Now we're onto the thorax stuff! These next few lectures were incredibly dense, so much so that one of them had extra slides tacked on the end for us to read in our own time >_>
Anatomy of the thoracic wall- bones, joints, muscles
The thoracic wall is made up of the rib cage, with its 12 ribs and sternum, as well as the gaps between ribs, the parietal pleura (the layer of the lungs that adheres onto the ribs- more on this later) and the muscles and bones of the upper limb.
Now I'm going to go into each of these elements in more detail, because as you've probably realised by now, anatomy seems to be a lot about dissecting the crap out of stuff and finding out as many details as you can before your head explodes. Anyway...
So back on the topic of the rib cage. There are 12 pairs of ribs which all attach to vertebrae, but not all attach to the sternum. The first seven ribs attach directly onto the sternum and are sometimes known as "true ribs" (well, there's some debate over whether the 7th rib counts as a true rib, but there you go). Ribs 8-10 attach to the cartilage of rib 7 and are sometimes known as "false ribs." The last two ribs, 11 and 12, do not attach to the sternum at all and are thus called "floating ribs."
The ribs also differ in their attachments to the vertebrae. Ribs have a head, a tubercle (which is a little bump near the head) and a body that ends in cartilage. The head articulates with facets on the body of the vertebrae. Most articulate with demifacets on two adjacent vertebrae as well as the disc in between them; however, ribs 1, 11 and 12 articulate with a single facet on T1, T11 and T12, respectively. The tubercle articulates with facets on the transverse processes (ribs 11 and 12 lack this joint). These joints are known as costotransverse joints (costo = rib), and are shaped differently as you go down the spine. The upper ribs tend to have more rounded costotransverse joints, whereas the lower ribs tend to have flatter surfaces at the joints. This also has implications in how they move: upper ribs tend to move up and forward in a "pump handle" motion, increasing the anterior-posterior diameter; lower ribs tend to move up and out in a "bucket handle" motion which increases the transverse diameter.
As mentioned before, ribs 1-7 attach to the sternum. The sternum itself can be broken down into various sections. The very top "notch" is known as the sternum notch (creative, I know). The top bony bit is called the manubrium, and it joins onto the body of the sternum at an angle known as the sternal angle (also very creative). This sternal angle is in line with T4. (Not sure what the significance of this is, but they keep drilling it in, so I assume it's important.) The very bottom part of the sternum is known as the xiphoid process, or xiphisternum.
Believe it or not, there are joints within the sternum. The joint between the manubrium and the body is a symphysis; however, partial ossification does occur. There is also a symphysis between the body and the xiphisternum, but this one might not ossify, which is why the xiphoid processes have fallen off some cadavers. There are also joints between the sternum and the ribs. Between the cartilage of the ribs and the sternum, there are small joints with cavities that are kinda like synovial joints. There are similar joints between the cartilages of the false ribs. Between ribs and cartilage, however, the matrix of the bone and cartilage just runs continuously into each other.
Now time to go into the muscles in a bit more detail! The true thoracic wall muscles are the external intercostals, internal intercostals and the inner layer of muscles, which consists of the transversus thoracis, innermost intercostals and subcostals. These all run between the ribs.
The fibres of the external intercostals run from top left to bottom right. When they contract, they lift the ribcage up and out. They are the second most important muscles in inspiration (the diaphragm is the most important muscle). The internal intercostals are the opposite: they run from bottom left to top right, and can pull the ribs down and inwards during forced expiration. Intercostal veins, arteries and nerves lie between the internal intercostals and the innermost layer. They are actually in that order (vein, artery, nerve) from top to bottom, and can be remembered by the acronym VAN.
Finally I'll just talk a bit about the diaphragm, because it is the divider between the thoracic and abdominal cavities. Also, it's important in breathing. The central tendon of the diaphragm is located in the centre. Around the edges, the diaphragm is attached to stuff. It is attached to the xiphisternum, ribs 7-12 and the lumbar vertebrae. At the point of attachment with the lumbar vertebrae, the diaphragm has two "legs," or crura. The left crus attaches to L1 and L2, whereas the right crus attaches to L1, L2 and L3. (Just in case you wanted more random details to remember!)
Lungs and pleural cavity
The lungs are covered by pleural membranes which surround a pleural cavity filled with a thin layer of fluid. This cavity originally developed from the coelomic cavity. The part of the pleural membrane that adheres to the lung is known as the visceral pleura, whereas the part of the pleural membrane that adheres to the chest wall is known as the parietal pleura. The pressure within the pleural cavity is lower than that of atmospheric pressure (around 756 mmHg as opposed to 760 mmHg) so sometimes it is said to have "negative pressure." This "negative pressure" also "sucks" the lungs towards the chest wall, thus expanding the lungs. If the pleural membranes become ruptured and the pressure becomes equal to that of atmospheric, the lungs will collapse. This is known as a pneumothorax.
Movements and their role in breathing
Breathing is all about changing the size of the thoracic cavity. Boyle's law states that pressure and volume are inversely proportional: as volume increases, pressure decreases and vice versa. Also, gases tend to move from an area of higher pressure to an area of lower pressure. Hence increasing the size of the thoracic cavity will decrease the pressure relative to atmospheric pressure, inducing air to "rush in." The opposite is true for decreasing the size of the thoracic cavity.
In inspiration (i.e. breathing in), the diaphragm and external intercostals are of utmost importance. The diaphragm moves downward in order to increase the size of the thoracic cavity. In doing so, the abdominal organs are compressed (this is why your belly moves out when you breathe in). When the abdominal organs cannot be compressed any further, the ribs are elevated. The balance between the use of the diaphragm (so-called "abdominal breathing") and external intercostals (thoracic breathing) can change depending on many factors, such as posture. In forced inspiration other muscles, such as the scalenes and sternocleidomastoid, may also be used to help lift the ribs.
No muscular effort is required during quiet expiration: instead, the muscles involved in inspiration simply relax. In forced expiration, however, other muscles such as the rectus abdominus (colloquially known as "abs") can help out.
Whew! That was long...
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