THORACIC CAVITY 83 



the parietal layer of the serous pericardium is reflected from 

 the fibrous layer of the pericardium on to the walls of the 

 aorta and the pulmonary artery, and the fingers, following it, 

 will pass downwards on the two great arteries and along the 

 fronts of the ventricles to the inferior border of the anterior 

 surface, so completing the circuit of the heart in the sagittal 

 plane, and demonstrating that it is covered on the posterior, 

 inferior and anterior surfaces by the serous layer of the 

 pericardium. 



It has been noted, previously, that the upper parts of the 

 anterior surfaces of the atria are concealed by the ascending 

 aorta and the stem of the pulmonary artery, from which they 

 are separated by the transverse sinus. It must be noted 

 now that the visceral layer of the serous pericardium which 

 surrounds the cavity of the transverse sinus covers the 

 posterior surfaces of the aorta and the stem of the pulmonary 

 artery, that it passes backwards from them across the inferior 

 surface of the right pulmonary artery and then downwards on 

 the anterior surface of the left atrium. It is immediately 

 behind the latter reflection that the upper border of the atrium 

 is not covered by serous pericardium in the angle between the 

 transverse sinus and the upper end of the oblique sinus. 

 (See Fig. 59, in which the prolongation upwards of the 

 fibrous layer of the pericardium on to the back of the right 

 pulmonary artery has been removed.) The fact that a finger 

 can be passed through the transverse sinus posterior to the 

 aorta and the pulmonary artery, but that it cannot be insinuated 

 between the two vessels, will indicate to the dissectors that 

 the two great arteries are enclosed in a tubular sheath of the 

 visceral part of the serous membrane. 



An examination of the venae cavae will show that the lower 

 inch of the superior vena cava lies within the fibrous peri- 

 cardium and that it is ensheathed, except along its postero- 

 medial border, by a covering of the serous layer, whilst the 

 inferior vena cava can scarcely be said to have any intra- 

 pericardial course, for it joins the lower and posterior part of the 

 right atrium immediately after piercing the fibrous layer, but the 

 margin of the orifice by which it enters is surrounded by the 

 serous layer except along a narrow line posteriorly (Fig. 32). 

 The left pulmonary veins are covered by the serous layer on 

 their superior, anterior, and inferior aspects, but not posteriorly; 

 and the right pulmonary veins, which enter the left atrium as 



