XIII.] STKUCTUEE AND ACTION OF THE HEART. 139' 



a. The thickness of the ventricular wall, the 

 projections of its inner surface or columnse 

 carneis. The band of muscle (moderator 

 band) running from wall to wall of the ven- 

 tricle across its cavity. 



b. The ventricular cavity does not extend to 

 the apex. 



c. The tricuspid valve, its form, and attach- 

 ment to the auriculo-ventricular ring, the 

 chordae tendineae, and their attachment to 

 the summits of the papillary muscles. 



9. Holding the heart vertically, pour water into the 

 pulmonary artery ; observe from below the form 

 of the semilunar valves, and their mode of 

 closing. 



10. To observe the valves from above, insert into the 

 pulmonary artery a short wide tube, fill it with 

 water, and cover it with a piece of glass, exclud- 

 ing air-bubbles. 



11. Prolong the incision of § 8 so as to lay open the 

 pulmonary artery. Note 



a. The form and attachment of the semilunar 

 valves. 



b. The small nodule of tissue in the middle of 

 the free edge of each valve, the corpus 

 Arantii. , 



c. The slight depressions in the arterial walls 

 opposite each valve, the sinuses of Val- 

 salva. 



