 I will draw your attention to these vessels here on either side of the sternum. This is the internal thoracic or the internal mammary artery and the accompanying vein. They are the branches from the subclavian artery on both the sides and they enter the thorax and they are coming down. This internal thoracic artery is accompanied by the internal thoracic vein. Normally the internal thoracic vein is on the medial side of the artery. But we noticed as we go lower down, it is not one but two. These are the venae cometantes of the internal thoracic artery. So from approximately the second or third rib onwards, it is not just a single vein but a venae cometantes. The next thing we notice is that the internal thoracic artery and the vein is disappearing inside this muscle. Even on this side also. And we have split over part of the muscle on this side and this side to show it is disappearing. This is the transversus thoracis muscle which I shall come to just a little later. This internal thoracic artery is the one which supplies the first to the sixth intercostal spaces by means of two arteries. Two anterior intercostal arteries. From the seventh space downwards, internal thoracic artery divides into a musculophrenic which supplies the diaphragm and a superior epigastric. The musculophrenic artery supplies the intercostal spaces from the seventh to the ninth anterior. Tenth and the eleventh intercostal space do not have any anterior intercostal arteries. And the twelfth one is the subcostal artery which also does not have any anterior. I will turn this specimen to show you the superior epigastric one side just now. We have lifted up the rectus abdominis on the left side and we can see an artery emerging through. This is the superior epigastric on the left side. This superior epigastric is one of the two terminal branches of the internal thoracic artery which goes to the abdominal wall. Now again I am turning it back to come back to where we were. The internal thoracic artery on the left side is used for coronary artery bypass grafting. Also referred to as left internal mammary artery bypass, lima bypass. That brings me to the next point. This internal thoracic artery is also referred to as internal mammary artery. Because especially in the second, third and fourth intercostal space it gives us to perforating arteries which perforate through and come to the anterior chest wall. And from there it supplies the breast in the female from the medial side as the medial mammary branches. And they are very important arteries and they have to be carefully dissected and ligated during mastectomy. And that is why this internal thoracic is also referred to as internal mammary artery. So that is about the internal thoracic artery. Now let me draw your attention to this muscle which I had mentioned just a little while back. Transversal thoracic muscle is present only on the anterior aspect of the chest wall. And this muscle becomes continuous with the transversal abdominis muscle. This actually belongs to layer number three of the chest wall. The outermost layer was the external intercostal muscle. The middle layer is the internal intercostal muscle. And the innermost layer actually is composed of three indistinctly divided muscles. One is called the subcostal muscle which is present only on the posterior aspect near the angles of the ribs. We cannot see it here. Then we can have something called the innermost intercostal which also is very vaguely defined. And only clearly well defined is this transversal thoracis. So this belongs to the third layer. And we know that the neurobascular plane is between layers number two and three. And that is the reason why the internal thoracic artery in the vein is disappearing deep to the transversal thoracis. Thank you very much for watching Dr. Sanjay Sanyal. Please like and subscribe if you have any questions or comments. Please put them in the comment section below.