108 TOPOGRAPHIC AND APPLIED ANATOMY. 



intersection of the left cardiac line with the upper border of the third left rib. The auriculoven- 

 tricular orifices are situated beneath this line. The area of absolute cardiac dulness (colored 

 black in Fig. 45) is bounded upon the right by a line drawn from the lower border of the sternal 

 attachment of the fourth left costal cartilage along the left sternal margin to the lower border 

 of the seventh left costal cartilage; it is bounded upon the left by a line commencing at the origin 

 of the right boundary and extending to the cardiac apex. The area of absolute cardiac dulness 

 has no sharp inferior limit on account of the neighboring hepatic dulness. 



The auriculoventricular orifices are situated beneath the transverse cardiac line. The tri- 

 cuspid orifice is between the insertions of the left fifth and the right sixth rib behind the lower 

 end of the sternum ; the mitral orifice is in the upper third of the transverse cardiac line. The 

 pulmonary orifice is covered by the sternal end of the third left costal cartilage ; the aortic orifice 

 is posterior to the pulmonary orifice and is behind the sternum at the level of the third intercostal 

 space. 



For physical reasons the areas for auscultating these valves do not correspond accurately 

 with the projections of the valves upoR the anterior thoracic wall. 



Pericardium. The heart does not lie exposed within the mediastinum, but is situated 

 within the pericardium. The pericardium with its contents is placed between the two layers of 

 mediastinal pleura in such a way that it projects more toward the left than toward the right 

 pleural cavity. It will be remembered that the left pleural cavity (and the left lung) is smaller 

 than the right one* and that the mediastinal surface of the left lung is more concave than that 

 of the right lung. That portion of the mediastinal pleura which is reflected over the pericardium 

 is also known as the pleura pericardiaca. Together with the pericardium it forms a thin lamella 

 which may be lifted up from the heart, and which is not so thick, at least during childhood, but 

 that the individual portions of the heart may be seen and felt through it. In making an autopsy 

 the pericardial pleura and the pericardium are usually described as "the pericardium." 



The intimate relation of the pericardium and pleura in this situation is of great practical 

 importance, since inflammations of the pericardium may easily extend to the pleura and, upon 

 the other hand, a pleurisy may be followed by a pericarditis. The pleural cavities and the lungs 

 are separated from the pericardial cavity and the heart by only a thin septum. It will therefore 

 be readily understood that diseases of the lung (such as tuberculosis) may extend to the pleura 

 and to the pericardium. The septum may even be perforated, so that pus from the pleural 

 cavity (in pyothorax) or air from an opened pulmonary cavity may enter into the pericardium 

 (pyopericardium, pneumopericardium). 



The lower surface of the pericardium is firmly attached to the diaphragm (see page 99). 



In the pericardium, as in the pleura, we differentiate a parietal layer (usually called simply 

 "the pericardium") and a visceral layer. Above the base of the heart the parietal layer is re- 

 flected upon the great vessels, so that the superior vena cava, as well as a large portion of the 

 aorta and of the pulmonary artery, are situated within the pericardium and are covered by the 

 visceral layer. The visceral pericardium is as firmly adherent to the heart as is the visceral 

 pleura to the lung. The visceral pericardium in this situation is also known as the epipericardium. 



Within the pericardium, the ascending aorta and the pulmonary artery are firmly united 

 by connective tissue and surrounded by a common sheath of visceral pericardium. The index- 



