300 THE HEAET. 



between the two layers of pleura which .bound it, and in this are contained 

 the vestiges of the thyinus gland ; behind the second piece of the sternum 

 the pleurae of opposite sides come into contact, and the anterior mediastinum 

 is reduced to a thin septum ; while lower down it is inclined to the left, and 

 widened out into an angular space of some breadth, by the margin of the 

 left pleura receding from the sternum : opposite the lower part of this space 

 the apex of the heart is situated, and in front of it is placed the triangularis 

 sterni muscle. 



THE PERICARDIUM. 



This membranous bag, in which the heart is contained, is of a somewhat 

 conical shape, its base being attached below to the upper surface of the 

 diaphragm, whilst the apex, or narrower part, surrounds the great vessels 

 which spring from the cavities of the heart, as far as their first subdivisions. 

 It consists of two layers, one external and fibrous, the other internal and 

 serous. 



The fibrous layer is a dense, thick and unyielding membrane, consisting 

 of fibres which interlace in every direction. At the base of the pericardium 

 these fibres are firmly attached to those of the central aponeurosis of the 

 diaphragm ; and above, where the pericardium embraces the large blood- 

 vessels, the fibrous layer is continued on their surface in the form of tubular 

 prolongations, which become gradually lost upon their external coats. The 

 superior vena cava, the four pulmonary veins, the aorta, and the right and 

 left divisions of the pulmonary artery, in all eight vessels, receive pro- 

 longations of this kind. 



The serous layer not only lines the fibrous layer of the pericardium and 

 the part of the diaphragm to which that layer is attached, but, like other 

 serous membranes, is reflected on the surface of the viscus which it invests. 

 It has, therefore, a visceral and a parietal portion. The parietal portion 

 adheres firmly by its outer surface to the fibrous membrane, and becomes 

 continuous with the visceral portion upon the arch of the aorta and other 

 great vessels, about two or two and a half inches from the base of the heart. 

 In passing round the aorta and pulmonary artery, it encloses both those 

 vessels in a single short tubular sheath. It is reflected also upon the 

 superior vena cava and on the four pulmonary veins, and forms a deep 

 recess or prolonged cavity between the entrance of the right and left veins 

 into the left auricle. The inferior vena cava receives only a very scanty 

 covering of this membrane, inasmuch as that vessel enters the right auricle 

 almost immediately after passing through the diaphragm, and is only 

 partially surrounded by a reflection of the pericardium in the narrow 

 interval between these parts. None of the vessels, indeed, joining the 

 heart, with the exception of the aorta and pulmonary artery where they are 

 united together, receive a complete covering from the pericardium, or can 

 be said to pass entirely through the sac : portions only of the membrane are 

 inflected round them more or less fully. 



On separating- the left pulmonary artery and subjacent pulmonary vein, a fold of 

 the pericardium will be seen between them, which has been termed by Marshall the 

 "vestigial fold of the pericardium." It is formed by a duplicature of the serous 

 membrane, including areolar and fatty tissue, together with some fibrous bands, 

 blood-vessels, and nerves. It is from half to three-quarters of an inch in length, and 

 from a half to one inch deep. Above the pulmonary artery it blends with the peri- 

 cardium, and passes onwards to the left superior intercostal vein. Below, it is lost, 

 on the side of the left auricle, in a narrow streak which crosses round the lower left 



