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The Thorax
Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is the thorax?

The thorax is the portion of your body enclosed by the rib cage. Some people might call it your chest, but the official name for it is the thorax, and it contains some of the most important organs in the body. Of course, every organ has a function which is important, but those held beneath the rib cage are sometimes considered the most essential for life; you could not last a second without them.

These are, of course, your heart and lungs. Essentially every cell in the body has to function, and it does this through metabolism of fuel (obtained from food) using oxygen (obtained from the air). The lungs are there to get the air in, and the heart is there to pump the fuel to where it's needed.

Obviously due to the essential role played by these two organs, they are heavily protected by the rib cage. Rather than a single plate of bone (which would break easily and allow virtually no movement), the rib cage has been cleverly designed to allow significant movement in both anterior and lateral directions.

The animation below, on the right shows a representation of the upper part of the torso when skin and fascia have been removed. Beside it is an explanation of what can be found at each level.

At level 1 we have some muscles that help out with respiration, including pectoralis major. These are very important, as they are attached to the sternum and the top six costal cartilages, and when they contract they aid elevation of the rib cage.

Level 2 also shows muscles important in aiding respiration, especially sternocleidomastoid and pectoralis minor. It's not a coincidence, and it's actually rather helpful when you're trying to learn muscle names. These muscles work together in aiding forced respiration - most of the time it's just the diaphragm that works.

Level 3 shows the intercostal muscles between the ribs. There are three layers - external, internal and innermost, with the innermost layer being incomplete. These contract during inspiration and expiration, to stop billowing in and out when pressure changes. The lifting of the rib cage during breathing is to decrease the pressure inside the thoracic cage, and these muscles make sure that it is only air that is sucked in.

Next up in level 4 is the rib cage, made up of the sternum, and costal cartilages linking ribs either side. The rib cage provides useful land marks for navigating around the contents, because the ribs can be felt from the other side of the skin (and seen in some people!) whereas the organs inside cannot. It is using the ribs, for example, that we know where the best places to listen to the heart sounds are.

Down on level 5 we have the lungs and heart, which are covered in greater detail elsewhere.

Finally we have the diaphragm at level 6. This is made up of many muscle fibres forming two domes - unoriginally named 'left' and 'right', and it is the descent of these domes when the fibres contract that lead to increase in volume in the thoracic cage, and ultimately a decrease in pressure that causes air to flow in: inspiration.

The structures around the thorax are very focussed upon breathing, but that's not to say that this is their only purpose. Central to the body as a whole, this section inevitably has to maintain posture, which is why there are several muscles involve in that. There are also muscles involved in movement of limbs scattered about, and the function of the heart must never be forgotten.

Ultimately the thorax is a good place to start when it comes to anatomy, because it's not the most complicated part of the body functionally. It's appearance is also quite helpful, as it becomes quite apparent that there is a cage protecting the vital organs within.

What is an intercostal space?

An intercostal space is the gap between two ribs. It is an important feature, especially surgically, because it's the way that surgeons can gain access to the thoracic cavity (the name given to the whole contents of the rib cage) if they don't want to lift off the entire rib cage.

In between the two ribs can generally be found three layers of muscle - the intercostal muscles. These are relevantly named the external, internal and innermost intercostal muscles (probably because they didn't know about the innermost intercostals when they were naming them!).

The external intercostal muscles have fibres running forwards and downwards - in the direction your hands in your pockets would make. The internal intercostals then do the opposite - upwards and forwards, as if you'd flipped your hands upwards. The innermost muscles form an incomplete layer on the inside of the ribcage, and are generally 'transverse' or horiztonal in direction.

There is also a neurovascular bundle just beneath each rib. A neurovascular bundle is a bundle of three things - an artery, a vein and a nerve. Since many things need a blood supply and a nervous supply, it makes sense that these things should go around together. However, it's important that nothing interrupts their function. If access must be gained through an intercostal space for something like an injection, it should be made in the middle or just above the top edge of a rib, to make sure none of the items are hit. Along the top of some ribs, there are collaterals from other neurovascular bundles, which is why it is sometimes recommended that the centre of the intercostal space is used. However, considering how small these spaces are, there's isn't much room for mistakes!

What is the rib cage?

The rib cage is like an in built shield for the important organs held within - the heart and lungs. However, it serves a variety of functions, and has been carefully constructed to make sure that it is capable of sufficient movement to achieve these.

It is made up of the central sternum, with ribs attached on either side via costal cartilages. These ribs go around the outside of the rib cage's contents, and meet the vertebral column at the back.

Gaps between the ribs ensure that effective movement can be achieved - although I have to admit, the movement is only slight, despite what the animations below may suggest!

The sternum is held between ribs by the costal cartilages, and is made up of three important parts - the manubrium (or head), the body, and the xiphisternum or xiphoid process. The manubrium is trapezium shaped, with the short side in contact with the body of the sternum at something called the sternal angle. At the top of the manubrium are point where other bones come into contact with it - points of articulation. These include the articulations with the clavicles, which are bones that pass from the sternum to the shoulder blades (scapulae).

The sternal angle (or Angle of Louis) provides an excellent place to begin examination of the chest, because there are several things that happen at that level. The second rib (of 12) articulates with the sternum at the sternal angle, the aortic arch begins and ends at this level, the trachea bifricates (splits into two) at this point, it is where something called the mediastinum is split into superior and inferior, and it is at the level of something called T4 and T5.

Fundamentally the sternum is a large bone that provides a wide area of protection - significantly, most of the heart is found behind the sternum. However, it also provides a point of contact for the ribs to aid their movement.

The movement of ribs has been under some debate. Unless you are particularly thin, you can't see the ribs through the skin, so seeing how they move isn't easy. However, some research has been done, and there is a widely accepted generalisation on how the ribs move.

It is said that the superior (upper) ribs have a pump handle movement. The idea of the ribs moving is to increase the volume of the thoracic cavity (i.e. to increase the volume inside the rib cage), so you need to increase the dimensions somehow. The pump handle movement of the superior ribs increases what is called the anterior-posterior dimension of the thoracic cavity, i.e. it increases the distance between the sternum and the vertebral column (spine). The distance front-to-back is increased, leading to an overall increase in volume.

The inferior (lower) ribs have what is called a bucket handle movement. You can see from the animation that the rib movement looks like a bucket handle swinging on a bucket. This increases the horizontal dimension of the thoracic cavity - it makes the chest slightly wider, increasing the volume within, enabling breathing.

When you breathe in, you have to increase the volume within the rib cage. This is achieved by moving the bones around the thoracic cavity - the ribs - and by descending the domes of the diaphragm. As the domes of the diaphragm move down, it leaves more space above them. As the ribs move (with their pump- or bucket-handled movement) they also increase the dimensions of the thoracic cavity. Their movement is achieved by the contraction of muscles such as pectoralis major. The muscles of the abdomen can also contract and relax to aid respiration, because when the diaphragm descends it pushes abdominal contents downwards; if the abdominal muscles relax, this makes it easier for abdominal organs to descend.

Another important consideration is the difference between typical ribs and true ribs. Typical ribs relate to the shape and features of the rib itself; true ribs relate to how the ribs attach to the sternum.

Those ribs described as typical have the features shown in the crude image on the left, and include ribs 1-9, sometimes 10. Atypical ribs vary. They all have only one articulation with the vertebrae, and in addition to this, ribs 1 and 2 have tubercles for the scalene muscles and are flatter; the 12th rib has no tubercle at all, and with only one facet on the head, it looks like a dagger.

True ribs are those which attach directly to the sternum via costal cartilages. As can be seen from the animation at the top of this section, this includes ribs 1 to 7. Ribs 8-10 attach to the sternum via costal cartilages again, but this time the costal cartilages joins to the the costal cartilage above before reaching the sternum; these ribs are described as false ribs. Ribs 11 and 12 don't attach to the sternum at all, and are described as free floating ribs.

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