154 THE CIRCULATION OF THE BLOOD 



point of wedge-shaped flap and expose cavity. Cut off pul- 

 monary artery about an inch above its origin and open right 

 auricle by cutting a piece of its wall at left of venae cavae. 



a) Pass handle of scalpel from ventricle into auricle, and 

 also from ventricle into pulmonary artery. Study out relations 

 of these openings. 



b) Slit open auricle. Note fleshy projections on its walls; 

 character of interior surface; apertures of venae cavae; and 

 entrance of 'coronary sinus below entrance of inferior cava. 



c) Pass probe through aperture along sinus and slit it open. 

 Note muscular layer covering it. 



10) Raise by its apex the flap cut out of ventricular wall, 

 and if necessary prolong cuts toward base of ventricle until 

 divisions of tricuspid valve come into view. Note fleshy 

 columns on wall of ventricle; muscular cord (not found in 

 human heart) stretching across its cavity; and prolongation of 

 ventricular cavity towards aperture of pulmonary artery. 



11) Cut away right auricle. Examine carefully tricuspid 

 valve, composed of three membranous flaps, thinning towards 

 their free edges; and tendinous cords which connect flaps to 

 muscular processes (papillary muscles) of wall of ventricle. 



12) Slit up right ventricle until origin of pulmonary artery 

 is disclosed. Looking carefully for flaps of semilunar valves, 

 prolong cut between two of them so as to open pulmonary 

 artery and spread it out. Note valves; pouch made by each 

 flap ; and slightly dilated wall of artery behind each flap. 



13) Open left ventricle and left auricle similarly. Cut aorta 

 off about y% in. above its origin. Note aperture between auricle 

 and ventricle; passage from ventricle into aorta; entry of 

 pulmonary veins into auricle; septum between auricles; and 

 septum between ventricles. 



14) Pass handle of scalpel from ventricle into auricle; 

 another from ventricle into aorta ; also pass probes into points 

 of entrance of pulmonary veins. Trace possible movements of 

 fluid through auricle, ventricle and aorta, with reference to 

 position and character of valves. 



15) Compare auricles and ventricles as to character of their 

 walls. 



16) Carefully raise wedge-shaped flap of left ventricle, and 

 <mt toward base of heart, until valve (mitral) between auricle 



