SPECIAL ANATOMY OF THE VISCEKA. 



THE THORACIC VISCEKA. 



The greater part of the thorax is occupied by the lungs, each of 

 which is invested by a serous membrane, the plnira. The heart, 

 enclosed by a membranous covering, the jierirardiwn, is situate between 

 the lungs in the middle of a space bounded laterally by the right and 

 left pleura3, and known as the mediastinal space, the septum formed by 

 the union of the two pleuras being termed the mctUa^timim. The 

 term anterior mediastinum is given to the part of this septum in front 

 of the heart and pericardium, and posterior mediastinum to the part 

 behind, while that portion which encloses the pericardium is sometimes 

 distinguished as tlie middle mediastinum. 



The anterioi' mediastinnm (fig. 1C3, 2), which is of no great 

 depth, is situate iu front of the pericardium, between it and the 

 sternum. At its superior part the two layers of pleurre separate some- 

 what to enclose the vestiges of the thymus gland ; behind the second 

 piece of the sternum they are in contact, but below this the left pleura 

 recedes from its fellow towards the left side, leaving an angular space of 

 gome breadth. The triangularis sterni muscle bounds this space in front. 



The posterior mediastinum, stretching from the pericardium to the 

 bodies of the vertebra?, encloses between its layers the lower part of the 

 windpipe and gullet (fig. 1G3 a) the thoracic duct {d), the descending- 

 aorta {a), the azygos vein (v. a), and the pneumogastric nerves {p.n), 

 together with some lymphatic glands. 



THE PERICARDIUM. 



This membranous sac, in which the heart is contained, is of a some- 

 what conical shape, its base resting on the diaphragm, whilst the upper 

 narrower part surrounds the great vessels as far as their first divi- 

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

 internal and serous. 



The fibrous layer is a dense, unyielding membrane, consisting of 

 fibres which interlace in every direction. At the base of the pericar- 

 dium this fibrous layer is attached to the upper surface of the dia- 

 phragm (fig. 164 d), partly to the central tendon, partly to the 

 adjoining muscular surface, especially on the left side. Over a small 

 space near the median line the fibrous structures are continuous, and 

 the connection very firm ; elsewhere the attachment is more lax, and is 

 effected mainly by areolar tissue. Above, where the pericardium 

 embraces the large blood-vessels, the fibrous layer is continued for some 



