Muscles – Thorax Muscles and Thoracic Wall

by Craig Canby, PhD

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    00:00 Now, we’ll shift our attention to the muscles of the thorax and when we look at muscles of the thorax, we can divide them or assign them into one of two categories. We have extrinsic muscles and we have intrinsic muscles. Extrinsic muscles will have an attachment to the thoracic wall, but then another attachment that lies outside of the thoracic wall such as the clavicle or the humerus. Intrinsic muscles will attach to the components of the thoracic wall, sternum to ribs, ribs to ribs, for example. Let’s focus first, then, on the extrinsic muscles of the thorax. There are just four of them that we’ll work through, the pectoralis major, the pectoralis minor, the serratus anterior and lastly, the subclavius. Two of these four muscles are depicted on this particular illustration. We have the pectoralis major which is responsible for providing the major contour to the anterior thoracic wall and we can also see these interdigitating muscle slips on the more lateral aspects of the wall and these belong to the serratus anterior. We will now spend a brief moment by looking at each one of these extrinsic muscles. The first one to take a look at is the pectoralis major. We want to understand its attachments, its innervation. And then we’ll also want you to understand what it does when it shortens, what are the functions or actions of this particular muscle. The pectoralis major is shown right in through here. Very prominent muscle, again, on the anterior thoracic wall. It is described as having two heads, a clavicular head because of its attachment to the medial third of the clavicle. Also has a sternocostal head, which is the major head, because of its attachment to the sternum and to the costal cartilages of ribs 2 through 7. It also has an attachment to the aponeurosis of the external abdominal oblique. Its distal attachment or its insertion is on the medial crest of the intertubercular groove or sulcus of the humerus. And so, this points of wide origin or proximal attachments, all of a sudden, will converge to have a very specific point of insertion or distal attachment to that medial breast. This muscle is innervated by the medial and lateral pectoral nerves. And now, what does it do when it shortens? What are its actions? And the best way to understand the actions of any muscle is to predict what the actions are based on the attachments. So, if we think about the attachments running from thoracic wall, clavicle, sternum, ribs, aponeurosis of the external abdominal oblique, to the humerus, the lateral crest, some of the actions of the pectoralis major are as follows. It is a very powerful producer of adduction of the humerus.

    03:29 So, if we start with our arm a little bit away from the midline of the body and the pectoralis major starts to shorten here on the right side, it is going to bring your humerus back to the midline of your body. Another function of the humerus is to pull your arm forward and that is called flexion. Another function yet of the humerus is to medially rotate your humerus or have internal rotation of your humerus. And if your arms are above your head, the pectoralis major muscle on both sides will help pull the trunk of your body up toward your arms. So, if you’re in the rock climbing and you have your hands above your body, the pectoralis major, in association with latissimus dorsi, will help pull your trunk back toward your humeri. The pectoralis minor lies deep to the pectoralis major and we can see the pectoralis minor in this illustration. It’s going to be attached to ribs 3, 4 and 5 for its more proximal attachments or its origin. And then its distal attachment or insertion is going to be onto the coracoid process of the scapula. The pectoralis minor is innervated by the medial pectoral nerve and there are two main actions of the pectoralis minor. Imagine if it was to shorten, it would pull the coracoid process downwards thereby depressing the angle of your shoulder, bringing it down. Another function of the pectoralis minor would be to pull the scapula forward thereby bringing the point of the shoulder into a more anterior place. That is referred to as protrusion of the scapula.

    05:28 The serratus anterior is a major muscle on the lateral side of the thoracic wall and we can see several of the slips from the serratus anterior taking their origin from ribs 1 through 9. The serratus anterior will wrap around the margin of your thoracic wall and then attach to the anterior surface of the scapula along its medial border.

    06:04 The serratus anterior is innervated by the long thoracic nerve and the actions of the serratus anterior are as follows. One action is to keep the scapula up against the thoracic wall, which is very important if you’re trying to push on a heavy object to move it forward.

    06:26 It will keep the scapula from winging out posteriorly.

    06:31 Another action of the serratus anterior is to produce abduction of the humerus above the horizontal. So, as you move your humerus superiorly away from the midline of your body and you reach the horizontal, you have to rotate your scapula so that it rotates up superiorly to get your humerus to move above that horizontal. And if the serratus anterior is paralyzed, there is difficulty in being able to abduct your humerus above the horizontal position. The trapezius is helpful in this movement as well.

    07:14 The subclavius is the smallest extrinsic muscle of the thorax. It is shown in this illustration attached to the inferior surface of the clavicle and then it attaches to the first rib. The innervation of the subclavius is via the nerve to the subclavius and the subclavius has two functions. The first is its most important function and that is to stabilize the joint or articulation that exists between the medial end of your clavicle and the manubrium. The other function is it can help depress the clavicle thereby depressing the point of the shoulder to some degree. Now, that we’ve finished our journey through the extrinsic muscles of the thorax, let’s take a look at the intrinsic muscles. The intrinsic muscles are listed on this slide. We have the external intercostal muscles, the internal intercostal muscles, innermost intercostal muscles, the transverse thoracis musculature, subcostal muscles and then we have these last two muscles, the serratus posterior (superior and inferior) as well as the levator costarum. These will be dealt with the lecture on the back because they’re on the back side of the thoracic wall. So, we’ll begin with the external intercostal muscles. We can see their muscle fibers right in this particular intercostal space and the external intercostal muscles do occupy the external most aspects of the intercostal spaces.

    09:05 Their muscle fibers will run down and in, as visualized here in the illustration. Within each intercostal space, they will begin posteriorly where the tubercles of the ribs are located.

    09:20 They’ll run, then, anteriorly and laterally until they get into the view that we see them here and then your external intercostals will end right typically where the costal cartilages of the ribs join the ribs themselves. And then, from that point, there is a membrane that will run from the costochondral junctions that we see here to the lateral edge of your sternum. Their attachments would be to the inferior border of the rib above to the superior border of the rib below. They are innervated by the intercostal nerves that travel within the intercostal spaces. And their actions are primarily to help with inspiration. So, they’ll help to lift the ribcage up. And they’re also very, very important in stabilizing the intercostal spaces during respiratory movements.

    10:23 The internal intercostal muscles will also occupy the intercostal spaces. The internal intercostal muscles lie immediately deep to the external intercostals. They will start posteriorly at the costal angles of the ribs and as they progress within the intercostal spaces, moving laterally and then anteriorly, they will terminate right along the lateral margin of the sternum on each side. And then they will lie immediately deep to the membrane that’s associated with the external intercostal muscles.

    11:04 If we take a look up here, in the first intercostal space, we can see the termination of your external intercostal in this first space, with the muscle fibers running down and in.

    11:14 And then here, running from that end of the exterior intercostal musculature to the sternal border, you can see the fibers of the internal intercostals running down and outwards. And so, those are oriented in a different direction than the externals. The internal intercostals are also innervated by intercostal nerves and their actions are to assist with expiration.

    11:46 And they will also contribute to the stabilization of the intercostal spaces during respiratory movements. The innermost intercostals are going to be the deepest intercostal muscle layer. They are not well discernible in this illustration, but they occupy the intercostal spaces like the intercostal muscles do. Their fibers are directed in the same orientation as those of the internal intercostals. Consequently, they’re also innervated by the intercostal nerves and they will assist in the same actions as the internal intercostal muscles. The transversus thoracis muscle is shown in this view. It’s on the deep surface of the anterior thoracic wall or its internal surface.

    12:47 Its muscle fibers are going to originate from the sternum, xiphoid process and the costal cartilages of ribs 4 through 7. And then the distal or points of insertion will be to ribs 2, 3, 4, 5 and 6. There is variability in these attachments. Innervation is also provided by the intercostal nerves and these muscles tend to be more active during expiratory movements.

    13:19 The subcostal muscles are not seen in this particular illustration, but they are located on the internal aspect of the posterior thoracic wall. And they would be lying in this general area, a little more developed more inferiorly along the posterior internal thoracic wall.

    13:42 They have the same orientation as do the internal and innermost intercostal muscles, but what’s unique about them, when you visualize them, is instead of running within just one intercostal space, your subcostal muscles may attach here, cross this rib below and then attach to the next rib below that. So, they’re going to span at least two intercostal spaces; also innervated by intercostal nerves traveling within the relative intercostal spaces. Actions are to most likely to help depress the ribs which will assist with expiration.

    About the Lecture

    The lecture Muscles – Thorax Muscles and Thoracic Wall by Craig Canby, PhD is from the course Abdominal Wall.

    Included Quiz Questions

    1. Pectoralis minor muscle
    2. Pectoralis major muscle
    3. Subclavius muscle
    4. Serratus anterior muscle
    5. Levator costarum muscle
    1. Subcostal muscle
    2. External intercostal muscle
    3. Internal intercostal muscle
    4. Innermost intercostal muscle
    5. Transversus thoracis muscle
    1. Pectoralis major.
    2. Deltoid.
    3. Serratus anterior.
    4. Subclavius.
    5. Intercostal muscles.
    1. Medial pectoral nerve.
    2. Thoracic nerves.
    3. Lateral pectoral nerve.
    4. Axillary nerve.
    5. Intercostal nerves.
    1. Subclavius.
    2. Pectoralis Major.
    3. Pectoralis Minor.
    4. Serratus anterior.
    5. Serratus posterior.
    1. Intercostal nerves.
    2. Lateral pectoral nerve.
    3. Medial pectoral nerve.
    4. Axillary nerve.
    5. Thoracic nerve.
    1. Pectoralis major, Pectoralis minor, Serrtaus anterior and Subclavius.
    2. Pectoralis major and Pectoralis minor
    3. Serratus anterior and Subclavius.
    4. Pectoralis major, Pectoralis minor, External oblique and Subclavius.
    5. Pectoralis minor and Serratus anterior

    Author of lecture Muscles – Thorax Muscles and Thoracic Wall

     Craig Canby, PhD

    Craig Canby, PhD

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