138 ELEMENTARY PHYSIOLOGY. [XIII. 



c. The septum auricularum. 



d. The fossa ovalis, or expression of the foetal 

 foramen ovale, which is early closed by the 

 growth of the septum auricularum. 



e. The Eustachian valve, a slightly projecting 

 membranous fold, immediately beneath the 

 entrance of the inferior vena cava, and again 

 beneath this. 



/. The opening of the comparatively large left 

 azygos vein. 



g. The auriculo-ventricular orifice. 



G. Cut open longitudinally the azygos vein, and 

 observe the coronary vein opening into it a 

 very short distance from the heart. 



7. Cut away most of the auricle, and holding the 

 ventricle in the left hand, pour water suddenly 

 into the auriculo-ventricular orifice. The auri- 

 culo-ventricular valve will float up and close the 

 orifice. Note the star-shaped junction of the 

 valve-flaps. 



8. Introduce a pair of scissors between two of the 

 valves, and cut through the wall towards the 

 apex. Having arrived at the bottom of the 

 ventricular cavity, turn the scissors sharp round 

 and carry an incision at an acute angle with the 

 previous one, alongside the septum, towards, but 

 not into, the pulmonary artery. Lifting up the 

 flap, note 



