242 THE VASCULAR SYSTEMS 



It contains four chambers, an auricle and a ventricle on each 

 side. 



It lies obliquely, the base being directed upward, backward, 

 and toward the right, and extending from the level of the fifth 

 to that of the eighth dorsal vertebra, and the apex looking 

 downward, forward, and to the left, its impulse against the 

 chest wall being felt in the fifth left interspace, about 3j inches 

 from the middle of the sternum. The heart lies more in the left 

 than in the right side of the chest, its base being held in position 

 by the great vessels which are connected with it; its posterior 

 or posteroinferior surface is flat, formed chiefly by the left 

 ventricle, and rests on the diaphragm; and its anterior surface, 

 formed chiefly by the right ventricle, but also partly by the left, 

 is convex and covered to some extent by the lungs. Of the 

 borders, the right is long and thin, and the left is shorter and 

 thick. The length of the heart is about 5 inches; its greatest 

 breadth is 3J inches; its thickness is about 2| inches. Its weight 

 is 10 to 12 ounces in the male, 8 to 10 in the female, and it 

 increases with age. 



Externally it presents a deep transverse groove, the auriculo- 

 ventricular, which marks off an upper or auricular and a lower 

 or ventricular portion; this latter part presents a longitudinal 

 furrow on the front and back, the former being somewhat to 

 the left, the latter to the right, marking off the right and left 

 ventricles. 



The interior of the heart is divided by a longitudinal septum 

 into a right and a left part, and these, in turn, are divided into 

 an auricle and a ventricle. 



THE RIGHT AURICLE 



The right auricle is larger than the left, its wall being about 

 one line in thickness and its capacity 2 ounces. Its cavity is 

 divided into two parts, the sinus venosus and the appendix 

 auriculae, the former lying between the entrances of the two 

 venae cavse, the latter overlapping the commencement of the 

 aorta. Within the auricle the following parts present them- 

 selves for examination : 



1. The orifice of the superior /r/m mm, looking downward and 

 forward. 



