IV 

 THE VASCULAR SYSTEM 



The Pericardium. The Pericardium is a fibre-serous sac 

 which encloses the heart and the roots of the great vessels. 

 It is shaped like a truncated cone, the base being directed 

 downwards and the apex upwards. Inferiorly, the outer 

 fibrous layer of the pericardium is blended with the central 

 tendon of the diaphragm, so that the possible amount of 

 lateral displacement of the heart is strictly limited. As the 

 great vessels pierce the pericardium, the fibrous layer is 

 prolonged upon them for varying distances. On the aorta, 

 it becomes blended with the pretracheal layer of the deep 

 cervical fascia, which descends into the thorax on the anterior 

 aspect of the trachea. This connexion may, perhaps, help to 

 produce the clinical phenomenon of " tracheal tugging," which 

 is found in association with aneurisms of the aortic arch (see 

 also p. 321). 



The serous pericardium consists of a parietal layer which 

 lines the fibrous pericardium, and a visceral layer which 

 is reflected on to the heart and constitutes the epicardium 

 (Fig. 104). In this way, the heart is enveloped in a completely 

 closed serous sac, so that its action becomes greatly impeded 

 when the sac is distended by effusions. As the aorta and 

 pulmonary artery leave the heart, they are surrounded by a 

 common tube-like continuation of the epicardium, about i inch 

 long, which becomes continuous with the parietal layer of 

 the sac. The superior vena cava does not possess a similar 



covering, and is only clothed on its anterior and lateral 



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