THE HEART 179 



guard the pulmonary orifice. They are semicircular, 

 their free margins being thick and tendinous, and 

 presenting at the middle a small fibrous nodule, the 

 corpus Arantii. On each side of this body, just behind 

 the free margin, the valve presents a small thinned- 

 out interval, and when the valves are closed 

 during diastole these valves are in contact, and so 

 also are the three nodules. These latter prevent 

 any leakage from the triangular space which would 

 otherwise be left. At the commencement of the 

 pulmonary artery are three pouches, the sinuses of 

 Valsaha, placed one behind each valve. They resemble 

 those of the aorta, but are smaller. 



The Left Auricle. This is smaller and thicker walled 

 than the right, and consists, like the right, of a sinus 

 and an appendix. The latter overlaps the pulmonary 

 artery. Within it presents the following features 

 of interest: 



The orifices of the pulmonary veins, opening two 

 into the right and two into the left side (sometimes 

 only three are seen); the auriculoventricular orifice; 

 and a few musculi pectin ati on the inner side of the 

 appendix. 



The Left Ventricle. This is longer than the right, and 

 forms the apex of the heart. Its walls are three times 

 as thick as those of the right. Within it presents for 

 examination : 



The auriculoventricular orifice, which is smaller than 

 the right and guarded by the mitral or bicuspid 

 valve; and the aortic opening, in front and to the 

 right of the preceding, guarded by the semilunar 

 valves. The mitral valve is attached, like the tricuspid, 

 on the right side. It consists of two curtains which 

 are larger and thicker than those of the tricuspid, 

 and of two smaller segments, one at each angle of 

 junction of the former. They are furnished with 

 chordae tendinese. The aortic semilunar valves are 

 similar to but larger and stronger than the pulmonary 



