

THE PERICARDIUM 



525 



In front, it is separated from the anterior wall of the thorax, in the greater part 

 of its extent, by the lungs and pleurae; but a small area, "some what variable in size, 

 and usually corresponding with the left half of the lower portion of the body of 

 the sternum and the medial ends of the cartilages of the fourth and fifth ribs of 

 the left side, comes into direct relationship with the chest wall. The lower extrem- 

 ity of the thymus, in the child, is in contact with the front of the upper part of 

 the pericardium. Behind, it rests upon the bronchi, the esophagus, the descending 

 thoracic aorta, and the posterior part of the mediastinal surface of each lung. 

 Laterally, it is covered by the pleurae, and is in relation with the mediastinal sur- 

 faces of the lungs; the phrenic nerve, with its accompanying vessels, descends 

 between the pericardium and pleura on either side. 



B. common carotid a. 



R. subclavian a. 



L. common carotid a. 



L. siibclavian a. 



Cut edges of serous 

 pericardium 



Sup. vena cava 



R. pulmonary/ 

 veins 



L. pulmonary veins 





FJQ. 489. Posterior wall of the pericardial sac, showing the lines of reflection of the serous pericardium 



on the great vessels. 



Structure of the Pericardium. -Although the pericardium is usually described as a single sac, 

 an examination of its structure shows that it consists essentially of two sacs intimately connected 

 with one another, but totally different in structure. The outer sac, known as the fibrous peri- 

 cardium, consists of fibrous tissue. The inner sac, or serous pericardium, is a delicate mem- 

 brane which lies within the fibrous sac and lines its walls; it is composed of a single layer of 



