ANTERIOR MEDIASTINUM. 113 



two venae mnominatae constitutes the superior vena cava. 

 {Sometimes the innominate veins are not united into one, 

 but descend separately to the heart, where both have dis- 

 tinct openings in the right auricle (p. 55). 



The SUPERIOR VENA CAVA is about three inches in length ; 

 it passes downward, piercing the pericardium, to enter the 

 upper part of the right auricle of the heart. Before enter- 

 ing the pericardium, it receives the vena azygos major. 



If the subject being dissected has been injected from the 

 aorta, the pericardium will have been laid open, in order to 

 perform that operation. 



The PERICARDIUM is the sac containing the heart; it con- 

 sists of a fibrous external and a serous internal layer. The 

 fibrous layer is attached to the great vessels of the heart 

 above, and to the diaphragm below. The serous layer in- 

 vests the heart, and is then reflected from it to the fibrous 

 layer, thus forming a shut sac, as a serous membrane always 

 does, with the heart in reality lying outside of it. The 

 student has only to imagine this sac a globular one to see 

 lat the heart, by making a protrusion into its cavity, will 

 ot only be covered by a serous surface, but lie in contact 

 with another which is free and external to it. 



The HEART occupies an oblique position in the chest, its 

 apex pointing to the space between the fifth and sixth ribs, 

 two or three inches from the sternum, and its base toward 

 the right shoulder. It should be observed that the apex is 

 formed by the left ventricle, as, when the organ is opened 

 and emptied of blood, this is not so apparent. The under 

 side of the heart is flattened, and rests lipon the diaphragm ; 

 its upper surface is rounded. It may be remarked that the 

 terms "right" and "left" ventricle might well be dispensed 

 with, and the terms "anterior" and "posterior" substituted 

 in their place. Monro states that the habit of describing 

 the two sides as right and left arose from the fact that the 

 earlier dissections were made upon animals, in whom the 

 position of the ventricles differs from that in man, and is 

 in fact right and left. 



By lifting the heart, and pulling it to either side, the vessels ema- 

 nating from or entering it will be made apparent, and are to be 

 examined both within the pericardium and after tlieir exit from it. 

 On the outside a little dissection will be necessary to separate them, 

 from one another, and from the bronchial glands and cellular tissue 

 which surround them; part of the pericardium must be cut away. 



10* 



