THORACIC CAVITY 59 



palpated through the posterior wall of the sinus. The 

 anterior wall of the oblique sinus is the posterior wall of 

 the left atrium (Fig. 27). If the dissector passes his left 

 index finger into the transverse sinus and the middle and 

 index fingers of his right hand into the oblique sinus, he will 

 be able to convince himself that the left atrium is the only 

 structure which intervenes between the cavities of the two 

 sinuses. When he has satisfied himself regarding this point, 

 he should note that the lower and posterior part of the 

 coronary sulcus of the heart extends across the lower part 

 of the base between the posterior end of the left ventricle 

 and the lower end of the left atrium, and that it is occupied 

 by the coronary blood sinus, which opens into the right 

 atrium immediately to the left of the upper end of the in- 

 ferior vena cava. 



A complete examination of the base of the heart cannot 

 be made until the heart is removed from the thorax at a later 

 stage of the dissection, and the dissectors should pass now 

 to a consideration of the relation of the serous layer of the 

 pericardium to the great vessels which are entering or leaving 

 the heart (see Fig. 24). They have previously noted (p. 51) 

 that the visceral layer of the serous portion of the pericardium 

 covers almost every portion of the heart, the only part left 

 uncovered being the upper border of the left auricle, which 

 is in contact with the lower border of the right pulmonary 

 artery. Along this border the visceral part of the serous layer 

 of the pericardium, ascending on the anterior aspect of the 

 left atrium, becomes continuous with the parietal layer which 

 passes anteriorly, in the roof of the transverse sinus, on the 

 lower wall of the right pulmonary artery, to the posterior 

 surface of the ascending part of the aorta, where it becomes 

 continuous with the -visceral layer which descends on the 

 posterior surface of the aorta, in the anterior wall of the 

 transverse sinus. Along the same border the visceral part 

 of the serous pericardium is reflected posteriorly in the roof 

 of the oblique sinus, to become continuous with the parietal 

 layer on the posterior wall of the pericardial sac. The 

 fact that he can pass his finger through the transverse sinus 

 posterior to the aorta and the pulmonary artery, but cannot 

 insinuate it between the two vessels, will indicate to the 

 student that the two great arteries are enclosed in a tubular 

 sheath of the visceral part of the serous membrane. An 



