DISSECTING MANUAL. 



valve. The aortic orifice, opening from the vestibule into the 

 aorta, is circular and lies in front and to the right of the mitral. 

 It has a valve (aortic) with three semilunar segments, anterior 

 and two posterior, like those of the pulmonary valve in struc- 

 ture and attachments. [790] 



Pericardium. This sac lies in the posterior mediastinum, 

 surrounding the heart, and consists of two parts, fibrous and 

 serous. The fibrous pericardium is a conical sac attached at its 

 base to the central tendon and muscle of the Diaphragm ; the 

 apex is lost on, and gives sheaths to, the great vessels of the base 

 of the heart. The serous pericardium is a closed sac which is in- 

 vaginated by the heart and therefore separable into two parts, 

 viz. : parietal, which lines and adheres to the inner surface of 

 the fibrous sac; visceral, which ensheathes the heart. These 

 parts are continuous where the serous layer is reflected onto 

 the great vessels as they pierce the fibrous layer. The superior 

 vena cava is covered in front and laterally by pericardium; 

 the pulmonary veins in front, above, and below; and the 

 inferior vena cava in front and laterally, for a very short dis- 

 tance. A single complete sheath encloses the aorta and pul- 

 monary artery; behind this there is a passage (great trans- 

 verse sinus) from one side to the other. Between the vessels 

 which are incompletely covered there are also pouches (sinu- 

 ses); the largest (great oblique sinus) lies between the in- 

 ferior vena cava and left inferior pulmonary vein. A small 

 fold (vestigial fold of Marshall) runs from the left pulmonary 

 artery to the left superior pulmonary vein, behind the left end 

 of the transverse sinus. [793] 



ARTERIES. 



Pulmonary. Arising from the infundibulum of the right 

 ventricle, this runs upward and backward toward the con- 

 cavity of the aortic arch, curving round the left side of the 

 ascending aorta. It divides, opposite the disc below the fifth 

 vertebra, into right and left branches. It is just over two 



[128] 



