 In this video, I will identify the axial muscles of the thoracic and abdominal regions and list the origin insertion and action of major muscles in the thoracic and abdominal regions. We'll start with the intercostal muscles. The intercostal muscles are located in between adjacent ribs. There are internal intercostal muscles as well as external intercostal muscles. As the name implies, the internal intercostal muscles are deep to the external intercostal muscles. As you can see over here in the focus, zoom in on the intercostal muscles. There is an internal intercostal muscle and then an even deeper layer of the innermost intercostal muscles. The internal intercostal muscles, when they contract, they will cause the thoracic cavity to decrease in volume. They will depress the ribs as they contract, leading to decrease the volume of the thoracic cavity, assisting with breathing outward or the action of expiration. The external intercostal muscles, located superficially to the internal intercostal muscles, have the opposite action. The external intercostal muscles elevate the ribs, causing the volume of the thoracic cavity to increase, assisting with the action of inspiration or to draw air into the lungs. Here we see an inferior view of the diaphragm. The diaphragm is a dome-shaped muscle that forms the border between the thoracic cavity and the abdominal cavity. There are cruces of the diaphragm that are attached to the lumbar vertebrae and enable the action when the muscle contracts. The muscle will pull the central tendon inferiorly so that the entire dome shape of the diaphragm flattens and moves inferiorly. The action of the diaphragm is to expand the thoracic cavity and the diaphragm is a primary muscle of inspiration. That is when the diaphragm contracts and the thoracic cavity volume increases, air flows into the lungs. Next we'll move down to the muscles of the abdominal wall. The most superficial is the external oblique, or the external abdominal oblique. When this muscle contracts it causes compression of the abdominal cavity and can cause the vertebral column to flex or rotate. Then deep to the external oblique is the internal oblique. The internal oblique muscle similarly causes compression of the abdominal cavity as well as flexion of the vertebral column or rotation of the vertebral column. Then along the midline we'll see the rectus abdominis. The rectus abdominis is located deep to the tendon of the external oblique surrounded by layers of connective tissue known as the rectus sheath. The rectus abdominis forms muscle bellies that are six to eight prominent bulges of the muscle, commonly known as the six pack of abs if they're highly toned. The action of the rectus abdominis is to flex the vertebral column, such as when you're doing a sit up you would be flexing the vertebral column through the lumbar and thoracic regions. Deep to the rectus abdominis and deep to the oblique muscles is the transversus abdominis, the deepest layer of the abdominal muscles that is primarily having the action of compressing the abdominal cavity. Compressing the abdominal cavity can both function to protect the abdominal organs as well as assisting with the respiration, forcefully breathing air out as the abdominal cavity is compressed. This can lead to decreased volume of the thoracic cavity and help force air out of the lungs. We'll go through the external abdominal oblique in a little more detail. It has an origin from the ribs, specifically ribs five through twelve, and it inserts along the midline at a band of connective tissue that extends down the midline of the abdomen, which is known as the linea alba. Linea alba literally translates to the white line. This is the insertion along the anterior of the abdominal cavity, and the other place of insertion for the external abdominal oblique is along the iliac crest. From the iliac crest down to the linea alba here is a region known as the inguinal ligament. In the inguinal region there's the inguinal ligament, which forms a canal, the inguinal canal, which is a pathway through which the vas deferens or ductus deferens in men travels to allow sperm to move into the pelvic cavity to reach the prostate gland from the testes. Similarly, in women there's a ligament that travels through that inguinal canal known as the round ligament that helps to stabilize the position of the uterus. And so the origin from ribs five through twelve and the insertion along the iliac crest and the linea alba, this orientation enables the external abdominal oblique to have the actions of flexing the vertebral column if both the right and the left external obliques contract. Or if the external oblique on just one side contracts, this would cause rotation of the vertebral column. Here we have the internal abdominal oblique which has an origin all along the crest of the ilium and an insertion along the costal cartilages here. Insertion along the costal cartilages but also out here onto the linea alba. And so the action of the internal abdominal oblique is similar to the action of the external abdominal oblique that it can perform flexion or rotation of the vertebral column. However, the rotation would be the opposite direction so that when the internal abdominal oblique contracts you would rotate to the same side as the contracting oblique. And if the external abdominal oblique were to contract you would have contralateral rotation, rotation towards the opposite side. Here we see the rectus abdominis which has an origin at the pubic symphysis. So an inferior origin down here at the pubis, the pubic symphysis. And then the insertion is along the xyphoid process of the sternum and onto the costal cartilage. And that orientation with the origin and insertion enables the rectus abdominis to have the action of flexing the vertebral column. Such as when you do a sit up in order to help tone your abdominal muscles. When you're doing a sit up that is the action of flexing the vertebral column.