552 



THE VASCULAR SYSTEMS 



of the third, fourth, fifth, sixth, and seventh ribs of the left side, in the interval 

 between the pleurae. The apex of the sac is directed upward, while the base is 

 applied to the central tendon of the Diaphragm. 



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, somewhat variable in size, and usually corresponding 

 with the left half of the lower portion of the gladiolus of the sternum and the inner extremities 

 of the cartilages of the fourth and fifth ribs of the left side, comes into direct relationship with 

 the thoracic wall. The lower extremity of the thymus gland in the child is in contact with the 

 front of the upper part of the pericardium. Behind, it rests upon the bronchi, the oesophagus, 

 and the descending aorta. Laterally, it is covered by the pleurae, and is in relation to the inner 

 surface of the lungs; the phrenic nerve with its accompanying vessels descends between the 

 pericardium and pleura on either side (Fig. 413). 



RIGHT 



PULMONARY. 

 VEINS 



PARIETA 

 PERICARDIUM 



FIG. 412. The heait with part of the serous pericardium attached. 



Structure pf 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 each other, but totally different in structure. The outer sac, known as the/6ro.v 

 pericardium, consists of fibrous tissue. The inner sac, or serous pericardium, is composed of a 

 delicate serous membrane which lies within the fibrous sac and lines its walls ; the heart invaginates 

 the wall of the serous sac from above and behind and practically obliterates its cavity, the space 

 being a potential one, except in front, where a small interspace exists below the apex of the heart. 



The fibrous pericardium forms a flask-shaped bag, the neck of which is closed by its fusion 

 with the external coats of the great vessels, while its base is attached to the central tendon and 

 to the muscle fibres of the left side of the Diaphragm. In some of the lower mammals the 

 base is either completely separated from the Diaphragm or joined to it by some loose areolar 



