles form the inner layer.
Now, the innermost intercostal muscles are fibers of the internal intercostal muscles that are the deepest fiber layer and lie internal to the intercostal nerve and vessels. All three intercostal muscles have the same origin and insertion. So their proximal attachment is at the inferior border of the ribs above and their distal attachment at the superior border of the rib below.
For example, the intercostals at the 3rd intercostal space all attach to the inferior border of the 3rd rib and the superior border of the 4th rib and so on. In addition, they are all innervated by the intercostal nerves traveling in their respective intercostal spaces. The main differences between the intercostals are their fiber orientation, and how far they span anteriorly to posteriorly.
So let’s look at the 11 pairs of external intercostal muscles that occupy the intercostal spaces from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly, where the most anterior muscle fibers are actually replaced by fascia called the external intercostal membranes. These muscles run infero-anteriorly from the rib above to the rib below.
To visualize this, the fibers run in the same way the fingers of your hand would when you placed your hands in the pockets of your jeans. The lower external intercostals are continuous anteriorly with the external oblique muscles in the abdominal wall. The external intercostal muscles elevate the ribs during forced inspiration.
Then, there are the 11 pairs of internal intercostal muscles, which run deep to and at right angles to the external intercostals. This means that their muscle fibers run infero-posteriorly from their origin compared to infero-anteriorly like the external intercostals.
To visualize this, the fibers run in the same way your fingers would if you were to cross your arms to hug yourself and place your hands on your hips. The internal intercostal attaches to the bodies of the ribs and their costal cartilages all the way to the sternum anteriorly and posteriorly as far as the angles of the ribs.
Between the ribs and medial to the angles, the internal intercostal muscles are replaced by fascia called the internal intercostal membranes. The inferior internal intercostal muscles are continuous with the internal oblique muscles in the anterior abdominal wall. The internal intercostal muscles mostly play a role during expiration.