2.11. Revision Tips
This might help you to remember the direction of the IC muscles: EXTERNAL INTERCOSTAL muscles are the hands-in-POCKETS muscles…and trouser POCKETS are always EXTERNAL!
Make sure you understand Boyle’s law – this says that if you INCREASE the volume of the thoracic cavity, there is a DECREASE in pressure. Plenty of people have explained this well on online forums, etc, so check these out. This is of fundamental importance to understanding respiration (Section 3).
Anatomy is (sometimes) logical – if you find it difficult to remember the axes of thoracic cavity movement, think about it logically…the diaphragm contracts and moves inferiorly, so it will increase the up-down (vertical) diameter of the thoracic cavity. Likewise, the external IC muscles will pull the ribs anteriorly and laterally, leading to an increase in the anteroposterior and transverse diameters.
If you haven’t studied the limbs yet then don’t worry about knowing every single detail of the musculature. However, making an effort now will definitely be of benefit later.
Wait until later...
The nerve impulses that control inspiration and expiration are discussed in Sections 3 and 6.
Have a quick think: if you wanted to insert a chest drain into the pleural cavity to drain air and/or blood, where exactly would you push the drain through? The answer is just superior to the rib below. So, if you wanted to insert a drain into the fifth IC space, where exactly would you choose? Remember that the fifth intercostal vein, artery and nerve are running in the costal groove of the fifth rib. To avoid damaging these structures, you would aim lower down, closer to the upper border of the sixth rib (more of this in Section 3).
Remember the diaphragm
The diaphragm is a muscle (not bone!) and the only structure to PIERCE it is the inferior vena cava (IVC).