DISSECTION OF THE BACK AND THORAX. 131 



Directions.—The cavity of the left auricle is to be exposed by au 

 incision from the right to the left pulmonary veins, and by another 

 from the first to the point of the appendix. 



The Cavity of the Left Aueicle is smaller than the right, but, like 

 it, consists of a sinus venosus and an ear-shaped appendage— the auricula. 



Fig 12. 



Heart seen from the Left Side, the Pericardium, Left Auricle, Left Ventricle, 

 AND Pulmonary Artery having been laid open. 



A. Left auricle ; B Left ventricle ; C. Unopened right ventricle ; D. Pericardium ; 1. Left 

 auricular appendix ; 2. Pulmonary veins ; 3. Musculi papillares ; 4. Mitral valve ; 4' Its chordai 

 tendinese ; 5. One of the trabecnlas carnese ; 6. Pulmonary artery; 7. Its semilunar valve; 

 8. Common aorta ; S' and S". Anterior and posterior aorta ; 9. Remains of ductus arteriosus ; 

 10. Right auricular appendix ; 11. Left Coronary Artery; 11' and 11". Its branches (Leisering and 

 Miiller.) 



The pulmonary veins open on the roof of the sinus venosus ; and most 

 commonly they have four openings — two from each lung, but they may 

 have as many as eight. They are not provided with valves. The 

 auricula and adjacent part of the sinus venosus show musculi pectinati 

 similar to these of the right auricle. In the floor of the cavity is the 

 left av.riculo-ventricular opening, by which, on contraction of the auricle, 

 the blood is passed into the left ventricle. 



