Hi. Let's talk about the heart. The heart, as you may recall, is an ever working pump that applies pressure to the blood that sends it through the arteries of the cardiovascular system. The heart is located in the thoracic cavity, that's the space that's bounded by the ribs, and the diaphragm, and the root of the neck. The thoracic cavity is broken up into three smaller sections. There are the paired pleura which contain the lungs, and then the central mediastinum. That mediastinum can be divided up a number of ways. One is between the superior and the inferior mediastinum, the heart occupying a space in the inferior mediastinum. Then the space occupied by the inferior mediastinum has three subdivisions. There's an anterior mediastinum, a middle mediastinum, and a posterior mediastinum. These three separate spaces are separated or defined by the outer limits of the pericardium which contains the heart, and thus the heart within the pericardium occupies and defines the middle mediastinum. The heart is oriented within the middle mediastinum with its apex pointing infero-laterally to the left with its great vessels, or at least the majority of its great vessels, entering and exiting the heart from the superior. The great vessels include the aorta, pulmonary trunk, and the superior and inferior vena cava. We can see that there are different borders and surfaces to the heart. So, for instance, there is a right and a left border, and an inferior border. There's a sternocostal surface that would be just deep to the sternum and ribs as well as portions of the lung. Then if we were to look at this inferiorly, there's also a diaphragmatic surface, and the pericardium of the heart is adhered to the diaphragm. Now, looking at this heart, it may not be evident because of the presence of so much pericardial fat, but there is a coronary sulcus that is a groove that's going to separate the atria above from the ventricles below. There's also an interventricular sulcus that is both on the anterior surface so that would be the anterior intraventricular sulcus, as well as the diaphragmatic surface which hosts the posterior interventricular sulcus. This intraventricular sulcus is a surface landmark that represents where the interventricular septum or the wall that divides right from left ventricle is. The heart is surrounded by the pericardium. The pericardium is a protective bag that creates a safe space called the pericardial cavity in which the heart may function. So, there are two parts to the pericardium. The fibrous pericardium is the outermost layer of the heart. It's rather rigid and inflexible, and then deep to the fibrous pericardium is the serous pericardium. The serous pericardium comes in two layers. There's the outer parietal layer of the serous pericardium and the inner visceral layer of the serous pericardium, and separating the two is the pericardial cavity. Now, this can sometimes be a difficult concept for students to grasp how this serous pericardium can exist. To help you along, I'd like to employ a prop from my lunch, so, this sandwich bag. So, this is going to represent the serous pericardium with its two layers both a visceral layer and a parietal layer. The parietal layer will be adhered to the fibrous pericardium and the visceral layer will be contained within the parietal layer, and come into contact with the heart represented here by my finger. So, this empty space of the sandwich bag would be the pericardial cavity in which the heart may expand and contract, expand and contract, expand and contract. Nothing safe a little bit of pericardial fluid should be within that pericardial cavity. In fact, when materials do enter that pericardial cavity, and sometimes these materials maybe blood, lymph, or just exudate, that can put pressure on the heart such that the heart can't fully expand, and that's a condition known as cardiac tamponade. That's basically a strangling of the heart. Beyond the pericardium, we have the actual layers of the heart proper, and what's interesting about this is one of these three layers is a part of the pericardium, and that's the epicardium here, which is actually the visceral serous pericardium that is so adhered to the heart surface that it becomes a part of it. Just deep to the epicardium is the myocardium. The myocardium is the muscular wall of the heart. Then contained within the myocardium is the endocardium which is going to line the various chambers of the heart. So, we talked a little bit about the heart, where it lives in the body, what it does, and the various layers that comprise it. Thank you.