140 ELEMENTARY PHYSIOLOGY. [XIII. 



12. Lay open the left auricle in a manner similar to 

 that employed for the right. Note that the 

 left auriculo-ventricular valve, the bicuspid or 

 mitral has but two flaps. Observe its manner 

 of closing (cp. § 7). 



13. Lay open the left ventricle in a manner similar 

 to that employed on the right side, carrying 

 the incision at first along the extreme left of 

 the heart. Note the thick walls, the mitral 

 valve, &c. 



14. Lay open the aorta, and examine its semilunar 

 valves, corpora Arantii, and the sinuses of 

 Valsalva, which are here very distinct. Note 

 that the coronary arteries open respectively 

 into the two anterior sinuses. 



B. Heart of Frog. 



1. Expose the heart of a just-killed frog in the 

 manner directed in Lesson xi. B. IL With the 

 pericardium intact, observe the pulsations of the 

 heart, noting the alternate beats of the auricles 

 and the ventricle ; and the synchronous beats of 

 the two auricles. 



2. Lay open the pericardium and observe 



a. The synchronous contractions of the two 

 auricles, followed almost immediately by 



6. The contraction of the ventricle, note that 



