 Now, let's come to the intercostal muscles itself. I will stretch the ribs here, and this is what I have done. And we can see the muscles which are immediately visible in front of us. This muscle is the external intercostal muscle. If you take a look at the direction of fibers, we can see two things about the direction of fibers. Number one, the direction of fibers are roughly parallel to the slope of the ribs, roughly, not exactly. And secondly, the direction of fibers are as if the person has got his hands in his pocket. Now, this is a very important concept. When the direction of fibers is a muscle is almost parallel or closely matching the direction of the sloping of the ribs, then when this muscle contracts, it is responsible for elevation of the ribs during forced respiration. Now let's continue with the external intercostal muscle. And here we noticed that the external intercostal muscle, as it comes closer to the caustic cartilage, it becomes membranous, and we have reflected that membrane here and here. This is called external intercostal membrane. So therefore the medial portion of the external intercostal muscle is not a muscle anymore, it is just a membrane. When we reflect this membrane, we see the next muscle underneath. This is the internal intercostal muscle. And I will show you the internal intercostal muscle in one more space. We have reflected a little bit of the external intercostal here, and we can see the internal intercostal here, apart from here. And we can see a little bit of that here also under the external intercostal membrane. The internal intercostal muscle, the direction of fibers, as you can clearly see here, is at right angles to the direction of fibers of the external intercostal muscle. And I'm holding it up here for your comparison. This is the external intercostal, this is the internal intercostal. Because the direction of fibers are opposite, this has been likened to as if the person has got his hands crossed to the opposite chest. These internal intercostal muscles, they have been divided into two parts. One portion is between the bony part of the ribs. That portion is referred to as the inter-ocious part. And there is yet another part, which is shown here, which is between the cartilagious part of the ribs. And that is referred to as the inter-condral part. The two of them have different functions, even though it's part of the same muscle. The interocious part of the internal intercostal muscle when it contracts, it is responsible for depression of the ribs during forced respiration. However, when the inter-condral part of the same muscle when it contracts, it is responsible for elevation of the ribs. And how does this work? As I mentioned, when the direction of fibers are at right angles to the long axis of the slope of the ribs, their contraction will cause depression of the ribs. But in this place, in the inter-condral part, we can see that the direction of fibers are closely matching the long axis of the cartilages. So therefore, when they contract, they are responsible for elevation of the ribs. So that's the unique point I wanted to mention about the function of the internal intercostal. This is another dissection to show the external and the internal intercostal muscles in another cadaver from a different perspective. In between the ribs, we can see these muscles here. These are the intercostal muscles. We have three layers of muscles, out of which the outer and the second layers are the most prominent, and we have reflected them in certain places. We have cut the intercostal space here, and we have lifted up this. This is the external intercostal muscle. The direction of fibers of the external intercostal muscle, as we can see, they are as if the hands are in the pocket. They go from the rib above, obliquely down to the rib below. And in forced respiration, they are responsible for elevation of the ribs. They start from the medial most margin, and anteriorly, they stop, and they become known as the external intercostal membrane. In this intercostal space, we can see it much more clearly, and I have separated them with beams of artery force. And we can see the external intercostal muscle clearly here. Under the external intercostal muscle, we have the next, that is the internal intercostal muscle. We can see that the direction of fibers of the internal intercostal muscle is exactly at right angles to the external intercostal muscle. The internal intercostal muscle, the direction of fibers are as if the hands are crossed to the opposite chest. Here, posteriorly, the internal intercostal muscle is deficient, and it is replaced by a membrane, which is referred to as the internal intercostal membrane. While anteriorly, it continues right up to the sternum. The internal intercostal is responsible for depressing the introsius part of the rib during forced respiration. Deep to that will be a very poorly defined muscle, but we cannot see that here, that is called the subcostalist muscle, which is in relation to the angle of the ribs, but on the inner surface. That is part of the innermost intercostal.