Dissection - Thorax; pleura and superior mediastinum

Welcome to the University of Michigan Dentistry Podcast Series promoting oral health care worldwide. As we begin our dissection in the thoracic region, you should first familiarize yourself with the skeletal anatomy of the entire thorax: the ribs, the sternum and the thoracic vertebral column. In addition to this, as you begin your dissection remove all of the structures that came down from the axillary region having an attachment in this upper chest location. Pectoralis major, pectoralis minor, serratus anterior and even the external abdominal oblique should be completely cleaned from this area so that we can continue to work. In the spaces between all of the ribs, there are intercostal muscles. And these intercostal muscles have the same fiber direction as those of the external and internal abdominal oblique muscles. Here we see muscle fibers that are passing between the costal cartilages. They have an oblique angle upward toward the midline similar to that of the internal abdominal oblique. Therefore these are the internal intercostal muscles. However in order to see the external intercostal muscles, we must go around to the side of the body, and here we can see the downward direction following the line of the external abdominal obliques. These are the external intercostal muscles. And as we follow these external intercostal muscles around to the front, they end at approximately the end of the ribs where the costal cartilage begins. And in this view, we can see the reason that we can visualize the internal abdominal oblique because the external intercostal muscle continues as a thin membrane that covers the deeper lying internal intercostal muscles. While conversely, the internal intercostal muscles, although they begin at the side of the sternum, and extend all the way around the side of the chest deep then to the external intercostal muscles. They do not go up to the vertebral column as do these external intercostal muscles but rather they end about an inch or two short of that area in a membrane which is called the internal intercostal membrane. Follow the dissection guide quite closely as to the instructions in this area and then we'll proceed to a dissection of the thoracic wall. The techniques involved here require that you get into the lateral aspect of the thoracic cavity on both sides. In order to do this, we must come in to see the side area and just cutting through bone and intercostal muscles would tend to cut down directly into the thoracic cavity. So we want to do first is to make an incision right along rib margin, in one intercostal space, and then carefully shred out the intercostal muscle in this area. The main thing is we don't want to go too deep in this dissectionů ůbecause now having gone through the external, we are down to the internal intercostal and this should be continued to be removedů ůso that we can get right down into the lining of the thoracic cavity. Keep shredding out the muscle until one and an adjacent space then is cleaned out. Here now the external and internal intercostal muscles have been removed from two adjacent spaces and carefully now as you get down to the non-muscular material, you must start pushing it away slowly so as not to rip it in both of these intercostal spaces. Then getting the probe or even better the back of the scalpel handle underneath the rib, slowly work this back and forth to separate the material underneath the rib from the deep rib surface. After you have clarified one area like this, then using the bone cutters, section the rib. And in this first piece of work take out approximately an inch segment of the rib. The rib is still held in place by some tissue. And then they should be cut and removed. Clarify this deep layer by removing any of the muscle tags from around it. And here we see a good example of an intercostal nerve passing forward along the lower edge of the rib margin. Now the best thing to do to determine the amount of pressure that you have inside from the attachment of this inner lining is to work this loose with your finger both up and downward and when it is removed from the inner surfaces of the ribs then you will make a cut in this rib and continue all the way up to the first rib and down as far as you can go on the side of the chest in order that we might reflect the whole anterior chest plate. Now that we have opened up the ribs all the way from the first rib on the left downward to the respiratory diaphragm, we continued the dissection by slowly with your finger pushing the material underneath the ribs away until you eventually will get almost towards the midline. It is a very slow, tedious procedure because if you push too hard you'll rip directly through the outer covering of the lung area. Now in order to turn the rib cage downward, we have to look at the base of the neck first. In the base of the neck, running down along the anterior scalene muscle is the phrenic nerve. This will be passing into the thoracic region and it should be preserved. Likewise, deeper in, as the component of the carotid sheath, is the vagus nerve. Most importantly in order to reflect the anterior chest wall, is the internal thoracic artery again rising from the first part of the subclavian and passing beneath the sternoclavicular joint and going down into the chest region. It is fused to the undersurface of the chest area and therefore if we just tried to reflect the anterior chest wall now we would rip this artery and its accompanying vein. This dissection that we've done so far is at least a two, if not a three man dissection and now in order to approach this area, the technique that we need is for our partner to begin to lift up the anterior chest wall and then getting underneath with the scalpel holding down the internal thoracic structures you will slowly dissect it out as the anterior chest sternomanubrial area is turned downward towards the abdomen. Continue this dissection on both sides to preserve these structures. As we look down on the upper portion the thoracic cavity, we can see some of the structures that we have previously dissected. First of all, we have the internal jugular vein on the left side joining with the subclavian vein of that same side to form the left brachiocephalic vein. On the opposite side, coming down from above is the internal jugular vein and the subclavian vein of the right side forming then the right brachiocephalic vein. As we now open the chest cavity, we can look down into it and we will see the full extent of the internal thoracic blood vessels that will pass down to the upper portion of the *** abdominis muscle and to the intercostal muscles throughout its course and to the respiratory diaphragm. Again back up in the neck, we previously studied the phrenic nerve aligned on the anterior scalene muscle and now when we reflect the venous system we can see phrenic nerve passing downward to go into the area alongside the pericardial sac. Here phrenic nerve. Medial to it is the quite large vagus nerve. And it disappears deeply within at the arch of the aorta. Let's now look at the left side of the chest. And we have removed the ribs, reflected it and we are looking down onto pleura. The thoracic cavity of each side is surrounded by the pleura. It is the parietal layer of pleura because the other layer, the visceral layer, is closely applied to the surface of the lung. I have cut into this pleura and you can see it here completely surrounding the inner aspects of the ribs which we have removed. And as we have cut through it, we can look directly down on to lung surface. The glistening appearance of the lung surface is because of the visceral pleura that is closely adherent to the lung and literally has to be ripped off it in order for you to be able to see it. So visceral pleura surrounds the lung, parietal pleura surrounds the cavity. The various visceral pleura have names, subdivisions, and so does the parietal pleura. If the plural layering surrounds the interior of the ribs it's called the costal pleura. If, on the other hand, it is high up in the neck, it is called the cervical pleura and that which lines the diaphragm below the lowest portion of the thoracic cavity, it is called diaphragmatic pleura and then there is one other area called mediastinal pleura. The chest cavity can be divided into three portions. A left chest area that is filled with the lung, the same thing on the opposite side, the right chest cavity, and a central group of midline structures extending from the vertebral column posteriorly all the way up to the sternum in front. This partition is called the mediastinum. And the mediastinum can be divided into several sections as we will see in a moment. When the pleura, the parietal pleura, passes from this anterior costal region and swings onto the surface of the mediastinum such as we have along here this is called the mediastinal pleura. And the mediastinal pleura will then pass directly on to the lung where the major structures of the lung attach. In other words, the bronchial tube, the pulmonary artery, and the pulmonary vein. This is called the hilum of the lung and it is at the hilum that there is a reflection of the pleura from the mediastinal surface onto the mediastinal surface of the lung so that the lung is free within and deeply and here we can see a beautiful example of this reflection where now this, the mediastinal pleura, is swinging off onto the rib of the lung at this point. If we were to dissect this area now we would see that we have the three major structures of the lung: pulmonary artery, pulmonary vein and the bronchial tube. This is the reflection now that we have. The lung, however, does not fill in the entire thoracic cavity. Because the diaphragm below is doomed upward. The lung does not normally penetrate into that area, nor does it get into the recess between diaphragm and parietal pleura laterally. These are called recesses and the one that we would see over at the mediastinal area and I'll place my finger in it now. This is at the base of the sternum and this is the costal mediastinal recess whereas if I swing my hand around and go deeply then down then laterally along the lateral border of the space of the lung area this is the costal diaphragmatic recess. The cut edge of the ribs here at the breastplate was reflected it's down laterally that you will find this area. The significance of course is if there's an infection in the lung area we can get infection pooling down in these recesses and this is also where one would do thoracic tap. Now we were talking about the mediastinum before as being that central group of structures that's located on the midline behind the sternum and in front of the vertebral column and we can divide the mediastinum into several areas. When we put the chest plate back on, theů illustration now will use some of our landmark the second rib and the sternal angle below being the body of the sternum and above the manubrium which we have removed. The superior mediastinum is that area extending from the first rib above to the sternal angle in front including the manubrium and a horizontal line drawn from that point directly backward which takes it to approximately the lower portion of the fourth thoracic vertebrae. Once again above the level of the first rib. Below a horizontal line that extends from the sternal angle posteriorly to the lower portion of the fourth thoracic vertebra. Laterally, would be mediastinum pleura. The second rib, the sternal angle is a tremendously important landmark in the thoracic area. Because we have several major structures in this region, which we can look at now. When we move the lung out of the way we are looking down on the pericardium and extending upward out of the pericardium into the base of the neck is the aorta. And the aorta gives off three major branches. These three branches we have seen already, at least in part. The first one to the right is the brachiocephalic artery. It passes upward behind the sternoclavicular joint and then divides into the internalů the common carotid and the jugular, along with the jugular vein and here is its other branch the subclavian. Once again brachiocephalic swinging into the shoulder as the subclavian artery and this, the common carotid directly up into the neck. On the left side, however, these two branches arrive separately. We do not have a brachiocephalic artery on the left but rather we have the common carotid rising directly as a second branch of the aorta and then separately rising from the aorta is the left subclavian artery. So we have three structures with the name "brachiocephalic": the right brachiocephalic vein, the left brachiocephalic vein, and the brachiocephalic artery. Now I'm going to put the brachiocephalic venous system back again and we can see the left brachiocephalic crossing the midline to get over to the right side and now it is joining the right brachiocephalic and right at that junction is the termination of the internal thoracic vein. When we removeů push the lung off to the side and this vein, we now can see again this junction of the two brachiocephalics and now inward going deep towards the heart in a downward direction is the superior vena cava. Turning our attention over the left neck area once again, we have coming out of the carotid sheath, the vagus nerve. And the vagus nerve on the left continues downward, around the arch of the aorta, and then will continue down throughout the chest and into the abdomen. But along with this you can see a branch coming off of it, deep within, and for the purpose of this demonstration I'll move the phrenic nerve out of way. Here is the main course of vagus and here a nerve that is attempting to wrap around the undersurface of the aorta. This small nerve is the recurrent laryngeal. And it will pass upward behind the aorta and then get into the area between the trachea on the midline and the esophagus behind it and go up to supply the muscles of the larynx and the esophagus as well. This is a continuation of that nerve, the recurrent laryngeal nerve. In addition to this, at this area, of the sternal angle, where we see the arch of the aorta. If we were to cut the aorta, reflect it, you would see that this is a terminal part of our trachea which is passing down on the midline where it is bifurcating into the left and the right bronchi. Continue the dissection as indicated following the dissection guide closely this gives you an overview to begin the work today. 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